You are doing a mapping experiment with bacteria. You have combined an Hfr strai

Question

You are doing a mapping experiment with bacteria. You have combined an Hfr strain (strs) with genotype arg+, his+, thr+. The recipient, F- cell is arg-, his- and thr-. You allow conjugation and select against the Hfr and the F- cell by plating on media containing streptomycin antibiotic, his and thr but not arg.

You collect 500 colonies of this genotype (arg+, strr) and then replica plate on several different plates to determine the genotype of the his and thr genes.

On minimal media (MM) plate, 420 of the 500 colonies grow

On MM plus his, 426 of the colonies grow

On MM plus thr: 472 colonies grow.

Give the numbers of cells of the following genotypes and briefly explain how you determined it:

arg+, his+, thr+

arg+, his+, thr –

arg+, his -, thr+

arg+, his -, thr –

Explanation / Answer

On minimal media (MM) plate, 420 of the 500 colonies grow

On MM plus his, 426 of the colonies grow

On MM plus thr: 472 colonies grow.

Colonies which were grown on the minimal media are the bacterial cells which have received all the three genes i.e. his, thr and arg.

Given that on minimal media 420 colonies were grown,So, these are the cells which have the genotype arg+, his+, thr+

Cells which have grown on  Minimal media plus his are the cells which have received arg and thr gene through conjugation.

So, arg+, his -, thr+ will be 476 cells.

Cells which have grown on Minimal media plus thr are the cells which have received arg and his gene through conjugation.

So, arg+, his +, thr- will be 476 cells

Cells which have not grown on Minimal media plus arg but unable to grow on minimal media are the cells which have received the only arg

So those cells will be 500-420 = 80.

arg+, his -, thr – genotype- 80 cells

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