Why Does DNA Replication Require a Lagging Strand?

[SAT2400]

1 hard(?!) Biology Question!HELP! :/

Homework Statement

If life had been "intelligently" design, and if an important design principle is conservation of material and efficiency of process, we would not expect DNA replication to involve the crazy, complex, intricate, Byzantine mechanism for lagging strand synthesis. For example, one way to design the process is to have both strands continuously synthesized. However, evolution is responsible for producing the lagging-strand synthesis mechanism, and evolution is notoriously constrained by ancestry. That is, because evolution works by modifying parts and processes that existed previously in an ancestor, what can exist in the descendent is limited by what existed previously. In general, evolutionary biologists call this "constraint" or "phylogenetic constraint" What is the major constraint that was likely involved in the evolution of the lagging-strand synthesis mechanism? (That is, what was the constraint most likely involved in limiting continuous polymerization to occur on only one strand at the replication fork?)

A. An RNA primer is required to start polymerization.
B. The DNA has to be unwound by a helicase, and this unwinding occurs in one rotational direction.
C. DNA polymerases only attach nucleotide monomers to the 3' end of the molecule.
D. The density of single-stranded binding proteins differs between the two strands.
E. Nucleotides have a particular polarity, with a phosphate at the 5' end and a 3' hydroxyl at the 3' end.

Homework Equations

Biology,,,gene replication?

The Attempt at a Solution

I think it's C..because 3' end is the part that should be open?!
Please helP~! thanks!

 

[Kalirren]

Yeah, pretty much. The helicase unwinds the DNA, resulting in two single strands of DNA running in opposite directions at the fork. If DNA synthesis can only proceed in one direction, then only one of these can be continuously synthesized, since the other strand must be being unwound in the wrong direction.

 

[k-rod AP 2010]

yes, C is the most likely answer b/c the DNA ploymerase inherited from ancestors can only go can only synthesize in the 5' to 3' direction. one strand, the leading, is going in the correct direction so there is no problem. but since the other strand is running in the opposite direction, cells have adapted to this problem by causing the Oozaki fragments to be formed and go "upstream" of the rest of the synthisis

 

[nobahar]

Isn't the 5' -3' limitation a consequence of the kinetics of the reaction, as oppose to an evoluitionary limitation?

 

[Kalirren]

Yes, it is a consequence of the reaction mechanism. The point of the "evolutionary limitation" is that evolution works with what it's got. What we -have- is a 5' -> 3' elongation mechanism, not a 3' -> 5' one. So this entire machinery surrounding Okazaki fragments gets invented to deal with the fact that we have to mimic 3'->5' elongation with a 5' -> 3' mechanism.

Ultimately that whole preamble doesn't really have much to do with the answer.