So now you know all about chromosomes and that they
replicate during S phase of the cell cycle.
Let us take a deeper look into what actually
happens during S phase in replication of
DNA. By the end of this lecture, you will be
able to diagram the manner in which nucleotides
assemble themselves in DNA as well as explain
the implications of semiconservative replication.
You will be able to describe the process of
DNA replication and the role of each of the
enzymes in that process as well as explain
modifications that are necessary for eukaryotic
chromosomes. We will begin with a quick review
of DNA structure.
You will recall that there are a phosphate
and sugar backbone. Much like a spiral staircase,
there are steps made of nitrogenous bases
and these steps and nitrogenous bases with
the phosphate, sugar backbone form a whole
molecular DNA, which has an antiparallel orientations.
That is each strand is in opposition to the
other strand, so they run in an antiparallel
fashion, which lends itself to some particular
situations during DNA replication that need
a little bit of adaptation. Looking at that
antiparallel orientation, we know that there
is a three prime OH group that is going to
be really important to us during DNA replication
and on the other strand that same end, you
have a five prime phosphate hanging off.
The reason this is important is DNA polymerase
can only read strands from the three prime
to five prime directions. It can only go if
we were counting 1, 2, 3, 4, 5. It can only
read the three prime to five prime. Keep that
in mind. You also will recall that the nitrogenous
base steps are held together with hydrogen
bonds. There are two bonds between adenine
and thymine and there are three bonds between
guanine and cytosine. The main point here
though is that it's hydrogen bonds that hold the
two strands that are antiparallel together.
And the backbone is held together by what
kind of bonds? Phosphodiester bonds that are
covalent bonds. And are they stronger or weaker
than the bonds between the strands, the hydrogen
bonds? Right, they are stronger. When we pull
on the DNA molecule and try to rip it apart,
it conveniently unzips right down the center,
so we have access to the nitrogenous bases,
DNA polymerase can do its job. What happens during
DNA replication? In synthesis
of the cell cycle S phase, you will recal
that cells spent most of their time in gap
1 doing their daily routine and they may not
be dividing again or if they are dividing,
they may spend a fair amount of time in gap
1. But once they are committed to divide,
it is time to get the machinery together for
synthesis phase and we are going to learn
all about the machinery in this lecture.
Synthesis phase is the highlight. Everything
in this lecture is about synthesis phase.
First of all, we need to understand that DNA
replication is semiconservative. That is,
the DNA will separate and then be replicated.
Each new molecule of DNA is composed of one
old strand and one new strand. The parent
chromosome separates and new strands are replicated
for each of the daughter chromosomes.