At the most basic level, some noncoding DNA is simply there for structural purposes. We call this constitutive heterochromatin, and it's structural DNA that always remains condensed. We usually find this around the centromere of our chromosome. This DNA always stays condensed. It does not contain genes, so it won't need to be opened up and read. We also find constitutive heterochromatin at the ends of our chromosomes around the telomeres. You can have it interspersed in the chromosome, but it's mostly found by the centromere and at the ends.
Now, we've already talked about another type of noncoding DNA, and these are segmental duplications. We've talked about these repeated sequences before. You can see an example of that happening right here, where we have this particular area, and it gets duplicated, so now we have it twice. We have also talked about these short tandem repeats. These microsatellites, which are again those short repeated sequences of DNA that vary between alleles and vary in number between individuals. So repeated sequences are very susceptible to unequal crossing over, which produces more repeats. These repeat sequences are actually self-propagating in a way. You can see an example of them right here.
Here we actually have a single nucleotide polymorphism unrelated to what we're talking about, but hey, it's there. We've talked about that before. The focus here is these short tandem repeats, and you see that we have the sequence CTA, and it gets repeated over and over again. In this one individual, he has 5 repeats, another individual has 6, and another 7, and so on and so forth. Some individuals can actually have many, many repeats, much bigger than you might expect.
Now, the reason these are so susceptible to unequal crossing over is, imagine that you are trying to do some crossing over, and you see that you have a sequence match here, CTA CTA, and oh, hey, look. There's CTA CTA over there. Well, have crossing over between those two points, boom. You have just created more repeats. So unequal crossing over is really, or repeated sequences, that is, are really susceptible to unequal crossing over, because they just have the same sequence repeated over and over and over again for a really long stretch of DNA. This makes it hard for the machinery involved in crossing over to match up the right points on the chromosomes.
Now, let's flip the page and talk about some other types of noncoding DNA.
