Q. 17

[mcat_premed3832]

OK, so you correctly stated what a circular chromosome would look like after the first replication: one labelled DNA strand and one unlabelled DNA strand.

For the next step, however, let's go a bit more slowly. Now, consistent with your knowledge of DNA replication and your understanding of the passage, a small part (the replication eye) of the 2 parental strands described above, open and permit labelled nucleotides to enter and create 2 new labelled strands of DNA. So, at the small replication eye, one brance will now have 2 labelled DNA strands while the other branch will be 1/2 labelled and 1/2 unlabelled. Now, recall that the parent strand was 1/2 unlabelled. Thus, one branch of the growing replication eye is twice as labelled as the rest of the chromosome (the parent strands) while the other branch is equally as labelled as the rest of the chromosome (thus answer choice D is true and C is contradicted by the above).

NB: passage 1 in GS-6 BS also explores this topic in great detail.

[jellywing_2058]

Exactly! This make choice D correct since you get 2 chromosomes (from one branch) out of four that are half labeled and the other 2 (from the other branch) are fully labeled, meaning they have double the label as the rest.

[mashbour8199]

I know this might be asking for a lot, but if anyone understands this and could possibly help with a picture please? My e-mail is mashbour@uoguelph.ca

If anyone else needs any help on other things, I'd be very pleased to help out! My problem is just wrapping my head around what they mean by all of these "strands". Ahh.

Thanks!

[deblinak1531]

I understood the concept, but what's the difference between choices C and D? If one chromosome is twice as strongly labeled as the other chromosome (D), then the other chromosome is half as strongly labeled (C).

[jellywing_2058]

This is already discussed previously by mcat_premed3832

Joined: 19 Oct 2006
Posts: 164

PostPosted: 10 Jul 2007 01:07 Post subject: Reply with quote

[rsandberg94836]

i completely understand the replication process, concept of semi-conservation, and all that crap but what I can't get is the radioactive marker crap. Why does it only mark the daughter strand? Is it because the parent strands are identical to their precursors? I just don't understand. Any help would be very appreciated!

[calena7178147]

See the diagram below. Radiolabelling (red strands in picture) of thymidine is used to differentiate between parental and newly synthesized strands. Newly made strands take up the labeled T.

[rsandberg94836]

thank you! the pic really helped =)

[hua8986059]

was trying to draw this one out with pencil during the test. Color helps so much more lol.

Semi-conservative replication.. I despise you.

at least now I understand it fully for GS-6 lol..

[bribercar2809]

If you use logic their is no difference in saying something is "1/2 as much as something else", or something is "2x as much as something else"

[mcat_premed3832]

It is very different.

A 2 door car has 1/2 the number of doors as a 4 door car. A 2 door car does not have twice the number of doors as a 4 door car. To make in true, you would have to exchange the order of the 2 door and 4 door car in the sentence. But that is not what was done among the answer choices. That is why answer choices C and D have completely different meanings with completely different implications.

[bribercar2809]

# One branch of the growing replication eye would be half as strongly labeled as the remainder of the chromosome
# One branch of the growing replication eye would be twice as strongly labeled as the remainder of the chromosome

It does not signify, which is which, it does not signify, a is x, and b is y, it just says a branch compared to the rest of the chromosome, and you can take this to mean what you wish, if you look at the picture you can see, that they are interchangeable

So yes a 2 door car has 1/2 the doors of a 4 door car,
and a 4 door car as 2x the doors of a 2 door,

[mcat_premed3832]

They are not the same.

branch of "growing" replication eye = daughter strand

rest of chromosome (what is left) = parent strand