Over on the thread The Chemical Structure of Double-Stranded DNA we're having a little discussion about exam questions related to the structure of DNA and reading frames.
I thought it might be fun to post some multiple choice questions from old exams to see if the Sandwalk readers are as smart as my third year molecular biology students. Here's a question from 1999.
Examples of overlapping genes that are transcribed in opposite directions (i.e., opposite strands serve as templates) are very rare in biology. Part of the coding region from the middle of two such overlapping genes is shown below. In one of these genes a mutation results in the substitution of valine for methionine in the polypeptide (i.e., the normal protein has methionine). What effect would this have on the polypeptide sequence encoded by the other gene? (the sequence of the normal or wild-type gene is shown)
a) no change
b) substitution of methionine for arginine
c) premature termination (shorter protein)
d) substitution of threonine for isoleucine
e) substitution of serine for phenylalanine
Oh man, I'd have to hunt for stop codons to determine the reading frame...too hard to do with the small font and wavy red line!
ReplyDeleteBut with too much free time on my hands, the answer is 'C'.
ReplyDeleteThe 3rd reading frame on the top strand (+3) is the only frame that does not contain a stop codon and therefore must be the ORF. ATG is the only codon for methionine, and assuming that there was only one base changed, the valine codon after mutation is GTG. In the -3 reading frame an ATT codes for isoleucine. The 'AT' part of this codon is the complement to the 'AT' from the +3 ATG, so the ATG -> GTG change produces an ATT -> ACT in the -3 reading frame corresponding to an I -> T change in the -3 reading frame.
...to see if the Sandwalk readers are as smart as my third year molecular biology students.
ReplyDeleteAn all-too-common bit of inward-looking silliness, I'm afraid.
This has absolutely nothing to do with being "smart." Try "educated on molecular biology," which is hardly the same thing.
scott belyea said,
ReplyDeleteAn all-too-common bit of inward-looking silliness, I'm afraid.
This has absolutely nothing to do with being "smart." Try "educated on molecular biology," which is hardly the same thing.
I'm just guessing here but I'm willing to bet that you don't know anything about the show Are You Smarter than a 5th Grader?.
If you do know about the show then I guess you're one of those sarcasm impaired readers that we encounter from time to time.
Do you get to use a codon table in the exam? I can cope with the concepts, but I can't commit things like codon tables to memory unless I use them every day for years.
ReplyDeleteYes, I make sure that students have a copy of the standard genetic code. In some cases they can bring all their notes to the exam as well.
ReplyDeleteA) No change
ReplyDeleteActually, it's d. Damn I feel stupid now
ReplyDeleteErrr... F) I don't have a copy of the genetic code with me. :(
ReplyDelete