Genetic Mutations, Part III: Information

Any change in DNA, whether is brought on by copy error or damage, we can call a mutation. In some cases, such as when mutations arise from mismatch errors, the change may be relatively small. We call those kinds of changes point-mutations. Mutations may also involve other relatively small, but potentially serious consequences, such as the insertion or deletion of a base-pair along the string of DNA.

Damage to DNA can actually cause much larger-scale changes. If the damage causes the DNA double helix to break entirely, then entire segments of the chromosome can be lost. We call those chromosomal deletions. Thousand of base-pairs can just be eliminated. Or, those segments that are broken out of the DNA molecule might get flipped around and put in reverse. We call those “inversions”. Or, those segments might actually be pulled out and moved to other part of the chromosome. We call those “translocation”. They might actually be pulled out and inserted in a number of different places. Those are “duplications”.

What are the consequences of these kinds of mutations? We talked briefly about the negative consequences that can happen if a critical gene in a cell is damaged. Not all mutations, however, have this kind of negative consequence. In fact, some may even have positive consequences, at least over the long run.

Mutations and Functionality of Proteins

The genetic code is redundant. That means that different combinations of bases code for the same amino-acid. For example, codons CCA and CCG both code for proline. If the A is somehow mutated and becomes a G, it doesn’t matter, we still have a code for proline. We call these kinds of mutations “silent mutations”. Unless we look at the DNA sequence itself, we would never know it is there.

If CCA, which codes for proline, is changed to UCA, which codes for serine, then we would have a change that affects the amino-acid sequence. Even a change in amino-acid sequence, however, may not be discernable. It may even have a slightly positive effect. The difference is hard to predict in advance, and it all depends on which amino-acid is substituted for the other, and how that substitution affects the shape, and therefore, the function of the protein.

It’s possible to have one amino-acid substituted for another and finding no noticeable change in the way the protein folds up. It’s also possible that an amino-acid substitution does cause a radical change in protein shape. These kinds of changes are what would lead to serious effects. For example, these are the kinds of change that may cause a cancer. If the amino-acid substituted happens to be a particularly critical one, the whole protein can be screwed up.

Interestingly, there is a third alternative. That is that the change of one amino-acid actually makes the protein work a little better. It is conceivable that a slight shape change would make it more functional. In this case, a mutation would have a positive effect.

Over the long run, mutations are important, because they change genetic information among individuals in populations. In that way, mutations add genetic variation, which is the stuff that natural selection works on. I have to point out, though, that the only mutations that matter are those that can be passed to offspring. Up to this point we were talking only about the cell. If a mutation occurs in a single celled organism, when it reproduces the mutations would be passed on to the offspring. For single celled organisms, any mutation is going to be passed on.

Mutations in Multicellular Organisms

If I have a mutation occurring in my skin cells, my future children don’t have to worry about that. Those mutations will die when I do. Those skin cells have no way of passing that genetic information on to my offspring. Instead, in multicellular organisms, such as ourselves, there are small groups of cells whose sole function is to produce reproductive cells. We call those germ cells. Only mutations occurring in germ cells can be passed on to subsequent generations.

I think that we this series of articles on mutations we’ve clarified the subject a little. Because of the ongoing debate over “information” in organisms, and how it is created in order for evolution to work, I think it is really important to share my little knowledge about the subject. In later articles I want to talk about other mechanisms of evolutionary change, like sexual reproduction and genetic drift. Be sure to check them out, and spread the word out, everyone needs to know this stuff. It's really eye-opening.