In section 6.5, your book talks about ways that DNA can somehow change. Changes in DNA might be beneficial, but more often, a change in one's DNA is detrimental. Since our DNA codes for who we are, if there is a change in it, that change can hurt us by preventing us from coding for our proteins properly. You already know that a problem in a particular gene can cause a genetic disease (like muscular dystrophy). Another example can be found in some types of cancers-- where a cell becomes cancerous after a mutation in a cell-cycle-controlling gene.
For a genetic disease to become widespread, like in MD, the problem in the gene had to have arisen a while back from a person who had something happen to that gene. If you go on to take Principles of Biology II, and study evolution, you will see that changes in DNA can lead to changes in our species. There are other good changes, too. I will try to describe both in this page.
Polyploidy and aneuploidy
These are changes in genetic content that occur due to errors mainly in meiosis (but could occur from mitosis, too). Polyploidy is when an organism has 3 or more of each type of chromosome (not just diploid, having 2 of every type of chromosome, but more). So in a polyploid cell, you would find more than just pairs of chromosomes, you would find trios or sets of 4 or 5 or 6! Some plants are polyploid and seem to survive better that way than their diploid ancestors. Your book points out that nearly half of all species of flowering plants are now polyploid-- an amazing statistic, and one that proves how advantageous this particular change is.
Note that in the examples above, polyploidy was so advantageous that it exists in all the individuals of certain species.
Aneuploidy is when the overall chromosome number is not quite right-- when it is off by one or two-- either by having extra or not enough chromosomes. This is different from having an extra, complete set of chromosomes as we saw in polyploidy. An example would be if a person had 47 chromosomes instead of 46. Most of the time in humans, aneuploidy is lethal. So, if a fertilized egg was aneuploid, it wouldn't grow and make it to birth. There is a prominent exception to this rule-- a Down syndrome person has one extra chromosome.
A change in a gene is called a mutation. Here are some ways that mutations can occur and some different types of mutations that can arise.
Transposable genetic elements can cause mutations.
This section explains that there is a natural phenomenon that exists for bits of DNA to get shifted around. Little bits of DNA, called transposons, can jump around and land elsewhere within DNA. This ends up being really serious. If the transposon lands in the middle of a gene for a particular protein, it will mess up the DNA sequence that codes for that protein... then the mRNA will get messed up, and the protein will not be made properly. This can happen in a cell at any time, but it doesn't happen a lot. Your book mentions that 1 in every 500 mutations is due to a transposon. Well, mutations are not that common, so one in 500 is even rarer. But it does exist as a way to change our genes. Therefore, transposons can cause mutations by jumping into the middle of our genes, interfering with proper transcription and translation.
Mutations of single DNA bases
If a mutation causes just one DNA nucleotide to change in some way, that is a point mutation. There are a few different ways that this can happen:
Your book describes a substitution first, and gives as an example, the mutation that causes sickle-cell anemia. You might remember that in a person with sickle-cell anemia, that person's hemoglobin has one amino acid that is different from a regular hemoglobin. How can just one amino acid be different? If one nucleotide were swapped for a different one, that would change the mRNA codon, and the amino acid would be different.
You probably noticed that there is typically more than one codon for each amino acid. So, if one nucleotide is swapped for another and that doesn't affect the amino acid that is added, that is called a neutral mutation. For example, if a DNA sequence includes "ATA," that would be transcribed into "UAU" and that codes for the amino acid tyrosine. If that DNA sequence mutated into "ATG," that would be transcribed into "UAC," which also codes for tyrosine. That would be a neutral mutation. But, if it were mutated into "AGA," that would be transcribed into "UCU," and that codon represents the amino acid, serine, so that would be a substitution.
A frame-shift mutation is a more extensive problem. If a nucleotide is lost or added, the sets of three codons will get messed up. Instead of 123 123 123 123 123, it could end up 123 121 231 231 231 in the case of a removal, or 123 122 312 312 312 in the case of an addition. Your book uses the expression "Joe ate hot dog" and that might be easier for you. It will also be easier for you to understand after you complete this week's project.
What can cause a point mutation?
It is hard to pinpoint this, but you have all heard about "cancer causing agents." And it is that stuff that tends to also cause mutations. So, too much sun or too many X-rays. Certain harmful chemicals. Smoking. The list goes on.
What does a mutation have to do with cancer? If the mutation is in one of the genes that codes for a protein that regulates the cell cycle, a cell might not be able to regulate its cell cycle any more and just grow out of control. Or, if the mutation is in a gene for a protein that helps a cell to know when it is squeezed in next to other cells, it may lose its ability to be inhibited by cell-to-cell contact and begin to grow out of control. There's no way to control where on the genome a mutation can occur. And if it occurs in certain places, cancer can begin. Remember, I told you a while ago that cancer can begin with one cell that has gone bad. A mutation can also occur in a place in the genome after which a cell can no longer survive, so it can't divide like mad, but it just dies. That's fine, we have lots of other cells.
I guess the more amazing thing is that cancer doesn't happen more often. Our cells are constantly checking on themselves and keeping mistakes like these to a minimum.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007