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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..

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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.

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  • $\begingroup$ except the sex chromosomes in males of course $\endgroup$ – WYSIWYG Nov 19 '14 at 5:37
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    $\begingroup$ Fascinating related video: Why Women Are Stripey $\endgroup$ – user4200 Nov 19 '14 at 8:27
  • $\begingroup$ And individual's genome includes both sets of chromosomes and mitochondrial DNA. So while most human cells have two full sets of 23 chromosomes and they are diploid, that is just a single copy of the genome. $\endgroup$ – clay Dec 3 '14 at 15:51
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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.

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  • $\begingroup$ An aside that doesn't warrant its own answer: chromosomes not splitting up correctly on division (hence one daughter cell having an extra chromosome the other lacks) doesn't only happen in germ cells but can happen on any cell dvision. While incorrect division of germ cells is usually deadly (except e.g. the already mentioned Down Syndrome), this is not always the case with [somatic cells] (en.wikipedia.org/wiki/Mosaic_%28genetics%29). $\endgroup$ – user66554 Apr 11 '15 at 11:18
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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.

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  • $\begingroup$ Some of the organisms on this list have really high chromosome numbers. I can't find a reference right now, but I've heard of plants with something like 8 copies of each chromosome, must make recessive alleles really hard to express. $\endgroup$ – user137 Nov 19 '14 at 7:42
  • $\begingroup$ Yes.. many plants including wheat and maize\ $\endgroup$ – WYSIWYG Nov 19 '14 at 7:58
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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.

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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.
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