Biology Forums - Study Force

DNA is read in the 3' to 5' direction, and synthesized 5' to 3'. I can see how this applies to the leading strand, but what about the lagging strand? It is oppositely oriented and thus progresses more slowly, requiring Okazaki fragments. Does this mean it is synthesized in the 3' to 5' direction? If not, why is it slower/more difficult? The many sources I've read don't seem to answer that question clearly.

Both strands have a leading and lagging strand. Remember, helicase is found on both ends of the DNA strand, so DNA polymerase is adding nucleotides to the 3' end of a growing chain ( i.e. , 5'- 3') at both ends. In other words, DNA polymerase always adds nucleotides to the 3' end, regardless of where it's growing the DNA strand.

Let me know if you need a better visual

Thanks for replying. Why is the lagging strand slower then?

Because more work is required. After the lagging strand is created, you have several of them that need to ligated together into one continuous strand. The leading strand -- the one that gets built towards to helicase enzyme -- doesn't have this issue. Speed-wise, it's the same polymerase enzyme.

Hi there, welcome to the site

Why do you suggest it's slower? Is that something u read in your textbook by some chance?

Thanks, let's see if I'm closer:

 The replication fork proceeds as the helix is opened, the leading strand is oriented in the same direction (3' to 5') so replication is simpler. The lagging strand is oriented 5' to 3', so there is a lag during which the polymerase has to wait for exposure of the nucleotides (100-200 in eukaryotes).

I've read on some q/a sites that the lagging replication is slower. Or must it occur at equal speed, which also explains the greater energy requirement?

@sighology

The lagging strand is only "slower" because it's technically built in chunks. I'll revise your statement:

The replication fork proceeds as the helix is opened, the leading strand is built in the same direction (adding nucleotides to the 3' end of the primer that was temporarily added) continuously. The lagging strand started at the other replication fork, so technically it's moving in the opposite direction towards the other helicase enzyme at the opposite replication fork. It also started with an RNA primer where DNA polymerase was able to add new nucleotides to the 3' end of the primer.

It's very important you understand this, because 9/10 biology students (& teachers) DO NOT know the process correctly. If you need more clarification, I'll illustrate in my next post.

@Duddy, I got this :)

Then what's the delay? If the helix is unwinding, and both strands are exposed at the same time, why would the lagging strand replicate slower merely because the polymerase was moving in the opposite direction?

It's the way the concept is taught; textbooks focus on one replication fork (at one specific frame) so they use words like leading and lagging strand. My two cents...

Ok thanks bio_man!