DNA Replication: The Process
This process is the foundation of who we are—without it, our cells could not reproduce, and we wouldn’t be able to live. I’ll run through the basic process, then let the proteins do the talking for me.
Essentially, in replication, the double helix is unwound in two separate strands by the enzyme helicases, which break the hydrogen bonds between the base pairs. The base pairs are explained, ready to serve as a template for the synthesis of new strands. The place where the helix is being unwound is called the replication fork. The replication fork has a bunch of enzymes swarming all over it, including DNA polymerase which synthesises new nucleotides to whack down like train tracks as the helix unwinds.
However, DNA polymerase can only add new nucleotides in a 5’ to 3’ direction, so one strand is going to get left out. An enzyme can just zip over the top strand and synthesise a matching one since it’s facing the right direction, and it’ll keep going as the strand keeps unwinding because the enzyme is moving left. But the bottom strand has to be synthesised in a 3’ to 5’ direction too, which is in the opposite direction to the way the helix is unwinding, away from the replication fork. So we’ve got these bits constantly being exposed at the fork.
Instead of synthesising continuously like the top strand, the bottom strand has to be synthesised discontinuously—little bits at a time, starting at the replication fork and moving until they reach the last fragment. These fragments are called Okazaki fragments, and in humans, they’re only made in stretches of 100-200 nucleotides. These fragments then have to be stitched together by an enzyme called ligase, creating one long continuous strand.
Because the bottom strand has to wait for the replication fork to open up a bit before it can start synthesising fragments, the process takes slightly longer—so it’s called the lagging strand, while the top strand is called the leading strand.
Another problem faced by the lagging strand is the creation of RNA. See, DNA polymerase, which synthesises new nucleotides and thus creates the new strands, actually lacks the ability to initiate the process. It can’t start strands out of nothing—it needs to have something to build off of. So, an enzyme called RNA primase is given the job of creating a short, initial stretch of nucleotides. Then DNA polymerase latches on and happily zips off.
In the leading strand, the RNA primase needs to do its job just once, and DNA polymerase chugs along continuously. But in the lagging strand, RNA primase constantly has to create new initial chains of nucleotides—one for each fragment. As you can probably imagine, this is a nightmare—because it means that between every Okazaki fragment, there are bits of RNA, disrupting your DNA strand.
So, before the Okazaki fragments can be stitched together by ligase, these RNA bits have to be removed.
Finally, once all this is done and the lagging strand has been patched up, we have two DNA molecules: each one with one parent strand, and one daughter strand, as per the semi-conservative model. Pretty neat, huh?
Next time: the super-long explanation of the mechanisms behind it all.
Further resources: Crash Course: DNA Structure and Replication