DNA Replication (i.e. Base Pair Porn!)

Could I come up with a more boring title? I don't think so! But how in the world do you write something interesting about how DNA copies itself? Maybe "base pair porn!" That would totally work! I'm putting that now...hee for me! Anyhow, on to today's question!

When DNA replicates semi conservatively, which of the following is true of each daughter DNA molecule?

A) Both strands are newly synthesized
B) One strand is newly synthesized, whereas the other is a strand from the parent DNA molecule
C) Both strands are the original strands of the parent molecule
D) One strand has more AT-rich regions than the other strand has
E) The newly synthesized strands are more susceptible to melting and renaturation than the parental DNA strands are

Ok, the big question in this question is "What is semi conservative replication?"

Remember that blog I did about complimentary base pairing? Yeah, me too! That was a good one. Sigh. Well, this is sort of a continuation of that last post. When DNA needs to copy itself, it undergoes replication. There are three methods the books talk about when discussing DNA replication: conservative, dispersive, and semi conservative.

Conservative DNA replication is when an entirely new double helix of DNA is replicated for the new (or daughter) cell. This works just like a copy machine--it's based on the mother cell's dna, and an exact copy is made. The two new strands are what are sent on to the daughter cell, while the strands they were copied from are left in the mother cell. This method of DNA replication has not been found to be biologically significant, so most people don't really care about it. And neither do we!

Dispersive replication is when bits and pieces of the mother strands are mixed up with new sections and all put together into a new double helix. The two daughter cells end up with a strange mix-and-match version of the DNA made up of both mother and daughter sections. Just like the last one, no one thinks this is a biologically significant method of replication.

Finally, the big one: semiconservative replication. This is the main way DNA is totally replicated during cell division. During this type of replication, the entire DNA double helix unzips. A new strand is made to match up with each original strand using complimentary base pairing. The result is two double helices where only one was before. Each double helix is made up of an old strand of DNA (the mother strand) and a new strand of DNA (the daughter strand). Each new daughter cell gets a double helix of DNA--one strand from the mother cell and one brand-spankin'-new strand. This is the only replication method of the three that is considered biologically significant (meaning, this is what we care about!)

Ok, back to the question:

When DNA replicates semi conservatively, which of the following is true of each daughter DNA molecule?

A) Both strands are newly synthesized
B) One strand is newly synthesized, whereas the other is a strand from the parent DNA molecule
C) Both strands are the original strands of the parent molecule
D) One strand has more AT-rich regions than the other strand has
E) The newly synthesized strands are more susceptible to melting and renaturation than the parental DNA strands are

Let's go through the answers. "A" is obviously incorrect, since we just learned that when both strands of a double helix are newly synthesized, that is called conservative replication. "C" is also wrong, because if both strands were of the parent molecule, no replication would have happened at all....the DNA would have just moved from one cell to another. "D" just doesn't make much sense. We know from complimentary base pairing, that each strand has exactly the same number of bases, so it's impossible for a semi conservatively replicated strand to have more AT regions than the other. "E" tries to throw you off by mentioning melting and renaturation, but we don't care about that.That leaves "B." This answer is the definition of semiconservative replication--one strand is newly synthesized, whereas the other is a strand from the parent DNA molecule.

There you go! Yay us!