24: Genes and Chromosomes - Biology

24: Genes and Chromosomes - Biology

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24: Genes and Chromosomes

24 and me: Discovery of gene of extra chromosome boosts zebra finch biology

A pair of Zebra finches at Bird Kingdom, Niagara Falls, Ontario, Canada. Credit: Wikipedia

In the zebra finch, an extra chromosome exists in the reproductive, or germline, cells. (Songbirds have 40 chromosomes and 41 with the extra chromosome.) Known as the germline-restricted chromosome, its sequence is largely unknown and none of its genes have been identified, until now. Using sophisticated genome-sequencing techniques, American University researchers have identified the first gene of the GRC. This finding could pave the way for further research into what makes a bird male or female.

"We don't know the function of this gene, and we don't know how many other organisms have genes like this," said John Bracht, assistant professor of biology at American University.

Bracht led the team of students on the genomics project. The idea originated when he started his lab at AU in 2014 and got to talking about the unusual genomics of zebra finch with Colin Saldanha, a co-author and collaborator on the study. Saldanha is an AU neurobiologist who studies how estrogen protects the brains of zebra finches from dangerous inflammation after traumatic injury. Both scientists agreed it would be worthwhile to try to sequence the mysterious extra chromosome in the germline of songbirds.

The work began three years ago, and since then, Bracht and his students have used computational biology to sequence, sort and filter genetic data obtained from Saldanha's finches. They decided to sequence RNA because much DNA can be highly repetitive and much of it is not used for the protein coding necessary for gene functions. The RNA represented a smaller target in which to find an unknown gene of an unknown chromosome, Bracht said.

Bracht and his students began the assembly process with 167,929 strands of RNA, eventually winnowing down that number through the computational process and verification work in the laboratory to eight proteins, one of which they confirmed as the first gene on the germline-restricted chromosome that they named 'GRC α-SNAP.' This is an exciting find because GRC α-SNAP is part of the SNAP family, genes crucial to membrane fusion in neuroscience and beyond. The fact that this new SNAP gene is found only in the germline immediately suggests several potential functions and directions for follow-up experiments.

Other findings from the filtering process are useful in filling in the gaps of finch biology. When the zebra finch genome was sequenced in 2010, some genes were missed. The AU team identified 936 of these missing proteins including another SNAP gene beyond the GRC α-SNAP. This makes the zebra finch the first known organism to display a gene duplication for this SNAP family of genes.

Furthermore, an evolutionary analysis showed GRC α-SNAP evolved for positive selection—evolution for changes to its protein sequence, rather than selection to maintain the status quo. This is suggestive of evolution toward a function, but much more research will be needed to determine what that function is and why it exists. For now the team can speculate: for example, a clear genetic determinant of sex is missing in birds. Could the gene play a role in sex determination?

"The discovery of GRC α-SNAP raises questions about sex determination in the zebra finch and the possibility that it is part of what makes a female bird a female, possibly downstream through the genetic expression in the ovaries," Bracht said. Next steps include sequencing the DNA and exploring functionality studies in the zebra finch.

Contributing authors to the paper are Michelle Biederman, Megan Nelson, Kathryn C. Asalone, Alyssa Pederson and Colin Saldanha. The paper is online today in Current Biology.


Humans have about 20,000 to 23,000 genes.

Genes consist of deoxyribonucleic acid (DNA). DNA contains the code, or blueprint, used to synthesize a protein. Genes vary in size, depending on the sizes of the proteins for which they code. Each DNA molecule is a long double helix that resembles a spiral staircase containing millions of steps. The steps of the staircase consist of pairs of four types of molecules called bases (nucleotides). In each step, the base adenine (A) is paired with the base thymine (T), or the base guanine (G) is paired with the base cytosine (C). Each extremely long DNA molecule is coiled up inside one of the chromosomes.

Structure of DNA

DNA (deoxyribonucleic acid) is the cell’s genetic material, contained in chromosomes within the cell nucleus and mitochondria.

Except for certain cells (for example, sperm and egg cells and red blood cells), the cell nucleus contains 23 pairs of chromosomes. A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct proteins.

The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar (deoxyribose) and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. In the steps, adenine is paired with thymine, and guanine with cytosine. Each pair of bases is held together by a hydrogen bond. A gene consists of a sequence of bases. Sequences of three bases code for an amino acid (amino acids are the building blocks of proteins) or other information.

Synthesizing proteins

Proteins are composed of a long chain of amino acids linked together one after another. There are 20 different amino acids that can be used in protein synthesis—some must come from the diet (essential amino acids), and some are made by enzymes in the body. As a chain of amino acids is put together, it folds upon itself to create a complex three-dimensional structure. It is the shape of the folded structure that determines its function in the body. Because the folding is determined by the precise sequence of amino acids, each different sequence results in a different protein. Some proteins (such as hemoglobin) contain several different folded chains. Instructions for synthesizing proteins are coded within the DNA.


Information is coded within DNA by the sequence in which the bases (A, T, G, and C) are arranged. The code is written in triplets. That is, the bases are arranged in groups of three. Particular sequences of three bases in DNA code for specific instructions, such as the addition of one amino acid to a chain. For example, GCT (guanine, cytosine, thymine) codes for the addition of the amino acid alanine, and GTT (guanine, thymine, thymine) codes for the addition of the amino acid valine. Thus, the sequence of amino acids in a protein is determined by the order of triplet base pairs in the gene for that protein on the DNA molecule. The process of turning coded genetic information into a protein involves transcription and translation.

Transcription and translation

Transcription is the process in which information coded in DNA is transferred (transcribed) to ribonucleic acid (RNA). RNA is a long chain of bases just like a strand of DNA, except that the base uracil (U) replaces the base thymine (T). Thus, RNA contains triplet-coded information just like DNA.

When transcription is initiated, part of the DNA double helix opens and unwinds. One of the unwound strands of DNA acts as a template against which a complementary strand of RNA forms. The complementary strand of RNA is called messenger RNA (mRNA). The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm (the part of the cell outside the nucleus—see Figure: Inside a Cell). There, the mRNA attaches to a ribosome, which is a tiny structure in the cell where protein synthesis occurs.

With translation, the mRNA code (from the DNA) tells the ribosome the order and type of amino acids to link together. The amino acids are brought to the ribosome by a much smaller type of RNA called transfer RNA (tRNA). Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules.

Control of gene expression

There are many types of cells in a person’s body, such as heart cells, liver cells, and muscle cells. These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells (and at different times in the same cell). The information about when a gene should be expressed is also coded in the DNA. Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood.

The mechanisms by which genes control each other are very complicated. Genes have chemical markers to indicate where transcription should begin and end. Various chemical substances (such as histones) in and around the DNA block or permit transcription. Also, a strand of RNA called antisense RNA can pair with a complementary strand of mRNA and block translation.


Cells reproduce by dividing in two. Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce (replicate) themselves during cell division. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two. After splitting, bases on each strand bind to complementary bases (A with T, and G with C) floating nearby. When this process is complete, two identical double-strand DNA molecules exist.


To prevent mistakes during replication, cells have a “proofreading” function to help ensure that bases are paired properly. There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in, and the complexity of, the protein synthesis process, mistakes may happen. Such mistakes may occur for numerous reasons (including exposure to radiation, drugs, or viruses) or for no apparent reason. Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations.

Inherited mutations are those that may be passed on to offspring. Mutations can be inherited only when they affect the reproductive cells (sperm or egg). Mutations that do not affect reproductive cells affect the descendants of the mutated cell (for example, becoming a cancer) but are not passed on to offspring.

Mutations may be unique to an individual or family, and most harmful mutations are rare. Mutations that become so common that they affect more than 1% of a population are called polymorphisms (for example, the human blood types A, B, AB, and O). Most polymorphisms have little or no effect on the phenotype (the actual structure and function of a person’s body).

Mutations may involve small or large segments of DNA. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced. If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal. For example, in phenylketonuria, a mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. This deficiency allows the amino acid phenylalanine (absorbed from the diet) to accumulate in the body, ultimately causing severe intellectual disability. In rare cases, a mutation introduces a change that is advantageous. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease. However, when a person inherits only one copy of the sickle cell gene (called a carrier), the person develops some protection against malaria (a blood infection). Although the protection against malaria can help a carrier survive, sickle cell disease (in a person who has two copies of the gene) causes symptoms and complications that may shorten life span.

Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring (and thus become less common in the population), whereas mutations that improve survival progressively become more common. Thus, beneficial mutations, although initially rare, eventually become common. The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution.

'Gay genes': science is on the right track, we're born this way. Let’s deal with it.

I n a recent Guardian article , Simon Copland argued that it is very unlikely people are born gay (or presumably any other sexual orientation). Scientific evidence says otherwise. It points strongly to a biological origin for our sexualities. Finding evidence for a biological basis should not scare us or undermine gay, lesbian and bisexual (LGB) rights (the studies I refer to do not include transgendered individuals, so I’ll confine my comments to lesbian, gay and bisexual people). I would argue that understanding our fundamental biological nature should make us more vigorous in promoting LGB rights.

Let’s get some facts and perspective on the issue. Evidence from independent research groups who studied twins shows that genetic factors explain about 25-30% of the differences between people in sexual orientation (heterosexual, gay, lesbian, and bisexual). Twin studies are a first look into the genetics of a trait and tell us that there are such things as “genes for sexual orientation” (I hate the phrase “gay gene”). Three gene finding studies showed that gay brothers share genetic markers on the X chromosome the most recent study also found shared markers on chromosome 8. This latest research overcomes the problems of three prior studies which did not find the same results.

Gene finding efforts have issues, as Copland argues, but these are technical and not catastrophic errors in the science. For example, complex psychological traits have many causal genes (not simply “a gay gene”). But each of these genes has a small effect on the trait so do not reach traditional levels of statistical significance. In other words, lots of genes which do influence sexual orientation may fall under the radar. But scientific techniques will eventually catch up. In fact there are more pressing problems that I would like to see addressed, such as the inadequate research on female sexuality. Perhaps this is due to the stereotype that female sexuality is “too complex” or that lesbians are rarer than gay men.

Genes are far from the whole story. Sex hormones in prenatal life play a role. For example, girls born with congenital adrenal hyperplasia (CAH), which results in naturally increased levels of male sex hormones, show relatively high rates of same-sex attractions as adults. Further evidence comes from genetic males who, through accidents, or being born without penises, were subjected to sex change and raised as girls. As adults these men are typically attracted to women. The fact that you cannot make a genetic male sexually attracted to another male by raising him as a girl makes any social theory of sexuality very weak. Genes could themselves nudge one towards a particular sexual orientation or genes may simply interact with other environmental factors (such as sex hormones in the womb environment) to influence later sexual orientation.

The brains of gay and heterosexual people also appear to be organised differently. For example patterns of brain organisation appear similar between gay men and heterosexual women and between lesbian women and heterosexual men. Gay men appear, on average, more “female typical” in brain pattern responses and lesbian women are somewhat more “male typical”. Differences in brain organisation mean differences in psychology and study after study show differences in cognition between heterosexual and gay people. Thus gay differences are not just about who you fancy. They are reflected in our psychology and the ways we relate to others. The influence of biology runs throughout our sexual and gendered lives and those differences, that diversity, is surely to be celebrated.

Some writers tend to wave off the scientific evidence by urging us to look to the history of sexuality or claim that homosexuality is a social construction (cue Michel Foucault and the like). But these accounts are mere descriptions at best and not scientific theories. Social constructionist accounts generate no hypotheses about sexual orientation and are not subject to systematic testing. So why should we take their claims seriously? Social constructionism and postmodernist theory question the very validity of empirical science in the first place. That makes it no better than climate science denial.

Some will argue that our common sense experiences are full of people who are “fluid” in their sexual orientations or change their sexualities. This won’t do either because our experience fools us all the time. Change is widely used to argue against biological explanations. Critics will say that if behaviour changes, or is “fluid”, then surely it can’t have a biological basis? This is false because it is our biology that allows us to learn, respond to socialisation, and helps generate our culture. So showing evidence of change is not an argument against biology. There is indeed some fluidity in sexuality over time, predominantly among women. But there is no “bell shaped curve” to sexual orientation. People may change the identity labels they use and who they have sex with but sexual attractions seem stable over time.

Remember, sexual orientation is a pattern of desire, not of behaviour or sexual acts per se. It is not a simple act of will or a performance. We fall in love with men or women because we have gay, straight, or bisexual orientations and not because of choice. So let’s stop pretending there is choice in sexual orientation. Who truly “chooses” anything of substance anyway? Surely our choices are the result of things we didn’t choose (our genes, personalities, upbringing, and culture).

People worry that scientific research will lead to “cures” for homosexuality (which is an odd worry to have if you don’t believe in the “born this way” argument). They worry more about this than the consequences of choice or environmental explanations, which are not without risk either. But clearly none of the direst predictions have materialised. Sexual minority identities have not been medicalised nor has there been any genetic testing. Genetic tests would never result in 100% accurate identification of LGB individuals because, as I said, genes are less than one-third of the story. On the social policy and legal front we’ve gone in the direction of more rights and more freedoms for LGB people (at least in the West) and not less.

So should the causes of sexuality influence how we view sexual minority identities? No. The causes of a trait should not influence how we see it. But the science shows us that sexuality has a biological basis: that is simply how the science turned out. It’s no use denying it. So let’s use it to supplement, but not replace, a discussion about LGB rights and social policy. The biology of sexuality diversity tells the world to deal with it. We are who we are, and our sexualities are part of human nature.

My worry about the claims of social construction, choice and such like is that it plays into the hands of homophobic ideology, into the hands of the “aversion therapists”, and into the hands of a growing culture which seeks to minimise gay differences. It reminds me of something Noam Chomsky alluded to : if humans were entirely unstructured creatures we would be subject to the totalitarian whims of outside forces.

Dr Qazi Rahman is an academic at the Institute of Psychiatry, King’s College London. He studies the biology of sexual orientation and the implications for mental health and is the co-author of Born Gay? The Psychobiology of Sex Orientation


The term genome was created in 1920 by Hans Winkler, [4] professor of botany at the University of Hamburg, Germany. The Oxford Dictionary suggests the name is a blend of the words gene and chromosome. [5] However, see omics for a more thorough discussion. A few related -ome words already existed, such as biome and rhizome, forming a vocabulary into which genome fits systematically. [6]

A genome sequence is the complete list of the nucleotides (A, C, G, and T for DNA genomes) that make up all the chromosomes of an individual or a species. Within a species, the vast majority of nucleotides are identical between individuals, but sequencing multiple individuals is necessary to understand the genetic diversity.

In 1976, Walter Fiers at the University of Ghent (Belgium) was the first to establish the complete nucleotide sequence of a viral RNA-genome (Bacteriophage MS2). The next year, Fred Sanger completed the first DNA-genome sequence: Phage Φ-X174, of 5386 base pairs. [7] The first complete genome sequences among all three domains of life were released within a short period during the mid-1990s: The first bacterial genome to be sequenced was that of Haemophilus influenzae, completed by a team at The Institute for Genomic Research in 1995. A few months later, the first eukaryotic genome was completed, with sequences of the 16 chromosomes of budding yeast Saccharomyces cerevisiae published as the result of a European-led effort begun in the mid-1980s. The first genome sequence for an archaeon, Methanococcus jannaschii, was completed in 1996, again by The Institute for Genomic Research.

The development of new technologies has made genome sequencing dramatically cheaper and easier, and the number of complete genome sequences is growing rapidly. The US National Institutes of Health maintains one of several comprehensive databases of genomic information. [8] Among the thousands of completed genome sequencing projects include those for rice, a mouse, the plant Arabidopsis thaliana, the puffer fish, and the bacteria E. coli. In December 2013, scientists first sequenced the entire genome of a Neanderthal, an extinct species of humans. The genome was extracted from the toe bone of a 130,000-year-old Neanderthal found in a Siberian cave. [9] [10]

New sequencing technologies, such as massive parallel sequencing have also opened up the prospect of personal genome sequencing as a diagnostic tool, as pioneered by Manteia Predictive Medicine. A major step toward that goal was the completion in 2007 of the full genome of James D. Watson, one of the co-discoverers of the structure of DNA. [11]

Whereas a genome sequence lists the order of every DNA base in a genome, a genome map identifies the landmarks. A genome map is less detailed than a genome sequence and aids in navigating around the genome. The Human Genome Project was organized to map and to sequence the human genome. A fundamental step in the project was the release of a detailed genomic map by Jean Weissenbach and his team at the Genoscope in Paris. [12] [13]

Reference genome sequences and maps continue to be updated, removing errors and clarifying regions of high allelic complexity. [14] The decreasing cost of genomic mapping has permitted genealogical sites to offer it as a service, [15] to the extent that one may submit one's genome to crowdsourced scientific endeavours such as DNA.LAND at the New York Genome Center, [16] an example both of the economies of scale and of citizen science. [17]

Viral genomes can be composed of either RNA or DNA. The genomes of RNA viruses can be either single-stranded RNA or double-stranded RNA, and may contain one or more separate RNA molecules (segments: monopartit or multipartit genome). DNA viruses can have either single-stranded or double-stranded genomes. Most DNA virus genomes are composed of a single, linear molecule of DNA, but some are made up of a circular DNA molecule. [18] There are also viral RNA called single stranded RNA: serves as template for mRNA synthesis [19] and single stranded RNA: serves as template for DNA synthesis.

Viral envelope [20] is a outer layer of membrane that viral genomes use to enter the host cell. Some of the classes of viral DNA and RNA consists of a viral envelope while some do not.

Prokaryotes and eukaryotes have DNA genomes. Archaea and most bacteria have a single circular chromosome, [21] however, some bacterial species have linear or multiple chromosomes. [22] [23] If the DNA is replicated faster than the bacterial cells divide, multiple copies of the chromosome can be present in a single cell, and if the cells divide faster than the DNA can be replicated, multiple replication of the chromosome is initiated before the division occurs, allowing daughter cells to inherit complete genomes and already partially replicated chromosomes. Most prokaryotes have very little repetitive DNA in their genomes. [24] However, some symbiotic bacteria (e.g. Serratia symbiotica) have reduced genomes and a high fraction of pseudogenes: only

40% of their DNA encodes proteins. [25] [26]

Some bacteria have auxiliary genetic material, also part of their genome, which is carried in plasmids. For this, the word genome should not be used as a synonym of chromosome.

Eukaryotic genomes are composed of one or more linear DNA chromosomes. The number of chromosomes varies widely from Jack jumper ants and an asexual nemotode, [27] which each have only one pair, to a fern species that has 720 pairs. [28] It is surprising the amount of DNA that eukaryotic genomes contain compared to other genomes. The amount is even more than what is necessary for DNA protein-coding and noncoding genes due to the fact that eukaryotic genomes show as much as 64,000-fold variation in their sizes. [29] However, this special characteristic is caused by the presence of repetitive DNA, and transposable elements (TEs).

A typical human cell has two copies of each of 22 autosomes, one inherited from each parent, plus two sex chromosomes, making it diploid. Gametes, such as ova, sperm, spores, and pollen, are haploid, meaning they carry only one copy of each chromosome. In addition to the chromosomes in the nucleus, organelles such as the chloroplasts and mitochondria have their own DNA. Mitochondria are sometimes said to have their own genome often referred to as the "mitochondrial genome". The DNA found within the chloroplast may be referred to as the "plastome". Like the bacteria they originated from, mitochondria and chloroplasts have a circular chromosome.

Unlike prokaryotes, eukaryotes have exon-intron organization of protein coding genes and variable amounts of repetitive DNA. In mammals and plants, the majority of the genome is composed of repetitive DNA. [30] Genes in eukaryotic genomes can be annotated using FINDER. [31]

Coding sequences Edit

DNA sequences that carry the instructions to make proteins are referred to as coding sequences. The proportion of the genome occupied by coding sequences varies widely. A larger genome does not necessarily contain more genes, and the proportion of non-repetitive DNA decreases along with increasing genome size in complex eukaryotes. [30]

Noncoding sequences Edit

Noncoding sequences include introns, sequences for non-coding RNAs, regulatory regions, and repetitive DNA. Noncoding sequences make up 98% of the human genome. There are two categories of repetitive DNA in the genome: tandem repeats and interspersed repeats. [32]

Tandem repeats Edit

Short, non-coding sequences that are repeated head-to-tail are called tandem repeats. Microsatellites consisting of 2-5 basepair repeats, while minisatellite repeats are 30-35 bp. Tandem repeats make up about 4% of the human genome and 9% of the fruit fly genome. [33] Tandem repeats can be functional. For example, telomeres are composed of the tandem repeat TTAGGG in mammals, and they play an important role in protecting the ends of the chromosome.

In other cases, expansions in the number of tandem repeats in exons or introns can cause disease. [34] For example, the human gene huntingtin typically contains 6–29 tandem repeats of the nucleotides CAG (encoding a polyglutamine tract). An expansion to over 36 repeats results in Huntington's disease, a neurodegenerative disease. Twenty human disorders are known to result from similar tandem repeat expansions in various genes. The mechanism by which proteins with expanded polygulatamine tracts cause death of neurons is not fully understood. One possibility is that the proteins fail to fold properly and avoid degradation, instead accumulating in aggregates that also sequester important transcription factors, thereby altering gene expression. [34]

Tandem repeats are usually caused by slippage during replication, unequal crossing-over and gene conversion. [35]

Transposable elements Edit

Transposable elements (TEs) are sequences of DNA with a defined structure that are able to change their location in the genome. [33] [24] [36] TEs are categorized as either as a mechanism that replicates by copy-and-paste or as a mechanism that can be excised from the genome and inserted at a new location. In the human genome, there are three important classes of TEs that make up more than 45% of the human DNA these classes are The long interspersed nuclear elements (LINEs), The interspersed nuclear elements (SINEs), and endogenous retroviruses. These elements have a big potential to modify the genetic control in a host organism. [29]

The movement of TEs is a driving force of genome evolution in eukaryotes because their insertion can disrupt gene functions, homologous recombination between TEs can produce duplications, and TE can shuffle exons and regulatory sequences to new locations. [37]

Retrotransposons Edit

Retrotransposons [38] are found mostly in eukaryotes but not found in prokaryotes and retrotransposons form a large portion of genomes of many eukaryotes. Retrotransposon is a transposable element that transpose through an RNA intermediate. Retrotransposons [39] are composed of DNA, but are transcribed into RNA for transposition, then the RNA transcript is copied back to DNA formation with the help of a specific enzyme called reverse transcriptase. Retrotransposons that carry reverse transcriptase in their gene can trigger its own transposition but the genes that lack the reverse transcriptase must use reverse transcriptase synthesized by another retrotransposon. Retrotransposons can be transcribed into RNA, which are then duplicated at another site into the genome. [40] Retrotransposons can be divided into long terminal repeats (LTRs) and non-long terminal repeats (Non-LTRs). [37]

Long terminal repeats (LTRs) are derived from ancient retroviral infections, so they encode proteins related to retroviral proteins including gag (structural proteins of the virus), pol (reverse transcriptase and integrase), pro (protease), and in some cases env (envelope) genes. [36] These genes are flanked by long repeats at both 5' and 3' ends. It has been reported that LTRs consist of the largest fraction in most plant genome and might account for the huge variation in genome size. [41]

Non-long terminal repeats (Non-LTRs) are classified as long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and Penelope-like elements (PLEs). In Dictyostelium discoideum, there is another DIRS-like elements belong to Non-LTRs. Non-LTRs are widely spread in eukaryotic genomes. [42]

Long interspersed elements (LINEs) encode genes for reverse transcriptase and endonuclease, making them autonomous transposable elements. The human genome has around 500,000 LINEs, taking around 17% of the genome. [43]

Short interspersed elements (SINEs) are usually less than 500 base pairs and are non-autonomous, so they rely on the proteins encoded by LINEs for transposition. [44] The Alu element is the most common SINE found in primates. It is about 350 base pairs and occupies about 11% of the human genome with around 1,500,000 copies. [37]

DNA transposons Edit

DNA transposons encode a transposase enzyme between inverted terminal repeats. When expressed, the transposase recognizes the terminal inverted repeats that flank the transposon and catalyzes its excision and reinsertion in a new site. [33] This cut-and-paste mechanism typically reinserts transposons near their original location (within 100kb). [37] DNA transposons are found in bacteria and make up 3% of the human genome and 12% of the genome of the roundworm C. elegans. [37]

Genome size is the total number of the DNA base pairs in one copy of a haploid genome. Genome size varies widely across species. Invertebrates have small genomes, this is also correlated to a small number of transposable elements. Fish and Amphibians have intermediate-size genomes, and birds have relatively small genomes but it has been suggested that birds lost a substantial portion of their genomes during the phase of transition to flight. Before this loss, DNA methylation allows the adequate expansion of the genome. [29]

In humans, the nuclear genome comprises approximately 3.2 billion nucleotides of DNA, divided into 24 linear molecules, the shortest 50 000 000 nucleotides in length and the longest 260 000 000 nucleotides, each contained in a different chromosome. [45] There is no clear and consistent correlation between morphological complexity and genome size in either prokaryotes or lower eukaryotes. [30] [46] Genome size is largely a function of the expansion and contraction of repetitive DNA elements.

Since genomes are very complex, one research strategy is to reduce the number of genes in a genome to the bare minimum and still have the organism in question survive. There is experimental work being done on minimal genomes for single cell organisms as well as minimal genomes for multi-cellular organisms (see Developmental biology). The work is both in vivo and in silico. [47] [48]

Genome size due to transposable elements Edit

There are many enormous differences in size in genomes, specially mentioned before in the multicellular eukaryotic genomes. The main reason why there is such a big variety of sizes is due to the presence of transposable elements. TEs are known to contribute to a significant change in a cell's mass of DNA. [29] This process is correlated to their long-term accommodation in the host genome, and therefore, to the expansion of the genome size.

Here is a table of some significant or representative genomes. See #See also for lists of sequenced genomes.

Initial sequencing and analysis of the human genome [90]

All the cells of an organism originate from a single cell, so they are expected to have identical genomes however, in some cases, differences arise. Both the process of copying DNA during cell division and exposure to environmental mutagens can result in mutations in somatic cells. In some cases, such mutations lead to cancer because they cause cells to divide more quickly and invade surrounding tissues. [95] In certain lymphocytes in the human immune system, V(D)J recombination generates different genomic sequences such that each cell produces a unique antibody or T cell receptors.

During meiosis, diploid cells divide twice to produce haploid germ cells. During this process, recombination results in a reshuffling of the genetic material from homologous chromosomes so each gamete has a unique genome.

Genome-wide reprogramming Edit

Genome-wide reprogramming in mouse primordial germ cells involves epigenetic imprint erasure leading to totipotency. Reprogramming is facilitated by active DNA demethylation, a process that entails the DNA base excision repair pathway. [96] This pathway is employed in the erasure of CpG methylation (5mC) in primordial germ cells. The erasure of 5mC occurs via its conversion to 5-hydroxymethylcytosine (5hmC) driven by high levels of the ten-eleven dioxygenase enzymes TET1 and TET2. [97]

Genomes are more than the sum of an organism's genes and have traits that may be measured and studied without reference to the details of any particular genes and their products. Researchers compare traits such as karyotype (chromosome number), genome size, gene order, codon usage bias, and GC-content to determine what mechanisms could have produced the great variety of genomes that exist today (for recent overviews, see Brown 2002 Saccone and Pesole 2003 Benfey and Protopapas 2004 Gibson and Muse 2004 Reese 2004 Gregory 2005).

Duplications play a major role in shaping the genome. Duplication may range from extension of short tandem repeats, to duplication of a cluster of genes, and all the way to duplication of entire chromosomes or even entire genomes. Such duplications are probably fundamental to the creation of genetic novelty.

Horizontal gene transfer is invoked to explain how there is often an extreme similarity between small portions of the genomes of two organisms that are otherwise very distantly related. Horizontal gene transfer seems to be common among many microbes. Also, eukaryotic cells seem to have experienced a transfer of some genetic material from their chloroplast and mitochondrial genomes to their nuclear chromosomes. Recent empirical data suggest an important role of viruses and sub-viral RNA-networks to represent a main driving role to generate genetic novelty and natural genome editing.

Works of science fiction illustrate concerns about the availability of genome sequences.

Michael Crichton's 1990 novel Jurassic Park and the subsequent film tell the story of a billionaire who creates a theme park of cloned dinosaurs on a remote island, with disastrous outcomes. A geneticist extracts dinosaur DNA from the blood of ancient mosquitoes and fills in the gaps with DNA from modern species to create several species of dinosaurs. A chaos theorist is asked to give his expert opinion on the safety of engineering an ecosystem with the dinosaurs, and he repeatedly warns that the outcomes of the project will be unpredictable and ultimately uncontrollable. These warnings about the perils of using genomic information are a major theme of the book.

The 1997 film Gattaca is set in a futurist society where genomes of children are engineered to contain the most ideal combination of their parents' traits, and metrics such as risk of heart disease and predicted life expectancy are documented for each person based on their genome. People conceived outside of the eugenics program, known as "In-Valids" suffer discrimination and are relegated to menial occupations. The protagonist of the film is an In-Valid who works to defy the supposed genetic odds and achieve his dream of working as a space navigator. The film warns against a future where genomic information fuels prejudice and extreme class differences between those who can and can't afford genetically engineered children. [98]

Crossing Over in Genes within Chromosome | Genetics

In this article we will discuss about:- 1. Meaning of Crossing Over 2. Double Cross-Over 3. Cytological Basis 4. Cytological Evidence 5. Somatic Crossover 6. Different Theories on the Mechanism 7. Theories that Explain the Happenings 8. Chiasma Formation—the Theories 9. No Cross-over in Drosophila Males 10. Experi­mental Conditions.

  1. Meaning of Crossing Over
  2. Double Cross-Over
  3. Cytological Basis of Crossing Over
  4. Cytological Evidence of Crossing Over
  5. Somatic Crossover
  6. Different Theories on the Mechanism of Crossing Over
  7. Theories that Explain the Happenings during Cross-Over
  8. Chiasma Formation—the Theories
  9. No Cross-over in Drosophila Males
  10. Cross-over Frequency under Experi­mental Conditions

1. Meaning of Crossing Over:

Linkage is an exception to Mendel’s principles of independent assortment and crossing over is in the same way an exception to linkage.

Crossing over means breaks in the link­age of genes within the chromosome and a bodily transshipment of genes from one chromosome to the corresponding position in its mate (Fig. 2.13). The phenomenon of crossing over closely resembles independent assortment of Mendel but it is a different thing.

Independent assortment is concerned with the whole chromosome while cross­ing over involves parts of chromosome. It is a sort of shuffling of genes between homologous pairs of chromosomes which always brings forth new combination.

The gametes containing the new combi­nations are known as cross-over or a-combination gametes. The gametes in which the linked genes remain in their original combinations are called non-cross-over gametes.

A case of crossing over in Drosophila:

A gray long female obtained by making a cross between gray long and black vesti­gial fly is back crossed to a black vestigial male. It is expected that in such a cross the two original kinds will be produced in the F2 generation.

But in actual experi­ment four kinds of offsprings—gray long and black vestigial like the grand parental combinations and two new com­binations gray vestigial and black long ap­peared. The percentage of these four types were: gray long 41 – 5, black vestigial 41-5, gray vestigial 8𔃿 and black long 8𔃿.

The percentages show that free and ran­dom assortment of all gametes have not occurred because had it been so the ratio would have been 1 : 1 : 1 : 1.

The appearance of new combinations is the resultant outcome of breaks in the linkage of the genes within the chromo­somes. This incompleteness of linkage leading to exchange of position of genes from one chromosome to the corresponding position of its partner is due to the pheno­menon of crossing over (Fig. 2.14).

From the experiment mentioned above it appears that there are 83 per cent (41 . 5 + 41 . 5) of non-crossing over and 17 percent (8 . 5 + 8 . 5) of crossing-over.

The percentage of cross-over varies bet­ween different genes. But for each pair of genes the percentage remains constant. According to Morgan the cross-over per­centage is related to the relative distance on the chromosomes between the two pairs of alleles.

Greater the distance, greater will be the amount of crossing over bet­ween them. In a simple way it may be stated that breaks occur more frequently in long chromosomes than in short one and between distant points on the same chromosome.

Interference and coincidence:

In crossing over, not only single pair of isolated genes are involved but also the whole blocks of genes which lie close together. Their proximity interferes mechanically with the crossing over of neighbouring genes owing to the limited flexibility of the chromosomes. In other words, crossing over at a particular region of a chromo­some tries to prevent another crossing over close to it.

This phenomenon is called interference. It is because of interference that there are no or few double cross-overs within a section of chromosome 10 units or less in length. The amount of inter­ference becomes less when the distance between two genes increases and there may be no interference when the distance is too great.

The double cross-overs are nothing but coming together or ‘coincidence’ of two single cross-overs. Thus when double cross­overs occur in expected numbers the coinci­dence is said to be 100 per cent and in such cases the interference is 0. When there is no double cross-overs the inter­ference is 100 per cent and coincidence is 0. Thus coincidence is inversely propor­tional to the amount of interference.

2. Double Cross-Over:

Crossing over just once is known as single cross-over and the resultant gametes are called single cross-overs. But sometimes crossing over occurs at two points in the same chromosome pair. This is known as double cross-over and the gametes so form­ed are called double cross-overs.

The amount of double cross-over between two loci increases with the distance apart of the loci. But as a rule double cross-overs are fewer than single cross-overs. Crossing over may also occur at three loci in the same chromosome pair (triple cross-over) but they are still fewer.

A case of double cross-over in Droso­phila:

A double cross-over involves three linked genes in the same chromosome. In Droso­phila yellow body (y), miniature wing (m) and forked bristles (f) are three recessive mutations in the X chromosome. The normal fly has gray body, long wings and straight bristles.

If we indicate the mutant genes by the symbols and their normal alleles by + signs then yellow, minature and forked female will be ymf/ymf, apure female will be represented a +++/+++, and a pure male will be represented as +++. A cross between ymf/ymf ♀ x +++ ♂ may give a female of genotype ymf/+++.

When reduction division takes place in the female, the following possibilities of gamete formation will be encountered (Fig. 2.15).

We can now calculate the distances between y m and f.

Percentage of single crossover between y and m = 30%.

Percentage of double crossover between y and m = 6%.

Total percentage of crossover between y and m = 36%.

Percentage of single crossover between m and f =14%.

Percentage of double crossover between m and f= 6%.

Therefore, total percentage of crossover between m and f =20%.

Thus the distance between y and m = 36 and the distance between m and f = 20. Since the genes are in the order y mf the distance between y and f = 36 + 20=56 (Fig. 2.16).

The above calculation shows that in getting the distance double crossovers have been counted twice. This appears to be little confusing. But it is to be remem­bered that a double crossover is equi­valent to two single crossovers—one bet­ween the genes y and m and another bet­ween the genes m and f. The double cross­overs are, therefore, considered twice in getting the total amount of crossovers between y and f.

3. Cytological Basis of Crossing Over:

During the prophase stage of first meiotic division, the two members of each pair of chromosomes, i.e., maternal and paternal chromosome come and pair. This pairing is called synapsis. Pairing occurs not only between homologous chromo­somes but also between homologous parts of the chromosomes. Each chromosome then becomes duplicated and as a result a tetrad consisting of four chromatids is formed.

During late prophase of first meiotic division the two centromeres tend to go apart. But the chromatids attached to the centromeres do not, as a rule, sepa­rate uniformly along their entire length. At one or more points along the tetrad, two of the four chromatids appear to lie across each other forming chiasma.

At each chiasma, two of the four chromatids break and then rejoin, so that newly oriented chromatids are formed out of sections of original ones. Because of this chiasma for­mation maternal and paternal chromo­somes cannot transmit as individual units.

They are compelled to exchange sections. The make-up of the chromosomes before and after meiosis gats changed to some extent because of this segmental inter­change. Walter has explained the pheno­menon as “Jack and Jill have exchanged heads and although nothing is missing they are now different individuals than they were before”.

4. Cytological Evidence of Crossing Over:

Crossing over involves segmental inter­change between homologous chromosomes. But normally crossing over cannot pro­duce permanent visible alternation in the structure of a chromosome. Thus, it is al­most impossible to differentiate between a non-crossover chromosome and a cross­over chromosome.

An experiment by Stern, however, gives cytological evidence in favour of crossing over. The experiment is a classic one and demonstrates visible results of crossing over. It forms a direct correlation of cytological and genetical crossover and was published in 1931.

The X chromosome of Drosophila is rod-shaped and a female possesses a pair of such rod-shaped X’s. But Stern obtained a female in which one X chromosome is broken into two. One part of this broken X houses the mutant gene carnation (Carnation = car which is recessive and imparts dark red eyes) and the gene Bar eye (Bar or Narrow eyes, dominant).

Both the broken segments had centromeres. In one it was the original centromere while the other derived its centromere probably from the fourth chromosome. Since these fragments had a centromere each they could be distributed in the normal manner in ceil division.

The unbroken X chromo­some had a fragment of the Y chromosome attached to one of its ends and contained the normal alleles (+’s) of Carnation and Bar (Fig. 2.17).

Now if there be no crossover the two X chromosomes will go to the two gametes in their changed (changed from normal) make-up and if there be crossover bet­ween Carnation and Bar the broken X chromosome bearing the Bar gene will have the Y chromosome attached to it and the unbroken one will lose the Y chromosome though it will have the car­nation gene.

The Y chromosome here will act as the marker and thus it will be possible to distinguish the crossovers microscopically.

Now if the X chromosome bearing Car + is added to each of those four classes of eggs produced by the hybrid female only female off-springs will result. When the chromosomes of these off-springs are exa­mined under microscope it is found that (Fig. 2.18).

(a) The offsprings which appear Car­nation Bar are with broken X chro­mosome.

(b) The offsprings which appear red round (Normal) are with the unbroken X with the Y chromosome attached to it.

(c) The red Bar offsprings are with broken X with Y chromosome attached to the part bearing the Bar gene.

(d) The offsprings which appear Car­nation round are with unbroken X without the attached Y chromo­some.

Thus it becomes evident that when two genetic non-crossover classes bear non-crossover X chromosomes of the mother but the two genetic crossover classes bear the crossover X’s of the mother. This is the cytological basis of crossing over.

5. Somatic Crossover:

Pairing of chromosomes is restricted to the germ cells and it takes place during the first maturation division. Somatic cross­over is a rare phenomenon. In Drosophila such a rare instance of somatic cross-over has been shown by Stern.

Such somatic crossovers occur in one or two cells during the course of development of the fly. But these cells give rise to a cluster of cells through the process of division re­sulting the formation of a patch or spot on the body with the crossover cells. The somatic cells on the other parts of the body will be normal. Thus, the fly will be a mosaic of crossover and non-crossover tissues.

A somatic crossover cannot be in­vestigated by taking into consideration the offsprings of the fly since the somatic cells do not give rise to offsprings. So in de­tecting somatic crossover in any organism it is the organism itself to be examined for any ‘crossover spot’.

In Drosophila it has been possible to detect such spots with the use of certain genes. The suitable genes for the purpose are yellow body colour (y) and ‘singed’, i.e., short and curly bristles (sn). Both the genes y and sn are recessive mutants and are located on the X chromo­somes. Normal alleles for the genes are indicated by + signs.

Stern got a fly of genotype y+ /+sn. That is one chromosome of the fly is with y+ and its homologous chromosome is with + sn (Fig. 2.19). Both the chromosomes are telo­centric (Note the dot end of the chromo­some).

Now let us assume that in the deve­loping fly crossover has occurred in one such cell and that between sn and the centromere. The split chromosome halves attached to a given centromere would no longer remain alike. In each instance one of the sister chromatids would be y + and the mates of these chromatids would be + sn.

Now if during the line-up of the chromosomes at metaphase the two chro­matids with y+ face one pole and the two chromatids with + sn face the opposite pole, at the end of the division we would get two cells—one with genotype y+/y+ and the other with genotype + sn/+ sn.

By further division, each of the cells would give rise to cluster of cells. If these two clusters lie close to the surface of the body, y+/y+ clusters would form a yellow spot and +sn/+sn would form a spot with singed bristles. The two spots would lie close together since they have been derived from sister cells and they would in this way give rise to twin spots. The causes for somatic crossover are not known.

6. Different Theories on the Mechanism of Crossing Over:

This theory ex­plains that during the early pro­phase stage of Meiosis the chromo­somes split up longitudinally. Each chromosome forms two sister chro­matids. The two non-sister chroma­tids of the homologous pairs of chromosomes coil round each other.

At their points of contact the chro­matids break first and cross. The theory thus states that crossing over does not produce chiasmata but actually chiasmata are caused by crossing over.

B. Chiasms type theory:

The theory states that breaking up of chroma­tids occurs at pachytene stage. After breaking up the chromatids unite again and form a chiasma. Thus according to it the chiasma is the result of crossing over.

This theory is based on the fact that synthetic activity and duplication of chromo­somes are intimately associated with recombination. In the mechanism according to the theory the sister chromatids duplicate their genetic parts and non-sister chromatids develop fibres at random.

The entire recombinants are formed from newly formed sections. The theory takes for granted that duplication occurs during late meiotic prophase but now it has been established that DNA replication occurs long before synapsis.

7. Theories that Explain the Happenings during Cross-Over:

According to this theory the chromatids destined to undergo cross-over touch each other first and then cross-over to give rise to chiasma. After this, breakage takes place at the point of contact and new attachment of chromatid parts takes place.

Breakage first theory:

Muller advocated the theory. According to him the chromo­somes destined to cross-over first break into two segments then reunion occurs between non-sister chromatids to give new arrangement.

This theory was ad­vocated by Darlington. The theory states that the chromosomes break as a result of strain at the time of pairing. A sort of strain develops when two chromatids pair, twist round each other and this results in breakage and reunion.

Belling believes that crossing over occurs between newly dupli­cated genes and that there is no breakage or reunion during crossing over.

Significance of crossing over:

a. Crossing over supports the fact that genes are arranged in a linear fashion on chromosomes.

b. Crossing over provides oppor­tunities for reshuffling of genes and thus brings variations which play major role in the process of evolution.

c. By calculating the cross-over fre­quency it is possible to plot the genes on the chromosomes.

8. Chiasma Formation—the Theories:

The process of chiasma formation was first correctly understood by a Belgian Cytologist, Janssens (1909). He suggested that a chiasma represents an exchange of parts between homologous chromosomes, lie thought that the exchange involves the whole chromosomes or in other words both chromatids of each homologous chromo­some exchange parts with both chroma­tids of the other.

But from the present-day knowledge, we know that the exchange at any point is between single chromatid— one of paternal and one of maternal origin—while the other two chromatids remain unaffected. Fig. 2.20 gives a schematic idea of exchange of genes between chromatids during crossing over.

Many speculations have been made to assign causes for breakage of chromatids. The followers of two-plane theory advocate that chiasmata causes crossing over. While the followers of one-plane theory claim that chiasmata is the consequence not the cause of crossing over.

But the real cause behind the breakage and subse­quent rejoining of chromatids is still not known. However, the process is a highly precise one because the two chromatids in a chiasma exchange mirror image seg­ments and no gain or loss of genes occurs (Fig. 2.21).

9. No Cross-over in Drosophila Males:

Recombination of linked genes occurs in most of the organisms that furnish materials for genetic studies. That is formation of chiasmata is universal in males and fe­males of these organisms. The situation is, however, different in case of Drosophila males where crossing over rarely or never occurs. This is because linkage is complete in male Drosophila.

A similar situation is encountered in silk-moth where no cross-over occurs in females. Cytological studies of spermatogenesis in Drosophila males show that homologous chromosomes pair as usual. But no chiasmata is estab­lished at least in the autosomal bivalents. At the first meiotic divisions the pairs of chromatids go straight to the two poles and at the second division single chroma­tid passes to each cell.

10. Cross-over Frequency under Experi­mental Conditions:

The cross over frequency in the chromo­somes may be influenced by a number of physiological and external environmental factors. In old females of Drosophila the amount of crossing over is less than what is encountered in its young age.

X’rays (Muller), temperature and chemical com­position of food affect the cross-over fre­quency. Frequency of crossing over which is nil in case of Drosophila males in normal circumstances is increased by X’rays.

Crossing over is a feature that appears during gametogenesis. Under certain cir­cumstances it has been seen in somatic cells. This somatic crossing over occurs in Drosophila (Stern) and in maize (Jones) Its significance is not yet understood.

Genomic DNA

Before discussing the steps a cell undertakes to replicate, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s complete complement of DNA is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle. The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth.

In eukaryotes, the genome comprises several double-stranded, linear DNA molecules (Figure 6.2) bound with proteins to form complexes called chromosomes. Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells (somatic cells) have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, a configuration known as diploid. The letter n is used to represent a single set of chromosomes therefore a diploid organism is designated 2n. Human cells that contain one set of 23 chromosomes are called gametes, or sex cells these eggs and sperm are designated n, or haploid.

Figure 6.2 There are 23 pairs of homologous chromosomes in a female human somatic cell. These chromosomes are viewed within the nucleus (top), removed from a cell in mitosis (right), and arranged according to length (left) in an arrangement called a karyotype. In this image, the chromosomes were exposed to fluorescent stains to distinguish them. (credit: “718 Bot”/Wikimedia Commons, National Human Genome Research)

The matched pairs of chromosomes in a diploid organism are called homologous chromosomes. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the different forms of a characteristic. For example, the shape of earlobes is a characteristic with traits of free or attached.

Each copy of the homologous pair of chromosomes originates from a different parent therefore, the copies of each of the genes themselves may not be identical. The variation of individuals within a species is caused by the specific combination of the genes inherited from both parents. For example, there are three possible gene sequences on the human chromosome that codes for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type (the trait) is determined by which two versions of the marker gene are inherited. It is possible to have two copies of the same gene sequence, one on each homologous chromosome (for example, AA, BB, or OO), or two different sequences, such as AB.

Minor variations in traits such as those for blood type, eye color, and height contribute to the natural variation found within a species. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosomes other than a small amount of homology that is necessary to reliably produce gametes, the genes found on the X and Y chromosomes are not the same.

24: Genes and Chromosomes - Biology

  • growth and repair
  • creation of gametes (sex cells)
  • method of reproduction in unicellular organisms

Binary Fission - type of reproduction that occurs in bacterial cells, single celled organism splits and becomes two identical organisms

Chromosomes are DNA wrapped around proteins to form an X-shaped structure.

The diagram will help you see the relationship .

1. Chromosomes are found in the nucleus
2. Chromosomes are made of DNA
3. Sections of chromosomes are called genes

DNA - deoxyribonucleic acid (it is the genetic code that contains all the information needed to build and maintain an organism)

Chromosome Structure

Each organism has a distinct number of chromosomes, in humans, every cell contains 46 chromosomes. Other organisms have different numbers, for instance, a dog has 78 chromosomes per cell.

Somatic Cells - body cells, such as muscle, skin, blood . etc. These cells contain a complete set of chromosomes (46 in humans) and are called DIPLOID.

Sex Cells - also known as gametes. These cells contain half the number of chromosomes as body cells and are called HAPLOID

Chromosomes come in pairs, called Homologous Pairs (or homologs). Imagine homologs as a matching set, but they are not exacly alike, like a pair of shoes.

Diploid cells have 23 homologous pairs = total of 46

Haploid cells have 23 chromosomes (that are not paired) = total of 23

Chromosomes determine the sex of an offspring. In humans, a pair of chromosomes called SEX CHROMOSOMES determine the sex.

If you have XX sex chromosomes - you are female

If you have XY sex chromosomes - you are male

During fertilization, sperm cells will either contain an X or a Y chromosome (in addition to 22 other chromosomes - total of 23). If a sperm containing an X chromosome fertilizes an egg, the offspring will be female. If a sperm cell containing a Y chromosome fertilizes an egg, the offspring will be male.

When two sex cells, or gametes come together, the resulting fertilized egg is called a ZYGOTE

Zygotes are diploid and have the total 46 chromosomes (in humans)

A karyotype is a picture of a person's (or fetus) chromosomes. A karyotype is often done to determine if the offspring has the correct number of chromosomes. An incorrect number of chromosomes indicates that the child will have a condition, like Down Syndrome

Compare the Karyotypes below

Notice that a person with Down Syndrome has an extra chromosome #21. Instead of a pair, this person has 3 chromosomes - a condition called TRISOMY (tri = three)

Trisomy results when chromosomes fail to separate - NONDISJUNCTION - when sex cells are created. The resulting egg or sperm has 24 instead of the normal 23.

Other conditions result from having the wrong number of chromosomes:

Klinefelters Syndrome - XXY (sex chromosomes)

Edward Syndrome - Trisomy of chromosome #13


Each species has its own characteristic number of chromosomes. Humans, for instance, have 46 chromosomes in a typical body cell, while dogs have 78. Like many species of animals and plants, humans are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologous pairs. Thus, the 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes see below).

Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad.

Image modified from “Karyotype,” by the National Institutes of Health (public domain).

The two chromosomes in a homologous pair are generally very similar to one another. They’re the same size and shape, and have the same pattern of light and dark bands, as you can see in the human karyotype (image of the chromosomes) shown above. Bands appear when the chromosomes are stained with a dye, and the dark bands mark more compacted DNA (usually, with fewer genes), while the light bands mark less compacted DNA (usually, with more genes). Most importantly, the two homologues in a pair carry the same type of genetic information. For instance, there is a gene found near the bottom of chromosome 15 that affects eye color [1] . A person might have the blue version, or allele, of this gene on one homologue, but the brown version on the other. Both homologues have the same type of gene in the same place, but they can (and often do!) have different versions of genes.

In humans, the X and Y chromosomes determine a person’s biological sex, with XX for female and XY for male. While the two X chromosomes in a woman’s cells are genuinely homologous, the X and Y chromosomes of a man’s cells are not. They differ in size and shape, with the X being much larger than the Y, and contain different mostly different genes (although they do have small regions of similarity). The X and Y chromosomes are known as sex chromosomes, while the other 44 human chromosomes are called autosomes.

Watch the video: DNA, Chromosomes, Genes, and Traits: An Intro to Heredity (January 2023).