A Genetics Question

[Guest Meninger]

I am currently enrolled in molecular genetics taught by Don Meninger (Harvard graduate) who has worked with scientists such as Messelson, Holliday, and such. This is one of his questions which I thought was a bit confusing at first. He seems to think that the question makes perfect sense. It may be a bit difficult for some of you nevertheless it is a practical problem having to do with practical logic skills which does not require one to be proficient in genetics.

 

After you answer the question, please comment on a scale of 1-5 on how difficult you thought the question was and if there was any ambiguity in how the question was phrased.

 

E coli has a base content of A=.247, G=.260, T=.236, C=.257. Recombination hot spots (X sites) are reported to exist every 5000 base pairs. Is the distribution of X sites random? Briefly explain your answer. The chi site sequence is 5' GCTGGTGG 3'.{for those of you who don't remember, dna is made up of four different kinds of nucleotides, each different nucleotide made up of either A, G, T, or C}

[Guest cjdf]

Interesting question...I just thought I would hazard an logical educated guess to it for a laugh....this how i would try and answer it....

 

E. coli - prokaryotic, mostly coding sequence.

Distribution of four bases is approximately equal - i.e each base accounts for approximately a quarter of the strands.

The chi sequence given is 75% G/C and 25% A/T, therefore presumably the X sites would be restricted to areas of high GC content and therefore NOT random.

 

Probably a messy answer but I'd be interested to see if its what you got, or to compare it to anyone elses...?

[Dr_666]

Let's assume that the frequency of each nucleotide is about 1/4 (0.25).

 

The probability of getting the GCTGGTGG sequence just by chance would be:

 

(1/4)^8 = 0.25 to the power of 8 = 0,000015

 

that means 1 every 65536 sequences of eight nucleotides.

 

You find that sequence every 5000, so they happen less often than they would be expected just by chance.

 

Hope this helps.

[Bluenoise]

Given this frequency of bases the sequence would appear randomly every 249151.8567

bases.