Can someone explain why the bolded answers are the correct choices? (A) If one w
ID: 86347 • Letter: C
Question
Can someone explain why the bolded answers are the correct choices?
(A) If one wants to know if two different rII point mutations lie at exactly the same site (nucleotide pair), one should:
i. coinfect E. coli K with both mutants. If phage are not produced they lie at the same site.
ii. coinfect E. coli K with both mutants. If phage are produced they lie at the same site.
iii. coinfect E. coli B with both mutants and plate the progeny phage on both E. coli B and E. coli K. If plaques appear on B but not K they lie at the same site.
iv. coinfect E. coli B with both mutants and plate the progeny phage on both E. coli B and E. coli K. If plaques appear on K but not B they lie at the same site.
(B) If one wants to know if two different rII point mutations lie in the same cistron, one should:
i. coinfect E. coli K with both mutants. If phage are not produced they lie in the same cistron.
ii. coinfect E. coli K with both mutants. If phage are produced they lie in the same cistron.
iii. coinfect E. coli B with both mutants and plate the progeny phage on both E. coli B and E. coli K. If plaques appear on B but not K they lie in the same cistron.
iv. coinfect E. coli B with both mutants and plate the progeny phage on both E. coli B and E. coli K. If plaques appear on K but not B they lie in the same cistron.
Explanation / Answer
E.coli B allows growth of rII point mutations whereas E.coli K doesnot allow . Hence rII mutations are called conditional mutants which are able to grow in specific conditions only. E.coli B is called permissive strain and E.coli K is called non-permissive strain. If the mutations are in different genes then it forms plaques in K strain because due to recombination the mutations would become wild type and hence lysis occurs. If the mutations in the same gene then recombination doesnot occur (as the 2 mutations travel alongside whereever they go) and hence lysis doesnot occur. Hence for A question option iii is correct. Similarly if point mutations in same gene , then recombination doesnot occur and hence no plaques are formed . Hence for B question option i is correct. Infecting B strain with mutants have no effect because it is permissive for rII mutants . Hence testing for K strain alone is needed as it is impermissive for mutants and allows only growth of wildtype strains.