Question

In: Biology

4. What is the function of the E, P and A site during polypeptide elongation? 5....

4. What is the function of the E, P and A site during polypeptide elongation?

5. What are polyribosomes and why do they not exist in eukaryotes?

6. Not all single nucleotide changes affect the amino acid sequence of a protein. Why is this and what is this type of mutation called?

7. Insertion and deletions of single nucleotides cause a frameshift mutation. Explain what frameshift means in this context.

Solutions

Expert Solution

4. Ribosomes has three site for binding amino acyl tRNA, P site (peptidyl site), A site (aminoacyl site) and E site (exit site).

P site - There is binding of the first charged tRNA and translocation movement of ribosome on mRNA, so peptidyl tRNA is at P site which take part in the peptide bond formation.

A site - There is binding of next charged tRNA at A site and a new animoacyl tRNA reaches the site which takes place in the formation of codon-anticodon binding.

E site - It is the exit and final site where the binding between codon and anticodon breaks and discharged tRNA leaves the ribosome.5. Polyribosomes are just a bunch of several ribosomes together translating the mRNA into polypeptide chain. Eukaryotes contain the sequence of a single polypeptide so one ribosome attach to the single initiation site whereas in prokaryotes it can be polycistronic meaning it have multiple initiation codon so a number of ribosomes attach to different initiation site independently. Hence, polyribosomes are not found in eukaryotes.

6. Silent mutation are those mutation that cause no change in the sequence of polypeptide chain even when there is replacement of one nucleotide with another in the chain. This happens because there is degeneracy of code that means some amino acids are coded by more than one codon. For example proline is coded by CCC but if the codon is changed to CCU due to change in single nucleotide then alao CU codes for proline.

7. The change in more than one nucleotide pair is called gross mutation. And it occurs by frame shift mutation in which there is a shift of frame of mRNA sequence due to one or more change in nucleotides. It is of two types :

Delection - Removal of one or more bases in a nucleotide chain.

Insertion or addition - Addition of one or more bases in a nucleotide chain.


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