Here are some notes and a practice question on bacteria...there's also a tetrad question, which I could not give you before because I wasn't able to make sure that it wouldn't be on the MT.
Please post all your questions on bacteria in this section!
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8 comments:
Hi Pam :)
For question 28 finding map units... 4 out of 1000 progeny are nicotinamide-requiring, so they themselves are mutants, but those that were mutants who reverted back to wt phenotype are also recombinants so thats why we have to multiply 2 to get m.u. of 8? Is it multiply by 2 all the time?
You are correct as to why we need to multiply by 2.
However, we don't always need to do that, it depends on whether the double mutant that is, in our case, nic2- sup+ is phenotypically WT or mutant.
Basically you need to remember that you have TWO classes that are recombinant. If they are both phenotypically mutant, then the % of the mutants give you directly the map units. If only one of the two recombinant classes is mutant, then you'll have to multiply its % by 2 to get the map units.
I'll put up a question that deals, in a way, with this issue....
Cheers
Pam
thanks for replying & the question :)
Shouldn't the answer key say tet^r instead of tet^s since tet^s would die in a medium containing tet?
Yes....(sorry, i copy-pasted the wrong genotype...)
Thanks for pointing this out, and good job on noticing it!
(It's now fixed)
I have question regarding "rollover Hfr lacZ" question.
Usually, the questions I've seen so far mention what alleles are present within the gene (ex. Hfr A+, B+, C+). Here, Hfr is said to have Z1 while F- is said to have Z2... but the answer shows that Hfr and F- both have Z1 and Z2; it's just the matter of having wt (+) or mutant (-). Do we know both Hfr and F- are heterozygous (ex. Z1-, Z2+ for Hfr) becuase otherwise nothing will grow on the medium containing glucose + streptomycin?
I hope I made my question clear enough... :O
also for that Hfr rollerover question: since it states that the Z represents two different alleles of the gene, shouldn't there be only one gene? Also, shouldn't they add lactose only in the medium to test if the gene for using lactose is transferred?
Yes, Z1 and Z2 are two different mutations in the Z gene. Let's say, Z1 may be a missense mutation at base pair number 5, and Z2 may be a missense mutation at base pair 315. So, you can actually get crossover between the 2 mutations and obtain one WT recombinant and one double mutant (with mutant phenotype). However, since there is 'little room' between the two mutations, the chances of such crossover are very slim. It's kind of like thinking of 2 genes that are right next to each other and so close that crossovers between them are very rare.
The message is that the other gene in question (I think it's "his"....I don't have the question in front of me right now), can't be in between Z1 and Z2...we know that the order must be either his, Z1, Z2 OR his, Z2, Z1. Based on the information in the question you should be able to figure out which one it is, and exaplain why you concluded that.
I hope this helps.
About the lactose in the medium, they aren't worried about it because they let the conjugation go for 45 minutes (which is almost enough to get the strep sensitive gene, which is far away from the Z gene, so the Z gene should definitely have a chance to go in). Also, the question they ask is not how many map units are between the various markers, but "out of all the his+colonies, how many are Z+ in cross #1, and how many are Z+ in cross#2?"
They are doing a comparison study, not an 'absolute', quantitative analysis.
Cheers!
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