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. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always feel free to email me:
