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I want to understand genetic mutation specially in the context of multicellular organisms like humans. I studied biology only till high school and I can’t fully understand wikipedia pages on this subject.

I know that genetic material is in chromosomes inside the nucleus of a cell. When that cell multiplies, the chromosomes too generates a copy of themselves and both child cells get one set of the chromosomes.

I would like to know:

  1. Does genetic mutation mean an error in chromosome generation at fission stage or will it occur later in the life of the cell?

  2. Multicellular organisms begin their life from a single cell (zygote). Under normal circumstances, every cell that grows from this single cell should have the same genetic material. Can genetic mutation eventually result in us having different genetic material in cells of different parts of our body? For example, when only a part of our body gets exposed to radiation.

  3. If answer to question 2 is yes, which genetic material will control the behaviour of the cells? Let’s say, for example, that the genetic material of cells in our stomach favour high growth rate of hair but the cells in the scalp themselves do not have that particular gene. What will be the result?

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  • $\begingroup$ Welcome to Biology.SE! 1-Genetic mutation means error in base sequence, not chromosome generation. 2- No. No effect would be so powerful that it would alter your genetic material. That'd mean that you are half human and half elephant (or something worse). 3- the result would be no hair at all (stomach would burn the hair on its lining, and scalp wouldn't generate hair at all). If you're satisfied with this, then I can post it as an answer with some more details. $\endgroup$ – another 'Homo sapien' Mar 25 '16 at 11:06
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    $\begingroup$ @another'Homosapien' I don't agree, there are many ways DNA can be altered, and smaller genetic changes in single cells happen due to errors in all the time. Of course this does not turn you into half-elephant, but it is certainly possible that the different cells in the body have differences in their genetic material. $\endgroup$ – Roland Mar 25 '16 at 12:22
  • $\begingroup$ @Roland I know, I just gave an introduction to these topics and said that I would give more details on this. However, iayork has already posted an answer which, I think, answers many parts of the question. :) $\endgroup$ – another 'Homo sapien' Mar 25 '16 at 12:25
  • $\begingroup$ More information on protein variation among despite little to no genetic variation: More variation in proteins than genes. Why? $\endgroup$ – Remi.b Mar 25 '16 at 12:32
  • $\begingroup$ @Kanj, I think your question (3) is causing some confusion. What do you mean by "which genetic material will control the behaviour" ? Each cell has its own copy of the genetic material (genome), and if the genome of one cell is changed by a mutation, various things can go awry in that particular cell, but it does not affect other cells in the body. Perhaps you can clarify your stomach--scalp example? $\endgroup$ – Roland Mar 25 '16 at 20:11
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The general answer is that no, it's not possible to have different genes in different parts of the body. There are two interesting classes of exceptions, but they are exceptions.

The first is that mutations can and do occur in cells during their growth. Most of these are probably neutral - they occur in the vast wasteland of genetic junk in the chromosome, and nothing happens. But some of them are harmful, and this is what puts cells on the pathway toward cancer. Outright cancers, that we can detect, are the result of at least a half-dozen mutations occurring in the same cell lineage, that cumulatively allow the cells in that lineage to grow in uncontrolled manner, to avoid immune clearance, to encourage their own blood vessel growth, and so on. (In fact, cancer cells generally have far more than a half-dozen mutations, because a route toward getting the mutations they need is to turn down DNA repair, allowing mutations to accumulate with minimal control. For example, Genomic landscape of DNA repair genes in cancer, Genomic profiling of pediatric acute myeloid leukemia reveals a changing mutational landscape from disease diagnosis to relapse.)

The second set of exceptions is in the immune system. Both B cells and T cells specifically modify their chromosomes during their development, making huge numbers (trillions) or new genes. The catch is that these are all in the same region, the antibody or T cell receptor genes. It's part of the mechanism by which the adaptive immune system "remembers" which pathogens it's interacted with in the past. The wikipedia article on V(D)J recombination is a starting point.

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    $\begingroup$ Good, but your examples show (in contrast to the first sentence) that the general answer is yes, it is possible to have different genetic material in different cell types of the body. "Different genes" sounds strange, but definitely different alleles of genes. $\endgroup$ – Roland Mar 25 '16 at 12:27
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  1. Genetic mutation usually refers to any errors made while copying DNA. This can certainly occur in the lead-up to cell division, when chromosomes are copied, although they can also occur when repairing DNA that has been damaged.

  2. I'm reading your use of the term genetic material as referring to DNA. Genetic mutation can change the sequence of DAN, but it doesn't really change what genetic material we have in any given cell. I think that it's important to understand that all of the cell in our bodies contain the same genome, or set of genes. What differs from one cell type to another is which genes are active (or "expressed" as biologists say). Mutations can alter gene activity. Say, for instance, that the deletion of a single nucleotide causes a frameshift; this can lead to the loss of that gene product in the affected cell and its daughters.

  3. Understanding that all genes are present, but only some genes are active/expressed should help clear up your third question.

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  • $\begingroup$ For (2), how do you figure that a genetic mutation does not change the genetic material? $\endgroup$ – Roland Mar 25 '16 at 16:34
  • $\begingroup$ @Roland - DNA remains DNA despite any mutations. At least, that was the way that I interpreted the OP's question. It sounded like he was confused about really basic things like what genetic material is. I felt that a more complicated answer risked straying too far off-target and becoming a broader biology lesson. I'll edit the answer to make that a little clearer, though. :-) $\endgroup$ – Forest Mar 25 '16 at 17:33
  • $\begingroup$ I agree that (2) is unclear. Personally, I also I find the expression "genetic mutation" a bit ridiculous and I would only expect non-biologist to use this term instead of just "mutation". $\endgroup$ – Remi.b Mar 25 '16 at 17:53
  • $\begingroup$ By definition, the DNA sequence changes with mutations, which means that the "genetic material" has changed. I think the OP's question 2 is pretty clear: the question is whether somatic mutations occur. (And yes, they do.) $\endgroup$ – Roland Mar 25 '16 at 20:05

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