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 1, define them and then number 2, 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. 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 nuclei
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 could imagine this would
be a really have book to open if it was one large book. And so it is 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. It would make it easier for them to copy the book. And that's really what chromosomes
are. They are 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 into 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 is 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, then we see this characteristic shape. Now
if you see it looking like an X, like this, that means that it has gone through interphase
and it has duplicated the 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 has copied all the information. So the information on one side is duplicated on the
other side. And so another thing that is 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 3 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 protein. And so
as we zoom in we start to see these beads that are would tightly around each other.
And those are called nucelosomes. And so as we look more deeply inside there, what we'll
find is it 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 will essentially wrap 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 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 would 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 is going to be all of our genetic
information. Not just the chromosomes in the nuclei but also the genetic information found
in the mitochondria and the chloroplasts. So that is all of our genes. The chromosome
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.