Video Transcript
DNA is wrapped around proteins and
coiled into loops to form chromatin. At what point will chromatin
condense to form visible chromosomes? (A) When cells are stimulated by
chemical messengers. (B) When cells become
fertilized. (C) As cells prepare for cell
division. (D) Shortly after cells have
completed cell division. Or (E) immediately after chromatin
has formed.
This question is asking us about
how DNA can be compacted in eukaryotes to form chromatin which can then be compacted
further, or condensed, to form visible chromosomes. Let’s first start by looking at
what chromosomes are and how they’re compacted.
In humans, our DNA can be found in
the nucleus of most cells. If you were to take the DNA out of
one of our cells and line it end to end, it would be about two meters in length. This is not one long continuous
piece of DNA. Instead, DNA is organized into 46
chromosomes in most of our cells. Each chromosome is a linear piece
of DNA, and if they’re lined up together, then it’s about two meters in length. To squeeze this amount of DNA into
a tiny cell requires a lot of compaction. Let’s look at how DNA can be
compacted to this degree.
DNA is first wrapped around special
proteins called histones to form a structure called a nucleosome. These nucleosomes are then coiled
around and around to form long dense fibers called chromatin. Chromatin can then be wrapped up
even more to form what’s called a condensed chromosome. This condensed chromosome is the
structure you may be most familiar with when talking about chromosomes. Before this, chromosomes exist
mainly as very long strings of somewhat loose chromatin. We can actually find these two
states of compacted DNA in our cells. On the bottom, the cell on the left
has its chromosomes in the condensed arrangement, while the cell on the right has it
in the chromatin arrangement. What determines whether or not the
DNA is in the chromatin or the condensed chromosome state has to do with what part
of the cell cycle the cell is in.
You recall that the cell cycle is a
cycle that cells go through as they divide. Interphase is a long stage where
DNA is copied, and mitosis is made up of several steps where this copied DNA is
separated into a new dividing cell. Chromatin is a state that DNA is in
during interphase as the DNA is being copied. Each chromosome, and there’s 46 of
them in human cells, is hard to make out because they’re long and all mixed up with
each other. This is what it might look like if
there were three chromosomes, each color blue, green, or orange in the chromatin
state.
If we stretched out one of these
chromosomes, the orange one, for example, it might look like this. Then, during interphase, it’s
copied. Then, it’s wrapped up and compacted
in preparation for mitosis and cell division to eventually give rise to the
condensed chromosome that’s visible under the light microscope. This familiar X-shaped chromosome
actually contains two separate copies of the chromosome. And these can be separated as the
cell divides. Therefore, chromatin compacts or
condenses to form visible chromosomes as the cell prepares for cell division.
