why does one strand have to lag? doesn’t DNA polymerase III bind to 3′ OH ends of RNA primers and then synthesize in a 5′ to 3′ direction? don’t all the RNA primers have a 3′ end for the polymerase to bind to? so why is one strand able to synthesize continuously while the other does it bit by bit?
please help ya girl broke down while studying in Starbucks because she couldn’t understand even after way too long
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The first half of problem is that the two strands are antiparallel which means that one strand has the 5′ end to the right, the other one has 5′ to the left.
You can imagine it if you think of a street with traffic jam: the right lane has a line of cars facing outbound, the left lane has a line of cars facing inbound.
The second half of the problem is that the entire replication machinery (incl. topoisimerase, helicase) goes in one direction, let’s say left to right. It practically means that on one strand it goes 5′ to 3′, and on the other strand it goes 3′ to 5′.
Note that this machinery is responsible for opening up the double stranded DNA and making it accessible for the polymerase. The result is that one strand can be replicated simply by following the opening machinery because it goes in the same direction. The other strand is in the opposite or “bad” orientation, so the replication must wait a little until some DNA is open and then the synthesis is going away from the opening. That’s why this strand goes in fragments.
Indeed it could have been possible that the DNA is being opened up in opposite directions and only one strand is synthesized on both sides. This is not chosen by evolution may because it would be an even bigger problem when the two replication directions collide.
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