Video Transcript: What are Chromosomes?

Hi. It's Mr. Andersen and a lot of my students are confused with chromosomes. And so in this short video, we're going to answer this question, what are chromosomes? And so let's begin with a pre quiz. Could you take these four terms chromosome base pair, genome and gene? And could you number one, define them? And then number two, could you rank them in order from the largest to the smallest? We're going to do that at the end of this video. And so these are the four topics that we're really going to revolve around.

But the first thing I want to get out there is that there are different types of cells. And so in a prokaryotic cell, like a bacteria, they really only have one chromosome, it's going to be one loop of DNA. And it's going to be bound tightly in what's called the nucleoid region on the inside of that cell. But generally, when we're talking about chromosomes, a lot of the time we're referring to those chromosomes inside the nucleus of a eukaryotic cell, like in your cells or the cells of a plant, or the cells of a fungi. And when you look closely inside that nucleus, you can see these characteristic X shaped chromosomes. And those are really sections of DNA that have been cut down to make it more usable. And so a good analogy is like an encyclopedia, this is an encyclopedia of cell biology. And you can imagine this would be a really hard book to open if it was one large book. And so it's easier to break it down into individual topics. So A to B, or C to E, it makes it easier for you to use the book and make it easier for them to copy the book. And that's really what chromosomes are. They're like individual sections of a larger piece of information, which is called our genome. Now remember, you get two copies of your genome, you get one from your mom, and you get one from your dad. And that's because we're diploid individuals.

And so now let's kind of zoom in to the level of genes. And let's start at the largest level, we're just looking at the nucleus itself. And so what is a genome? A genome is going to be all the genetic information inside a cell. And so it's going to be contained within these chromosomes on the inside of the nucleus, you're also going to have a little bit of information in the mitochondria of an animal cell and in the mitochondria and chloroplasts of a plant cell. But it's all the genes that an individual cell has.

Now if we were to isolate on one of those, what we call a chromosome, and we see this characteristic shape. Now, if you see it looking like an X like this, that means that it's gone through interphase. And it's duplicated as two sides of the chromosome. And so in the center, you have what's called the centromere, and we're not going to have a lot of genes there. But then you're going to find what are called the two sister chromatids. And so during interphase, it's copied all the information. So the information on one side is duplicated on the other side.

And so another thing that's important to note at this point that the DNA has been tightly bound, so it can actually fit inside and easily divide inside the nuclei. The typical genome inside a cell is going to be, you know, about three meters long. And so there's no way it could fit inside the nuclei if it wasn't tightly bound. 

And so again, a genome is broken down into chromosomes. But let's isolate on some of this bound DNA and proteins. And so as we zoom in, we start to see these beads that are wound tightly around each other. And those are called nucleosomes. And so as we look more deeply inside there, what we'll find is, is really made up of two things. You're going to have the DNA, which is going to be this thread. And then you're going to have these histone proteins. And so we're going to have eight of these histone proteins, and the DNA is essentially wrapped around those and then you eventually get what's called a nucleosome. So now let's zoom into this. Let's zoom into the DNA itself. And we'll find this is that characteristic double helix that we have. Now, again, we're really zoomed in.

And we're finally at the level of the gene. And so what is a gene? A gene is simply going to be a section of that DNA that codes for one specific protein. In other words, humans are made of proteins, all things are made of proteins. And so the genes are simply the recipe on how to make one protein. So here would be one gene here would be another gene here, it'd be another gene. And so we're going to have around 20,000 genes. Now you'll find that inside all this DNA, there's a lot of sections between the genes, and those are going to be regulatory portions that are actually turning those genes on or off. But if we isolate on one of these regions, then we finally get down to the level of the base pairs. And so these are going to be the nitrogenous bases, adenine always bonds to thymine, and cytosine always bonds to guanine. And so what are these, these are the smallest bits of information. Now these will come together to code for specific amino acids, which eventually make the proteins which eventually make us.

And so again, let's go through and review this. First of all, could you put these in order from the largest to the smallest, take a second and try to do that. So it should go in order like that. So we've got the genome, what is the genome? Remember, it's going to be all our genetic information, not just the chromosomes in the nuclei, but also the genetic information found in the mitochondria and the chloroplasts. So that's all of our genes; the chromosomes are going to be just different segments of that genome cut into more usable forms that we can easily divide between cells. Genes are going to be parts of the chromosome, each of those code for one specific protein. And then finally, the building blocks of those genes are going to be the base pairs. And so those are chromosomes and I hope that was helpful.