Is it possible to have different genes in different parts of our body?

Question

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?

Answer 1

This is a little difficult to answer because there are so many factors that could contribute to genetic mutations and this is by no means a comprehensive list, but here goes:

To answer 1, both. DNA can definitely mutate during replication, and it can mutate without replication.

There are billions of nucleotides in the human genome, so while the chances of the enzymes responsible for replication accidentally adding in the wrong base aren’t super high, it does happen. Even so, the body usually takes care of this via a squadron of genetic spell checkers, and even if the mutation is not caught, the mutation might not interfere with anything. If the mutation is in non-coding DNA (DNA that doesn’t contain any actual genes), generally nothing happens. Similarly, if one base is changed to another, but it still codes for the same amino acid (for instance CUA and CUG both create the amino acid leucine), nothing generally happens. If the mutation isn’t caught, but does disrupt a gene, the cell it’s in may suffer and die. If the mutation isn’t caught and disrupts a gene, it may cause cancer. Or, hopefully the body recognizes the malignant cell and triggers cell death via the immune system. There are tons of possibilities that could occur.

Several things can happen to DNA that don’t involve replication, however. For instance, viruses can actually incorporate their own genome into the host. This is what HIV does to T cells, and what HPV does to cervical cells. In the first, HIV high jacks the immune system via the T cells, eventually causing immunodeficiency in people (AIDS). In the second, cervical cells with incorporated HPV can become cancerous. Viruses essentially “live” with one goal in mind, which is to replicate as much as possible. Sometimes this interferes with the cells they infect, causing uncontrollable replication as well (cancer).

Other random hazardous things that can destroy DNA are radiation (ie sunlight, or I guess Cherynobyl) and carcinogens. In the first case, UV radiation can physically break the DNA backbone (the supports of ladder) or the bonds between the base pairs (the rungs of the ladder). This could either kill the cell (since it may need that gene to live), the cell could repair the damage, or the cell could replicate with a mutated version of the gene since replication post-damage is more susceptible to mutations. When your skin peels after a sunburn, that’s actually your body’s way of trying to get rid of affected cells that may have mutated/broken DNA. Carcinogens can cause any number of mutations, depending on what the chemical is. For instance, cigarettes often cause a point mutation (single base pair swap out) in the gene p53. This gene is responsible to tumor suppression, and without it tumors can easily grow. Like in the lungs.

From these examples, you can see how mutations can be in certain parts of the body, but not others. With all of your cells replicating at once, mutations are being made and fixed in random combinations all over the body. UV radiation generally affects the DNA in skin cells since those are exposed to the sun, whereas smoking affects the lungs through inhalation. Cancer cells can migrate through the blood and lymphatic system, but the mutation originated where the cell was first exposed to the damage.

Now for question three, what genes are expressed or not is dependent on what kind of cell it is. Everything starts as a STEM cell, but eventually differentiates into a specific cell line (like kidney cells, stomach cells, T cells, nerve cells, etc). All of these cells have the same DNA, they just differ based off which genes are turned on or off. This is controlled by external signals throughout development (simplified, chemicals come to the cell surface and tell it what to become). So stomach cells and hair follicle cells both have the gene to make hair, it’s just that it’s turned off in stomach cells but not in hair follicle cells. Or, a more interesting thing to think about, chickens have a lot of the same DNA as t rexes, it’s just that several different genes are turned off/on. So, it’s possible, maybe, that someone could turn back on chicken teeth.