Professor Dave here, I'm going to tell you about DNA replication.
We now know what DNA is, it's a two-stranded polymer of nucleotides
and each strand has a backbone made of identical sugar and phosphate groups
with different nitrogenous bases pointing inwards, pairing in base
specific manner, A with T and C with G. These long molecules are coiled around
histones and then supercoiled to give compact chromosomes, each with many
millions of base pairs. All of your genetic material, 23 pairs of chromosomes
to be exact, is present in every single cell in your body. But your cells are
dividing all the time.
Old ones die and new ones are generated to take their place. In fact, apart from
female egg cells, there's not a single cell in your body that was there when
you were born.
So how does each new cell get all of the genetic material? As it happens, all of it
is copied through a process called DNA replication, so that when a cell divides
each resulting cell keeps a copy of all of your chromosomes. So how does DNA
replication work? It's an impressive operation, with about a dozen enzymes
working in tandem.
Let's look at a few of these enzymes and see what they do. Helicase is an enzyme
that unwinds the double helix and disrupts the hydrogen bonds between the
bases, thus separating DNA into individual strands and creating a
replication fork. The unwinding of the helix generates strain further ahead in
the chain so as we go
topoisomerase will break, untwist, and reconnect the DNA, always ahead of the
replication fork. With the strands separate we can begin to copy each one,
but the enzyme that copies the strand needs a place to start, so an enzyme
called primase will anneal an RNA primer at a specific location to kick-start the
replication. This primer is about five to ten nucleotides long. Then, another enzyme
called DNA polymerase III binds to the primer and
begins to generate a whole new complementary strand, adding nucleotides
to the new chain that was initiated by the primer. Nucleotides enter the enzymes
active site and polymerase catalyzes formation of the phosphodiester bond
that joins each new nucleotide as it is added to the complementary strand.
This process will be different for each strand because polymerase will always
add nucleotides to the 3' end of the existing strand, not the 5'
end, and the strands are antiparallel so the direction of
replication must be in opposite directions for the opposing strands.
On the leading strand, DNA replication moves along with the replication fork
continuously synthesizing the complementary strand and requiring only
the initial primer. But on the lagging strand, polymerase has to go one chunk at
a time as new template is made available. These chunks are called Okazaki
fragments and they are around 100 to 200 nucleotides long. Each one will require
its own primer in order for polymerase to bind and copy the new fragment.
After each fragment is synthesized, DNA polymerase I will go through and replace
the RNA nucleotides from the primer with DNA nucleotides to make sure its DNA all
the way through.
Lastly, because polymerase can't join the last nucleotide of one fragment to the
first nucleotide of another, a separate enzyme called ligase has to go through
and make sure everything is connected.
So to summarize, helicase unwinds and separates the DNA into two strands.
Primase anneals primers to start things off, and polymerase III copies each strand.
On the leading strand we need just one primer and everything goes continuously.
On the lagging strand we need a primer for each Okazaki fragment.
Then, polymerase I replaces the primers with DNA nucleotides, and ligase seals
everything up. Boom. Two identical copies of the original DNA molecule.
This whole process, which is happening in billions of cells in your body at this very moment
goes very fast, about 50 base pairs per second.
Moreover, polymerase is very good at getting the code right.
It almost always puts the correct base across from the template strand, and when
it makes a mistake it can usually backtrack and correct it in a process
called proofreading. Even with this, around one in every ten billion base
pairs, an error ends up in the final sequence.
Luckily there are enzymes that can recognize these errors and perform
mismatch repair, swapping out the incorrect base for the correct one, just
like other enzymes that repair damage caused by external sources. These enzymes
minimize the possibility of mutation, and we will learn about them later, but as we
said, polymerase almost always gets it right. In this way, each strand in the
double helix acts as the template for its complement, and we end up with two
identical copies of all the genetic material. When a cell divides, each new
cell retains one of these copies, and when the cell cycle gets to a certain
point, these new cells go about copying everything again, to be ready for another
division when the time comes.
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