Hi. In this video, we're going to talk about eukaryotic chromosome structure. So eukaryotic chromosomes have a specific structure, and the first term that you need to know is chromatin, and chromatin is a combination of DNA and protein. There are two types of chromatin: heterochromatin and euchromatin. Heterochromatin means that it's tightly packed, and euchromatin means that it's loosely packed. So, looking at this, this would be heterochromatin. The DNA is here in black, and the proteins are here on these colorful circles. This is going to be euchromatin.
Proteins that package with the DNA or associate with the DNA often are really important for condensing that DNA into a small chromosome that actually fits into the nucleus. Chromosomes require a lot of packaging, and there are four different levels. The first level, and the one that's most talked about in your book, is the nucleosome. This is made up of a group of proteins called histone proteins and DNA. There are a lot of different types of histone proteins, and they do different things in the nucleosome. The nucleosome has a histone core, with two copies each of histones H2A, H2B, H3, and H4, forming a core that the DNA wraps around. Here you can see nucleosomes, and inside them, these are the histone cores, represented by these balls. In between the histone core, you get a histone linker. This has a histone protein called H1, which lies in between and connects those histone cores together.
From here, these nucleosomes are condensed further into what is known as a 30 nanometer fiber, which has multiple nucleosomes in it. This then progresses into a 250 nanometer fiber, and this is more condensed together and begins to look like a chromosome. Eventually, it becomes even more condensed and turns into the chromosome.
At the chromosome level, there are specific structures as well. The first is the centromere, which is the constricted region of the chromosome where spindle fibers attach during division, allowing chromosomes to be separated properly into the gametes. The chromosome is associated with a protein complex called the kinetochore, a group of proteins that link centromeres to the spindle fibers. The centromere is likely to have heterochromatin because it's condensed. At the centromere, there's actually a histone variant, rare in occurrence, called CENH3, only found at centromeres.
Another significant structure on a chromosome is the telomere, which is the end of the chromosome and is characterized by containing telomeric sequences. It's repeats of sequences like TTAGG, which can vary by species but typically consist of a's and t's followed by g's. Telomeric sequences contain a ton of repeats - up to like 250 kilobases - but proteins, especially Shelterin, bind to these sequences and prevent DNA from degrading. At the very end of the telomere is a G-rich 3' overhang, a single-stranded region that is crucial for various biological processes like replication.
