Basic Genetics: The Genome & Chromosomes

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  • 0:06 Chromosomes
  • 3:01 Chromosomes in a Cell
  • 4:24 Homologous Chromosomes
  • 5:37 Diploid Complements
  • 8:30 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
There are approximately 3 billion bases in the human genome. In this lesson, you'll learn more about the genome and its organization into chromosomes. We think about DNA often as the blueprint for life, but we can also think of it as the recipe for life with the nucleus being the library.

Chromosomes

Genome is the name that we give to the DNA content of the cell.

Definition of genome
Genome Definition

Now, if we're thinking about DNA as a recipe for life organized into a library...what happens in a library? You have a lot of organization. You have the information organized into discrete packages - books. The DNA is organized into discrete structures and those structures are known as chromosomes.

Definition of chromosomes
Chromosome Definition

If we think about the chromosomes as, say, cookbooks, since we're working with this library analogy, each cookbook has recipes in it, right? Now, you might think that the biological information is organized by process or cell or something like that. In terms of a cookbook, we might think about that in terms of having a book for a specific type of food - Chinese, Italian, Japanese, things like that.

Actually, it's more complicated than that. The cookbooks are more like general cookbooks, so each cookbook has an assortment of recipes. So I might have Chinese, Italian and Japanese all in the same cookbook, and another cookbook might also have Chinese, but also have Korean and Thai.

So say I want to have a three course meal. I might have to access three different cookbooks if these are all general cookbooks. By the same token, if I'm trying to make a mitochondria, I might have to access three different chromosomes. Now, it's important to note that this is a super oversimplification; obviously, it's going to take many more than three chromosomes and three products of those chromosomes to be able to make something as complicated as a mitochondria. But just for the sake of argument, let's just use that example.

As you can see here, it's really important then for the cell to be able to prevent loss of any information. If I was to lose one of these chromosomes, I'm not going to be able to make this mitochondria. As I was saying before, the chromosome is coding from more information than just one process or one structure. So by the same token, now maybe I'm not able to make red blood cells or something like that. The loss of even one chromosome is usually considered a lethal event. So the cell employs many mechanisms to prevent this loss from happening.

Most multicellular organisms like humans possess more than one chromosome. Humans process 46 chromosomes in most of their cells. The exception to that rule is gametes. Gametes are going to be either egg cells that are produced by females or sperm cells that are produced by males. The gametes only possess half as many chromosomes as most of the rest of our cells do, so they possess 23 chromosomes.

Understanding gametes
Gametes

The number of chromosomes that a gamete possesses is referred to as the haploid number. So in the case of humans, the haploid number is 23. Haploid is often abbreviated 'n.' What happens now is because we have only 23 chromosomes in gametes, when you have your egg and your sperm, when those two cells fuse, we can reproduce the normal chromosome count of 46. So it takes 46 chromosomes to cook up a human, so to speak.

Homologous Chromosomes

We have 23 chromosomes in the egg and we have 23 chromosomes in the sperm. There are 23 unique chromosomes in each of these cells. What that's saying is that both mom and dad are contributing some of their DNA. On chromosome four, mom might be coding for freckles, but dad might be coding for a completely different trait. He might be coding for no freckles. So these copies of chromosome four are similar, yet different versions of the same information.

Examples of homologous chromosomes
Homologous Chromosomes

If we think about that in terms of the recipe book, think about this as two different versions of the same cookbook. The two different versions of the chromosomes are referred to as homologous chromosomes, also known as homologs.

Diploid Complements

You might ask, 'Why have multiple sets of chromosomes?' Essentially what I've said is we have two complete sets of chromosomes. If I was a chef, why bother to have two complete sets of recipe books? If I already have Martin Yan's cookbook, why have the exact same cookbook that he published a year later?

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