I've attached the picture of the problem.

SO, based on the information I managed to assemble two large cistrons. (if the combination of two mutants yield (-) mutations are on the same chromosome)

A)so my cistrons are:



Note: somehow 4 and 7 is present in both cistrons

B) they ask us what mutation could cause two different cistrons have the same mutants

I was thinking maybe an insertion? i just dont get how it's even possible since if they have the same mutants doesnt it mean they overlap and thus are on the same chromosome??

I would appreciate if you could explain me what type mutation could cause it and check if I did the complementation correctly

enter image description here


1 Answer 1

  1. bacteriophage T4 has a single chromosome (approx. 200 kb in length). The chromosomes are terminally redundant, so the genetic map is circular, even though the physical map is linear (as I recall).
  2. A mutation in E. coli (or its viruses) that affects two adjacent protein-coding genes is said to be 'polar'. This indicates that the two genes in question form part of an operon. A strong loss-of-function mutation in the promoter for the operon will typically affect the expression of all the genes in the operon. In your example there are two cistrons, or genes, in the rII locus of T4.

It is possible that the data for this question are taken directly from the published literature. I suggest you search PubMed for papers by Seymour Benzer, from the 1950s.

  • $\begingroup$ Sorry Perry, I just don't understand what you mean. So you're saying this problem can't be solved or the mutation on operon is the cause? $\endgroup$
    – Jx1
    Commented Nov 30, 2015 at 3:41
  • $\begingroup$ I was trying to say that these two cistrons form an operon in the phage genome, and that by knocking out the promoter for that operon you would reduce the expression from each cistron. Another way to say this is that there are two genes bedside each other on the chromosome. The upstream gene has a promoter and the downstream gene does not have its own promoter. The downstream gene uses the upstream gene's promoter. this means that the mRNA from that promoter will be polycistronic and contain 2 open reading frames. Breaking one promoter kills the expression of 2 genes. $\endgroup$
    – mdperry
    Commented Nov 30, 2015 at 13:32
  • $\begingroup$ So how can this mutation be called then? 1 promoter per 2 cistrons? I've never encountered anything like this in my class before $\endgroup$
    – Jx1
    Commented Dec 1, 2015 at 4:55
  • $\begingroup$ On second thought, the answer they probably want is that 4 and 7 are deletions, or deficiencies, that remove some part of both genes. That is going to be a simpler explanation than the operon with two cistrons. $\endgroup$
    – mdperry
    Commented Dec 1, 2015 at 18:23

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