So having had a quick tour of the language of chromosomes,
we can now talk about how those chromosomes are composed.
So DNA as we know is a long linear strand. And in order
for it to function in the cell, it needs to be unwound,
and it is in its chromatin form.
In order for chromatin to wind up into chromosomes, and pair
as sister chromatids as we've seen in our chromosome language,
we need to take this DNA and quill it up quite tightly
which of course makes it inaccessible.
So this is not going to happen until after S phase.
So one of the things that's happening during G2, the growth phase 2,
is we're beginning to pack up the chromosomes from chromatin.
So DNA is our double helix, and we move from DNA and
wrap it around a core of four different histone proteins.
It goes around there about two and a half times.
So this green strand goes around two and a half times
around the core of eight histone proteins
and it's snapped on by the red guide one other histone protein.
So these are like beads on a necklace string.
If we were to take those beads on a necklace string and
put them into a cylinder and just drop them in one by one,
they would stack up on each other.
This is our next level of DNA packing.
So all of these beads on a string packed together in a tube
and then those beads on the string pack together in the tube
will then fold and coil upon each other,
so we have these looping structures here.
And then each of those looping structures will loop again
and we call this supercoiling,
and then that supercoiling becomes tightly packed and
supercoils stack on supercoils to form the whole chromosome, right.
So we have here a replicated chromosome.
It's composed of two sister chromatids.
Both of them have to pack up in this
fairly compact means of chromosome packing.