Question

Translocation is the collective movement of the mRNA and tRNA during translation. These steps occur in the following order:

	A.	P-site tRNA moves to the E-site and the A-site tRNA moves to the P-site
	B.	E-site tRNA moves out of the ribosome and the P-site tRNA moves to the E-site
	C.	E-site tRNA moves to the P-site and the P-site tRNA moves to the A-site
	D.	A-site tRNA moves to the P-site and the P-site tRNA moves to the E-site

Answer 1

Correct option is (D)

D) Ribosome consists of 4 sites

1 mRna binding site and 3 tRna binding site.

Three tRna binding sites are A-site,P-site and E-site.

A-site - it is the acceptor site where it binds the aminoacyl tRna and holds the new aminoacid that to be added to polypeptide chain.

P-site- It is a peptidal site where it hold the growing peptide chain of amino acids.

E-site- It is exit site where aminoacid leaves the chain and forms protein.

The aminoacyl tRna binds initially to codon of mRna(this initial bind of tRna is the A-site) then the aminoacids transfered from aminoacyl tRna of A-site to the growing polypeptide chain of P-site and then tRna moves to E-site and leaves the site then again aminoacyl tRna enters the A-site.

hence this process is continued for many times to from a protein.

Incorrect options

A) The question is about collective movement from mRna to tRna during translocation.

so it starts from the A-site not the P-site(A-site-->P-site-->E-site).It starts from the middle of translocation at P-site.

B) It strats with E-site (exit site) that means last step of translocation and it tells about the p-site entering the E-site.so,the option does not start with initiation step of translocation and it incorrect.

C) E-site tRNA moves to the P-site and the P-site tRNA moves to the A-site.In this E-site does not move to the P-site and also P-site doesnot move to the A-site.so, the option is incorrect.