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Do human cells contain only one copy of the genome?

Do human cells contain only one copy of the genome?


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Does each cell contain only a single copy of its genome? Or are there ever 'extra' copies present.

Reason behind question: Wondering whether gene mutations could be corrected by referencing a 'backup copy'… If such a thing exists…


As a quick answer, yes, most human cells carry 2 copies of the genome and are known as diploid cells. One copy comes from each of your parents, so they aren't identical, but usually pretty close. Sperm and egg cells only carry one copy of the genome and are known as haploid. During fertilization the 2 cells merge their copies and make a diploid zygote. At the chromosomal level, humans have 23 chromosomes, so a diploid cell has 2 copies of each so a total of 46.

As far as repairing damaged DNA, using one chromosome to repair its damaged counterpart is known as Homologous Recombination.


Diploid cells contain two copies of the genome. Additionally, each copy of the genome can have multiple copies of certain genes. Which can provide a level of protective redundancy.

However, there are a number of potential problems with having multiple copies of portions of the genome such as trisomy disorders (e.g. Down Syndrome). Which is why therapy like you describe is still highly experimental.


Addition to other answers.

Usually a human cell is diploid. Sometimes there might be extra copies of a chromosome and this phenomenon is called aneuploidy (Downs Syndrome, Klinefelter's syndrome etc). This always has deleterious effects because of changes in stoichiometries of different gene products (dosage effect).

Some other organisms also tolerate polyploidy (more than two copies of the entire genome) - salamanders and a lizard whose name I can't remember right now.

Would an extra copy help in providing a functional gene when the other copy is mutated?

Yes and it is the case with all recessive mutations. The effect of loss of function is not seen because the other allele is providing a functional copy (sometimes the dosage is reduced and it affects the system in subtle ways).

However, it the mutation is dominant then extra copies won't help much. For example, a mutation that leads to the formation of a toxic or hyperactive protein (gain of function), then extra copies may at best reduce the overall relative concentration of the bad allele but cannot eliminate its ill effects.


To address the "reason behind the question" - no, this doesn't work as you seem to assume it does. It is the whole point of sexual reproduction to have two slightly different copies of the genome. Mutations are not "corrected" within an organism, because they are not considered a bad thing per se. Mutations are "corrected" across generations, in the sense that maladaptive mutations are selected out of the gene pool, because their carriers don't manage to reproduce well.

If there were a mechanism in the cell which would decide that "this copy of the genome is 'correct', this is 'incorrect', I will replace all differing places in the 'incorrect' copy with the information from the 'correct' copy", evolution would never have happened.

An individual has two genome copies, and each of them gets transcribed from. If one of them contains a mutation with negative effects, then it's bad luck for the individual. The purpose of the two genome copies is to increase genetic variability, not to reduce it.


Most of the above answers are incorrect.

The definition of the genome is all of the encoding genetic material. An individual human genome includes two full sets of 23 chromosomes as well as mitochondrial DNA. There is one copy of this genome in most human cells. A few exceptions:

  • Red blood cells don't have nucleuses and have no chromosomes. They don't have a copy of the genome.
  • Human sperm cells have only one set of 23 chromosomes. This is not the complete genome of the individual.
  • Despite comments above, human egg cells have a full two sets of chromosomes until ovulation. The fertilized zygote gets one set of chromosomes from an egg cell and one set from a sperm cell. However, the actual egg cell in the ovaries has a full two sets of chromosomes which don't all get transferred to the zygote.
  • There is genetic variation between cells due to damage and copying mistakes.
  • Some living humans are chimeric and are often unaware of this. They are individuals formed from distinct egg/sperm combinations. That adds extra complexity to this answer.

Why does every cell in our body contain DNA?

Not every cell in the human body contains DNA bundled in a cell nucleus. Specifically, mature red blood cells and cornified cells in the skin, hair, and nails contain no nucleus.

As part of the maturation process, human red blood cells destroy their cell nuclei. They do this in order to carry as much oxygen as possible and still stay small enough to fit through narrow blood capillaries, thereby maximizing the oxygen delivery. In fact, humans have some of the smallest red blood cells of all vertebrates, thanks in part to the destruction of the nucleus. Most mammals have red blood cells without nuclei, while all other types of vertebrates do have nuclei in their red blood cells. However, all red blood cells, including human, must start with DNA, as DNA contains the code that tells each cell how to construct itself in the first place. Human red blood cells simply destroy their nucleus once it is no longer needed as part of the maturation process. A ring of actin within a maturing red blood cell pinches and splits the cell into two parts: one part with the DNA and one part without. Red blood cell enucleation is therefore a special type of cell division. Macrophages then come along and gobble up the parts containing DNA, leaving only the red blood cell parts that don't have DNA. Note that there is much more in blood than red blood cells. As a result, a blood sample does contain DNA due to the presence of other kinds of cells.

Cornified cells in the skin, hair, and nails also contain no cell nucleus. Like red blood cells, these cells start out with cell nuclei in order to develop properly, but then destroy their nuclei as part of the cornification process. They do this in order to maximize the space in the cell filled with the structural protein keratin. Keratin is a strong protein that gives hair, skin, finger nails, and toe nails their toughness. Cells that undergo cornification experience a form of programmed, controlled cell death in order to achieve their strength. The cell nucleus and other internal parts of the cell are destroyed and their space is filled by keratin. Once cornification is complete, these cells are dead and carry out no biochemical processes. But dead does not mean useless. Cornified cells fulfill their end purpose of giving structural strength and warmth to surrounding tissues despite being dead. The fact that cornified cells are dead means that you can cut your hair, clip your nails, and rub off the outer layer of skin without causing any damage or killing cells. The lack of nuclear DNA in cornified cells means that forensic biologists can rarely extract DNA from hair clippings in order to help determine the culprit.

Aside from red blood cells and cornified cells, all other cells in the human body contain nuclear DNA. Also, all cells start with nuclear DNA. The reason for this is that DNA contains the basic code that tells each cell how to grow, function, and reproduce.


Explainer: What are genes?

We inherit genes from our parents. Genes are like a recipe that’s passed along from one generation to the next. But the number of genes doesn’t explain why we are more complex than simple animals or bacteria.

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February 8, 2019 at 6:30 am

Genes are the blueprints for building the chemical machinery that keeps cells alive. That’s true for humans and all other forms of life. But did you know that with 20,000 genes, people have almost 11,000 fewer genes than water fleas? If the number of genes doesn’t predict complexity, what does?

The answer is that our genetic material contains much more than the units we call genes. Just as important are the switches that turn a gene on and off. And how cells read and interpret genetic instructions is far more complex in people than in those water fleas.

Genes and the switches that control them are made of DNA. That’s a long molecule resembling a spiral ladder. Its shape is known as a double helix. A total of three billion rungs connect the two outer strands — the upright supports — of this ladder. We call the rungs base pairs for the two chemicals (pair) from which they are made. Scientists refer to each chemical by its initial: A (adenine), C (cytosine), G (guanine) and T (thymine). A always pairs with T C always pairs with G.

In human cells, the double-stranded DNA doesn’t exist as one gigantic molecule. It’s split into smaller chunks called chromosomes (KROH-moh-soams). These are packaged into 23 pairs per cell. That makes 46 chromosomes in total. Together, the 20,000 genes on our 46 chromosomes are referred to as the human genome.

The role of DNA is similar to the role of the alphabet. It has the potential to carry information, but only if the letters are combined in ways that make meaningful words. Stringing words together makes instructions, as in a recipe. So genes are instructions for the cell. Like instructions, genes have a “start.” Their string of base pairs must follow in a specific order until they reach some defined “end.”

Explainer: What’s on your genes

If genes are like a basic recipe, alleles (Ah-LEE-uhls) are versions of that recipe. For instance, the alleles of the “eye color” gene give directions for making eyes blue, green, brown and so on. We inherit one allele, or gene version, from each of our parents. That means most of our cells contain two alleles, one per chromosome.

But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.

Scientists Say: Chromosome

By combining two sets of 23 chromosomes — one set from the egg, one set from the sperm cell — that new person ends up with the usual two alleles and 46 chromosomes. And her unique combination of alleles will never arise in the exact same way again. It’s what makes each of us unique.But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.

A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.

To make organs and tissues, the cells use the instructions on their DNA to build tiny machines. They control reactions between chemicals in the cell that eventually produce organs and tissues. The tiny machines are proteins. When a cell reads a gene’s instructions, we call it gene expression. A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.

How does gene expression work?

Gene expression relies on helper molecules. These interpret a gene’s instructions to make the right types of proteins. One important group of those helpers is known as RNA. It’s chemically similar to DNA. One type of RNA is messenger RNA (mRNA). It’s a single-stranded copy of the double-stranded DNA.

Making mRNA from DNA is the first step in gene expression. That process is known as transcription and happens inside a cell’s core, or nucleus. The second step, called translation, takes place outside of the nucleus. It turns the mRNA message into a protein by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids.

All human proteins are chains with different combinations of 20 amino acids. Some proteins control chemical reactions. Some carry messages. Still others function as building materials. All organisms need proteins so that their cells can live and grow.

To build a protein, molecules of another type of RNA — transfer RNA (tRNA) — line up along the mRNA strand. Each tRNA carries a three-letter sequence on one end and an amino acid on the other. For example, the sequence GCG always carries the amino acid alanine (AL-uh-neen). The tRNAs match up their sequence with the mRNA sequence, three letters at a time. Then, another helper molecule, known as a ribosome (RY-boh-soam), joins the amino acids on the other end to make the protein.

One gene, several proteins

Scientists first thought that each gene held the code to make one protein only. They were wrong. Using the RNA machinery and its helpers, our cells can make way more than 20,000 proteins from their 20,000 genes. Scientists don’t know exactly how many more. It could be a few hundred thousand — perhaps a million!

Explainer: What are proteins?

How can one gene make more than one type of protein? Only some stretches of a gene, known as exons, code for amino acids. The regions in between them are introns. Before the mRNA leaves a cell’s nucleus, helper molecules remove its introns and stitch together its exons. Scientists refer to this as mRNA splicing.

The same mRNA may be spliced in different ways. This often happens in different tissues (perhaps skin, the brain or the liver). It’s like the readers “speak” different languages and interpret the same DNA message in multiple ways. That’s one way the body can have more proteins than genes.

Scientists Say: DNA sequencing

Here’s another way. Most genes have multiple switches. The switches determine where an mRNA begins to read a DNA sequence, and where it stops. Different start or end sites create different proteins, some longer and some shorter. Sometimes, transcription doesn’t start until several chemicals attach themselves to the DNA sequence. These DNA binding sites may be far away from the gene, but still influence when and how the cell reads its message.

Splicing variations and gene switches result in different mRNAs. And these are translated into different proteins. Proteins also may change after their building blocks have been assembled into a chain. For example, the cell may add chemicals to give a protein some new function.

DNA holds more than building instructions

Making proteins is far from DNA’s only role. In fact, only one percent of human DNA contains the exons that the cell translates into protein sequences. Estimates for the share of DNA that controls gene expression range from 25 to 80 percent. Scientists do not yet know the exact number because it’s harder to find these regulatory DNA regions. Some are gene switches. Others make RNA molecules that aren’t involved in building proteins.

Controlling gene expression is almost as complex as conducting a large symphony orchestra. Just consider what it takes for a single fertilized egg cell to develop into a baby within nine months.

So does it matter that water fleas have more protein-coding genes than people? Not really. Much of our complexity hides in the regulatory regions of our DNA. And decoding that part of our genome will keep scientists busy for many, many years.

Power Words

allele One of two or more alternative versions of a gene.

amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic building blocks of proteins.

base pairs (in genetics) Sets of nucleotides that match up with each other on DNA or RNA. For DNA, adenine (A) matches up with thymine (T), and cytosine (C) matches up with guanine (G).

biology The study of living things. The scientists who study them are known as biologists.

cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chromosome A single threadlike piece of coiled DNA found in a cell’s nucleus. A chromosome is generally X-shaped in animals and plants. Some segments of DNA in a chromosome are genes. Other segments of DNA in a chromosome are landing pads for proteins. The function of other segments of DNA in chromosomes is still not fully understood by scientists.

coding (in genetics) The instructions contained in DNA (or its genes) that allow a cells to know what proteins to make and when to make them. (in computing) A slang term for developing computer programming — or software — that performs a particular, desired computational task.

decoding Figuring out a message hidden in some code.

development (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape. (v. develop)

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

egg The unfertilized reproductive cell made by females.

exon Part of a DNA or RNA molecule that holds the directions for creating part of a protein or peptide.

expression (in genetics) The process by which a cell uses the information coded in a gene to direct a cell to make a particular protein.

gene A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.

guanine One of four substances that organisms need to produce DNA.

helix An object with a three-dimensional shape like that of a wire wound uniformly in a single layer around a cylinder or cone, as in a corkscrew or spiral staircase.

intron A section of DNA or RNA that does not carry the blueprints for making some protein.

messenger RNA (or mRNA) A type of genetic material that is copied from DNA. It carries the instructions for building a cell’s proteins.

molecule An electrically neutral group of atoms that represents the smallest possibleamount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nucleus Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.

organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

RNA A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.

sequence (in genetics) n. The precise order of the nucleotides within a gene. (v.)
To figure out the precise order of the nucleotides making up a gene.

sibling An offspring that shares the same parents (with its brother or sister).

sperm The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

splice Originally meant to weave the ends of two pieces of rope together so that it becomes one longer piece of rope. It can now also mean to take two long things (movie film, pieces of lumber or pieces of DNA, for instance) and make them a single longer one.

stitch A length of thread that binds two or more fabrics together.

tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

translation (in genetics) The process of turning the mRNA message into a protein. A cell does this by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids. Translation occurs outside of a cell’s inner core, or nucleus.

transcription (v. transcribe) To copy something down, word for word. (in genetics) The first step in gene expression. It's where an enzyme copies a selected piece of DNA into RNA (especially messenger RNA). Both DNA and RNA are made up of base pairs of nucleotides.

transfer RNA (tRNA) A type of RNA (ribonucleic acid) molecule that a cell uses to read a section of messenger RNA. This takes place during the production of a cellular protein.

unique Something that is unlike anything else the only one of its kind.

About Silke Schmidt

Silke Schmidt is a freelance science writer with degrees in biostatistics and journalism. She enjoys covering the environment, engineering and medicine. She has two kids and two places she calls home, Wisconsin and Germany.

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Aluelements and genome evolution

Alu elements are ancestrally derived from the 7SL RNA gene [35, 36]. Although the details of the origin are not known, it seems likely that a relatively inefficient retrotransposon was formed by a deleted version of the 7SL RNA gene sometime before the primate/rodent evolutionary divergence. This precursor then evolved into B1 repeats in rodents, and into FLAM (free left Alu monomer) and FRAM (free right Alu monomer) sequences in the primate lineage [36, 37]. A dimer of FLAM and FRAM eventually took on the highly efficient amplification characteristics of the Alu elements.

Large-scale sequencing studies of primate genomes have provided a great deal of detail on the evolution of Alu elements. Because there is no specific mechanism for removal of Alu insertions, Alu evolution is dominated by the accumulation of new Alu inserts. These new Alu inserts accumulate sequence variation over time and are rarely removed by non-specific deletion processes. Different periods of evolutionary history have given rise to different subfamilies of Alu elements with a very limited and homogeneous group of subfamilies active in any given species because of a very limited number of source, or master, Alu loci (Figure 3) [38, 39]. The earliest Alu elements were the J subfamily, followed by a very active series of S subfamilies. The dominant S subfamilies included Sx, Sq, Sp and Sc (Alu subfamilies and their nomenclature are defined in [40]). More recently, most of the Alu amplification in old world monkey and ape lineages has been from a series of Y subfamilies, with Ya5 and Yb8 dominating in humans. The Alu amplification rate peaked with the S subfamilies [38]. Comparisons between chimpanzee and human genomes have shown that, since their divergence about 6 million years ago, there have been about 2,400 and 5,000 lineage-specific insertions fixed, respectively [41, 42]. There are 110,000 lineage-specific insertions in the Rhesus macaque genome [43]. However, this estimate was measured over a longer period of time than the estimates for human and chimpanzee insertion rates. Thus, we are unable to compare rates over the same period of time. The orangutan has only acquired approximately 250 lineage-specific insertions in the last 12 million years [44], demonstrating a marked decrease in amplification rate in that lineage. L1 elements do not show a significant difference in their lineage-specific insertions between human, chimp and orangutan, and it therefore appears that changes in Alu source elements or other Alu-specific amplification changes have occurred to cause the slow rate in orangutan. Further studies from incomplete, large-scale analyses of other primate genomes [45] show that the overall rates of Alu insertion in the marmoset lineage were generally lower than towards the human lineage, supporting the idea that Alu amplification rates vary in a species-specific or lineage-specific manner. Subfamily analysis and these rate studies suggest that the bottleneck events that occur during speciation can result in altered levels of Alu activity, probably through fixation of different numbers or levels of activity of source elements.

Evolutionary impact of Alu elements in primates. An approximate evolutionary tree is shown for various primate species. The approximate density of Alu elements in the genomes of those species is shown as the number of Alu elements per megabase (MB). For specific evolutionary time periods, marked by thicker lines, the number of lineage-specific Alu insertions (Lsi) is marked. Data of Alu/Alu recombination causing deletions (Dels) between the human and chimp genomes are also shown. Note that the rate of Alu insertion, as well as recombination, seems to vary with different lineages and different evolutionary time periods.

Alu elements have an even larger impact than that provided by their insertional mutagenesis through their influence on genome instability by providing the most common source of homology for non-allelic homologous recombination events leading to disease [23, 46]. The bioinformatics required to analyze these types of rearrangements from comparative genomic data is technically more difficult than characterizing insertions. However, studies of the human and chimpanzee genomes show that approximately 500 deletion events have occurred in both genomes (Figure 3) [47, 48]. It has not been possible to assess the duplication events that are also caused by this type of recombination, but it is likely that there is approximately the same number of events, and these events have also been suggested to contribute to genomic inversions [49] and segmental duplications [50]. The lower number of apparent non-allelic Alu/Alu recombination events between human and chimpanzee relative to the number of Alu insertion events (Figure 3) suggests that the recombination events cause a stronger negative selection because there are many more Alu recombination events than insertions causing disease [23]. Thus, they contribute more to disease, but are less well fixed in the population. This is consistent with the relatively short length of the fixed deletions relative to the longer deletions commonly found associated with disease [46].

Alu elements are preferentially enriched in regions that are generally gene rich, whereas L1 elements are enriched in the gene-poor regions [1]. This also correlates with Alu elements being enriched in reverse G bands [51], as well as in G+C-rich genomic isochores [52]. However, younger Alu and L1 elements do not show much disparity in their locations, making it most likely that the differences in location are the result of losses of L1 and Alu elements in different genomic regions. It is easy to understand why the much larger L1 elements might have more negative selection when located in genes, making Alu elements much more stably maintained within the genes. It is more difficult to understand why Alu elements seem to be preferentially lost between genes over evolutionary time compared with L1. It is most likely that the tendency of Alu elements to participate in non-allelic homologous recombination events might allow loss of these elements when not under selection [53, 54].


Function

DNA directs a cell ’ s activities by specifying the structures of its proteins and by regulating which proteins and how much are produced, and where. In so doing, it never leaves the nucleus. Each human cell contains about 6 ft (2 m) of highly condensed DNA, which encodes some 30, 000 genes. If a particular protein is to be made, the DNA segment corresponding to the gene for that protein acts as a template (pattern) for the synthesis of an RNA molecule in a process known as transcription. This messenger RNA molecule travels from the nucleus to the cytoplasm where it in turn acts as the template for the construction of the protein by the protein assembly apparatus of the cell. This latter process is known as translation and requires an adaptor molecule, transfer RNA, which translates the genetic code of DNA into the language of proteins.

Eventually, when a cell divides, its DNA must be copied so that each daughter cell will have a complete set of genetic instructions. The structure of DNA is perfectly suited to this process. The two intertwined strands unwind, exposing their bases, which then pair with bases on free nucleotides present in the cell. Because of the base-pairing rules, the sequence of bases along one strand of DNA determines the sequence of bases in the newly forming complementary strand. An enzyme then joins the free nucleotides to complete the new strand. Since the two new DNA strands that result are identical to the two originals, the cell can pass along an exact copy of its DNA to each daughter cell.

Sex cells, the eggs and sperm, contain half the number of chromosomes as other cells. When the egg and sperm fuse during fertilization, they form the first cell of a new individual with the complete complement of DNA — 46 chromosomes. Each cell (except the sex cells) in the new person carries DNA identical to that in the fertilized egg cell. In this way the DNA of both parents is passed from one generation to the next. Thus, DNA plays a crucial role in the propagation of life.


Allele

An allele is a viable DNA (deoxyribonucleic acid) coding that occupies a given locus (position) on a chromosome.

Usually alleles are sequences that code for a gene, but sometimes the term is used to refer to a non-gene sequence.

An individual's genotype for that gene is the set of alleles it happens to possess.

In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype.

An example is the gene for blossom color in many species of flower &mdash a single gene controls the color of the petals, but there may be several different versions (or alleles) of the gene.

One version might result in red petals, while another might result in white petals.

The resulting color of an individual flower will depend on which two alleles it possesses for the gene and how the two interact.

An allele is an alternative form of a gene (in diploids, one member of a pair) that is located at a specific position on a specific chromosome.

Diploid organisms, for example, humans, have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene.

An organism in which the two copies of the gene are identical &mdash that is, have the same allele &mdash is called homozygous for that gene.

An organism which has two different alleles of the gene is called heterozygous.

Phenotypes (the expressed characteristics) associated with a certain allele can sometimes be dominant or recessive, but often they are neither.

A dominant phenotype will be expressed when at least one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type.

However, there are exceptions to the way heterozygotes express themselves in the phenotype.

One exception is incomplete dominance (sometimes called blending inheritance) when alleles blend their traits in the phenotype.

An example of this would be seen if, when crossing Antirrhinums &mdash flowers with incompletely dominant "red" and "white" alleles for petal color &mdash the resulting offspring had pink petals.

Another exception is co-dominance, where both alleles are active and both traits are expressed at the same time for example, both red and white petals in the same bloom or red and white flowers on the same plant.

Codominance is also apparent in human blood types.

A person with one "A" blood type allele and one "B" blood type allele would have a blood type of "AB".

A wild type allele is an allele which is considered to be "normal" for the organism in question, as opposed to a mutant allele which is usually a relatively new modification. (Note that with the advent of neutral genetic markers, the term 'allele' is now often used to refer to DNA sequence variants in non-functional, or junk DNA.

For example, allele frequency tables are often presented for genetic markers, such as the DYS markers.) Also there are many different types of alleles.


DNA and Genetic Material in Viruses | Genetics

In this article we will discuss about the evidence that DNA is the genetic material in viruses.

The conclusion reached by Avery, Macleod and McCarty was not immediately accepted. Some investigators suggested that the DNA preparations contained a mutagen which caused mutation to the S form. Others pointed out that transformation was due to traces of some specific protein remaining in the DNA samples used.

One apparent reason why the significance of this work was only gradually recognised was the absence of knowledge about bacterial genetics at that time. The existence of a chromosome capable of exchanging genes was not fully known in bacteria.

Moreover doubts continued as to whether DNA or protein was functioning as the genetic material. DNA was mostly ruled out because its only variable components were the four nitrogenous bases (the sugar and phosphate being identical in all nucleotides).

On the other hand the molecular complexity of a protein, due to twenty different amino acids in various sequences could better account for the diversity required in a genetic material for performing the varied functions in an organism.

Looking back it seems that it was perhaps due to lack of knowledge that the great achievement of Avery, Macleod and McCarty was not rewarded with a Nobel Prize. Their discovery however, did have the impact of initiating studies in molecular genetics with the use of micro-organisms.

One of the finest demonstrations that DNA is the genetic material came in 1950’s through investigations on bacterial viruses by A. D. Hershey and Martha C. Chase. They were studying the mechanism by which the virus infects E. coli.

Bacterial viruses known as bacteriophages are obligate parasites which must infect a host cell in order to reproduce. The bacterial viruses most studied are those that infect E. coli bacteria particularly those classified as T2. It was known at that time that T2 consists exclusively of DNA and protein.

In EM the T2 virus has a hexagonal body and a protruding tail. When a suspension of these viruses is mixed with a suspension of E. coli and left at 37°C, the viruses infect the host cells by becoming attached by their tails to the bacterial cell wall. After about 30 minutes of this attachment, the bacteria burst (lyse) releasing several hundred newly synthesised viruses.

The noteworthy point in the above experiment is that the original infecting viruses can still be seen attached to the bacterial membrane. Hershey and Chase became interested to find out what molecular event occurred between the time of T2 attachment to E. coli and the release of new virus progeny.

Obviously, some material contained in the infecting viruses must have passed into the host bacterium where it caused the formation of new viruses. This must be the genetic material of the virus. Hershey and Chase now prepared bacterial viruses in which either the phosphorus or sulphur were radioactively labelled.

By doing so they could distinguish between viral DNA and viral protein and trace them during the process of infection. They did this by first growing E. coli cells in a nutrient medium containing radioactive phosphate ( 32 P) and sulphur ( 35 S).

The labelled E. coli cells were used as hosts for un-labelled viruses. The resulting virus progeny labelled with 32 P and 35 S was used to infect un-labelled E. coli cells in a series of experiments popularly known as the waring blender experiments (Fig. 13.2).

Since sulphur is present only in protein and not in DNA and phosphorus only in DNA but not in protein, it was clear that in the labelled viruses, the protein component was selectively labelled with 35 S and the DNA with 32 P. When a suspension of labelled viruses was mixed with un-labelled E. coli cells and left for a few minutes, the viruses were observed in EM attached to the bacteria.

By stirring the mixture in a warring blender, the attached viruses were broken away from the host cell. The suspension was divided into two parts. One portion was incubated for some more minutes and the bacteria burst to liberate the newly formed progeny virus. The remaining suspension was cooled and centrifuged to separate the bacteria from the remains of attached viruses.

The amounts of radioactive sulfur and phosphorus were assayed in each. They found that 85% of 32 P of the viruses had passed into the bacteria whereas 80 % of 35 S had remained in the viruses. It was apparent therefore that most of the 32 P containing DNA had entered the host bacterium whereas bulk of the proteins did not.

Experiments performed in a more refined way showed that almost all the virus DNA enters the infected E. coli cell and only about 3 per cent of the protein. Evidently the genetic material of the virus is contained in the DNA rather than in the trace of protein. With conclusions drawn from Hershey and Chase experiments, DNA was indisputably recognised as the hereditary material.


The virus’s genome is less than 30,000 genetic “letters” long. (Ours is over 3 billion.) The infected cell reads the RNA and begins making proteins that will keep the immune system at bay and help assemble new copies of the virus.

Antibiotics kill bacteria and do not work against viruses. But researchers are testing antiviral drugs that might disrupt viral proteins and stop the infection.


Genes

A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood.

Genes are the basic unit of genetics. Human beings have 20,000 to 25,000 genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.


Genetic Code

Although the genetic code is not a "code" in the sense normally used in intelligence and espionage terminology, a fundamental understanding of the genetic code is essential to understanding the molecular basis of advanced DNA and genetic tests that are increasingly important in forensic science and identification technology.

The genetic information that is passed on from parent to offspring is carried by the DNA of a cell. The genes on the DNA code for specific proteins that determine appearance, different facets of personality, health etc. In order for the genes to produce the proteins, it must first be transcribed from DNA to RNA in a process known as transcription. Thus, transcription is defined as the transfer of genetic information from the DNA to the RNA. Translation is the process in which genetic information, carried by messenger RNA (mRNA), directs the synthesis of proteins from amino acids, whereby the primary structure of the protein is determined by the nucleotide sequence in the mRNA.

The genetic code is the set of correspondences between the nucleotide sequences of nucleic acids such as deoxyribonucleic acid (DNA), and the amino acid sequences of proteins (polypeptides). These correspondences enable the information encoded in the chemical components of DNA to be transferred to the ribonucleic acid messenger (mRNA) and then used to establish the correct sequence of amino acids in the polypeptide. The elements of the encoding system, the nucleotides, differ by only four different bases. These are known as adenine (A), guanine,(G), thymine (T) and cytosine (C), in DNA or uracil (U) in RNA. Thus RNA contains U in the place of C and the nucleotide sequence of DNA acts as a template for the synthesis of a complementary sequence of RNA, a process known as transcription. For historical reasons, the term genetic code in fact refers specifically to the sequence of nucleotides in mRNA, although today it is sometimes used interchangeably with the coded information in DNA.

Proteins found in nature consist of 20 naturally occurring amino acids. One important question is, how can four nucleotides code for 20 amino acids? This question was raised by scientists in the 1950s soon after the discovery that the DNA comprised the hereditary material of living organisms. It was reasoned that if a single nucleotide coded for one amino acid, then only four amino acids could be provided for. Alternatively, if two nucleotides specified one amino acid, then there could be a maximum number of 16 (4 2 ) possible arrangements. If, however, three nucleotides coded for one amino acid, then there would be 64 (4 3 ) possible permutations, more than enough to account for all the 20 naturally occurring amino acids. The latter suggestion was proposed by the Russian born physicist, George Gamow (1904 – 1968) and was later proved to be correct. It is now well known that every amino acid is coded by at least one nucleotide triplet or codon, and that some triplet combinations function as instructions for the termination or initiation of translation. Three combinations in tRNA, UAA, UGA and UAG, are termination codons, while AUG is a translation start codon.

The genetic code was solved between 1961 and 1963. The American scientist Marshall Nirenberg (1927 – ), working with his colleague Heinrich Matthaei, made the first breakthrough when they discovered how to make synthetic mRNA. They found that if the nucleotides of RNA carrying the four bases A, G, C and U, were mixed in the presence of the enzyme polynucleotide phosphorylase, a single stranded RNA was formed in the reaction, with the nucleotides being incorporated at random. This offered the possibility of creating specific mRNA sequences and then seeing which amino acids they would specify. The first synthetic mRNA polymer obtained contained only uracil (U) and when mixed in vitro with the protein synthesizing machinery of Escherichia coli it produced a polyphenylalanine — a string of phenylalanine. From this it was concluded that the triplet UUU coded for phenylalanine. Similarly, a pure cytosine (C) RNA polymer produced only the amino acid proline, so the corresponding codon for cytosine had to be CCC. This type of analysis was refined when nucleotides were mixed in different proportions in the synthetic mRNA and a statistical analysis was used to determine the amino acids produced. It was quickly found that a particular amino acid could be specified by more than one codon. Thus, the amino acid serine could be produced from any one of the combinations UCU, UCC, UCA, or UCG. In this way the genetic code is said to be degenerate, meaning that each of the 64 possible triplets

have some meaning within the code and that several codons may encode a single amino acid.

This work confirmed the ideas of the British scientists Francis Crick (1916 – ) and Sydney Brenner (1927 – ). Brenner and Crick were working with mutations in the bacterial virus bacteriophage T4 and found that the deletion of a single nucleotide could abolish the function of a specific gene. However, a second mutation in which a nucleotide was inserted at a different, but nearby position, restored the function of that gene. These two mutations are said to be suppressors of each other, meaning that they cancel each other's mutant properties. It was concluded from this that the genetic code was read in a sequential manner starting from a fixed point in the gene. The insertion or deletion of a nucleotide shifted the reading frame in which succeeding nucleotides were read as codons, and was thus termed a frameshift mutation. It was also found that whereas two closely spaced deletions, or two closely spaced insertions, could not suppress each other, three closely spaced deletions or insertions could do so. Consequently, these observations established the triplet nature of the genetic code. The reading frame of a sequence is the way in which the sequence is divided into the triplets and is determined by the precise point at which translation is initiated. For example, the sequence CATCATCAT can be read CAT CAT CAT or C ATC ATC AT or CA TCA TCA T in the three possible reading frames. Sometimes, as in particular bacterial viruses, genes have been found that are contained within other genes. These are translated in different reading frames so the amino acid sequences of the proteins encoded by them are different. Such economy of genetic material is, however, quite rare.

The same genetic code appears to operate in all living things, but exceptions to this universality are known. In human mitochondrial mRNA, AGA and AGG are termination or stop codons. Other differences also exist in the correspondences between certain codon sequences and amino acids.