Don't understand how multiple replication bubbles work

Question

I'm not exactly sure how multiple replication bubbles work, assuming were working with a linear, eukaryotic chromosome.

This is a diagram for reference:

It appears that the DNA is being synthesized "towards" the centre. What I mean is that in one replication bubble, the top strand is being replicated from left to right, whereas for the other replication bubble, the top strand is being replicated from right to left.

I don't understand why this is. Aren't the directions reversed? Isn't synthesis always supposed to be from 5' to 3'?

Thanks!

Answer 1

You mistake replication direction with polymerase synthesis direction. Indeed, the polymerase synthesizes new strands 5' -> 3' but if the replication of each strand was continuous, there would be no such structure as a replication bubble whatsoever. Take a good look at the drawings:

The thing is the direction of replication is consistent with the direction of leading strand synthesis, so the bubbles will "spread" outwards. Lagging strand is synthesized in the form of fragments 5' -> 3'. If you look closely fragments that are closest to the center of the replication bubble are synthesized first, then polymerase detaches and works on the next fragment which is actually closer to the 3' end of the lagging strand. Here's an article on Wikipedia about those fragments: https://en.wikipedia.org/wiki/Okazaki_fragments

Hope this makes things clear.

Cheers and happy learning,

Krzysztof