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It's my understanding that various hazards can damage the DNA in our cells, causing mutations.

But whenever I picture this, I see the damage being done to one of our tissues (for example, our lungs due to smoking, or our skin due to UV rays).

When I think about this, I see that... many cells in a smoker's lungs, or many cells on the back of a beach-goer's neck, may have mutations in their DNA. But only the cells in that tissue have these mutations... the other cells in our body would not have the same mutations.

In particular, sperm and egg cells would not have the same mutations, so the mutations due to smoking and UV rays shouldn't pass on to children.

Are there instances where mutations that occur over the course of our life are spread to every cell, including sperm and egg cells, so that every cell reflects the mutation, and the mutation is passed onto our offspring?

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This usually happens when the mutations occur at an early stage of development, such that a precursor cell with the mutation gives rise to all other cells in the body. Are you asking about cell-to-cell "transmission" of mutations? –  dd3 Apr 29 '13 at 3:04
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Sperm/egg can be affected by the general body physiology, such as in case of starvation etc, by epigenetic mechanisms. These epigenetic changes that can be passed to next generation. See Transgenerational Epigenetic Inheritance –  WYSIWYG Apr 29 '13 at 6:09
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Goods question! Only mutations that occurred when we were in the early stages of development will affect all cells. That's why pregnant mothers shouldn't smoke. The reason for this is that one cell goes on to divide and become all our cells so any mutations in that cell are passed on to cells formed when it divides. That same principle explains your smoker example. In the lung we have two types of cells which are called Type 1 and Type 2. Type 1 cells are constantly dying as they get old and they get replaced by type 2 cells whose job it is to divide continuously to replace Type 1 cells. So we have lots of type 1 and few type 2. When type 2 divide they make one type 1 cell and one type 2 cell, so type 2 cells never run out. If smoke causes a mutation in type 1 cells generally they're okay because they'll die before enough mutations occur. Of course something that causes lots of mutations could make it cancerous before it dies but that's rare. Now if it occurs in Type 2 cells every subsequent type 1 or type 2 cell that cell makes is mutated.

The last thing you mentioned are mutations that occur to eggs and sperm. When making sperm you have the same as above. One cell makes sperm and a sperm maker cell. The sperm is like type 1 cells and the sperm maker cell is like type 2. So mutations affect them as they do above, where a mutation in a sperm maker cell is way worse. As for eggs women are born with all the eggs they'll ever have and there's no egg maker cells. So mutations just occur, and the older women are the more mutations. That's why the older a mother is the much higher risk of genetic diseases like downs. But an older man only has a slight increase in risk.

Mutations that affect eggs and sperm tend to be really harmful, as they affect every cell, so these cells or "babies" die before the mother even realises she's pregnant or as a miscarriage. I've not given the scientific names for things but if you request them, I'll give you the real names, but hope you enjoy the principles.

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Hi AndroidPenguin, thank you for the explanation. I really appreciate your examples with Type I and Type II cells in the lung, as well as the analogous sperm and sperm maker cells. I didn't know that some cells were designated to be the producers of others, kind of like the factory pattern in software design. –  ktm5124 Apr 29 '13 at 17:07
    
I wonder whether these 'factory' cells always replicate through meiosis? I know that sperm maker cells do, since they produce haploid cells, but not sure about type II cells in the lung. A more general question would be, if cell A replicates to produce cells A and B, does it always do so through meiosis? As far as I understand, it's not possible to achieve this with mitosis. –  ktm5124 Apr 29 '13 at 17:08
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Type 2 cells replicate through mitosis: one of the daughter cells differentiates into a type 1, the other stays undifferentiated as a type 2. Type 1 cells can't divide because they're so far down the maturity process, also I imagine its a protective mechanism. As for sperm maker cells, as I call them here, they divide mitotically into one sperm maker and one cell that's a pre-sperm cell (still diploid). This divides again as per meiosis to give rise to 4 haploid sperm cells. You need it to divide again for crossover (when the genes swap around so you get a random selection from your dad) –  AndroidPenguin Apr 29 '13 at 20:29
    
I see, thanks for your time :-) –  ktm5124 Apr 29 '13 at 21:36
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Yes, all cell division in the body is through mitosis. Meiosis is a very special form that occurs only in the ovaries and testes to produce egg and sperm cells - absolutely nowhere else! Do note that mitosis can be asymmetrical, i.e. producing two different kinds of daughter cells rather than simply multiplying the parent (as in the example with type II cells). –  Armatus Apr 30 '13 at 11:48
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