Tutor profile: Ronnie L.

Please make the appropriate correction, if any, to the following quoted phrase: If either of these events "happen, it is" likely to slow down traffic. (A) NO CHANGE (B) happen, they are (C) happens, it is (D) happens; they are

Looking at the sentence, it is possible that either event could happen, and "either" suggests one outcome out of two or more. Therefore, "either of these events" is singular, not plural, and should correspond with a singular verb, "happens." Because we are talking about one event and not many, the phrase after the comma should stay "it is." The correct answer, then, is (C).

How do you differentiate between an (R) or (S) configuration?

First, find the chiral carbon of interest, and then order the four different groups attached to it by branching out from the chiral center one atom at a time. Once you're one atom out, rank the group based on atomic number, with the highest atomic number atom being highest priority. If you encounter two of the same atom (such as two carbons), continue to move out one more atom at a time until there is a difference. If there is a double bond, count that as higher priority than a single bond to the same atom; for example, C=O would have a higher priority than C-OH. Finally, once you got all the four groups in order of priority, rotate the molecule so that lowest priority group points away from you into the page. Notice which way your hand turns if you go from priority #1 to #3. If it turns clockwise, that center is (R); if it turns counterclockwise, that center is (S).

Is the lac operon an example of an inducible or repressible operon?

The lac operon is probably the most well-characterized inducible operon. Being an inducible operon (i.e. transcription of the genes can be induced) means that a specific molecule, in this case allolactose, must bind to a regulatory protein and allow transcription to occur. Here, the regulatory protein is binding to the stretch of operon DNA and preventing transcription from taking place. When allolactose binds, however, the protein changes shape and falls off the DNA, therefore allowing transcription.