Parts of a Chromosome & Their Roles

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  • 0:48 Chromosome
  • 1:22 Homologs
  • 2:07 Sister Chromatids
  • 2:36 Three Parts
  • 4:18 Summary
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Lesson Transcript
Instructor: Greg Chin
Learn all about chromosomes, including the different parts of a chromosome and the types of chromosomes in diploid organisms like humans. Learn the vocab used with chromosomes.


As we've seen so far, chromosomes are pretty much the key to life. The information that's stored in a chromosome provides recipes to produce vital cellular components. The importance of this information is underscored by the fact that this cell dedicates an entire process, known as mitosis, to accurately separating or segregating the chromosomes into daughter cells. Before we get into mitosis, let's talk some more about chromosomes.

Have you been able to keep track of all this chromosome terminology that we've been throwing around? Let's see if we can write everything down and clarify everything in our minds before we move on.

Chromosome Cookbooks

Basically a chromosome is a single molecule of DNA that represents a specific subset of information in the genome. We said that we were going to remember this as each chromosome being a different cookbook, right? We also said the each gene is located on a specific chromosome. That also makes sense, because if we think about each gene as a recipe, each recipe is only found in a specific cookbook.

Humans are among the organisms that have two copies of each chromosome, called homologs
Homologous chromosomes

Diploid organisms, like humans, possess two copies of each chromosome. We call those copies homologous chromosomes, or homologs. We can think of them as backup copies of the cookbook that might vary slightly, since we have different editions of the same cookbook.

In preparation for cell division, a cell must make a copy of its DNA in a process known as replication. So, replication yields two copies of every chromosome, which are known as chromatids. It's important to note that replication doesn't produce more chromosomes; rather, it just produces two copies of a given chromosome.

Following replication, there are two chromatids associated with each chromosome, and we often refer to them as sister chromatids because they're basically just copies of the same chromosome. We can think of these sister chromatids as being exact copies of the same cookbook. Now that we got all that straight, let's go ahead and introduce some new terminology.

The Parts of a Chromosome

Okay, so we keep seeing this representation of a chromosome, but why are we depicting it this way? It turns out that chromosome can be divided into three different parts: the centromere, the arm and the telomere. Notice that the chromosome is often depicted as an X-shaped structure with a constriction in the middle. This constriction point is called a centromere, and the centromere is a physical location where the sister chromatids are held together throughout most of cell division.

The point where two chromatids are held together as a chromosome is called the centromere
Center of chomosome

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