Methionine is one of the two amino acids that is specified only by a codon. How...

Methionine is one of the two amino acids that is specified only by a codon. How do prokaryotes use this single codon to specify the initiator residue of a polypeptide and the internal methionine residues in a polypeptide chain?

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Expert Solution

DNA as an acidic substance present in the nucleus was first observed by Fredrich Meischer in 1869. It serves as the genetic material in all living organisms. It contains 2 anti-parallel strands joined via H-bonding.

A forms 2 H-bonds with T and Gforms 3 H-bonds with C.

Protein synthesis occurs on molecular machines called ribosomes. tRNA serves as an adapter molecule which reads the sequence of mRNA and brings the corresponding amino acid.

The genetic code is read as triplet nucleotide sequences called codons. This genetic code is not punctuated and nearly universal. Each codon codes for 1 amino acid but 1 amino acid can be encoded vy >1 codons. AUG encodes for met and also as a start codon. Of the total 64 codons, 3 are stop codons which terminate transcription (UAA, UAG, UGA).

Ribosome contains 3 pockets namely E, P, A sites. The initiator tRNA binds to P site and each incoming amino acyl tRNA binds to A site. The amino acid bound P site initiator tRNA is joined to the amino acid in A site. This reaction is called peptidyl transferase reaction. The p site tRNA moves to E site while A site tRNA moves to P site.

Several bacteria use alternate initiator codons which code for Val instead of Met. However in those where AUG serves dual purpose, the Met bound to the initiator tRNA is formylated after vharging. This distinguishes the initiator tRNA and elongating Met tRNA.

Initiator tRNA: Formyl Met (AUG)

Elongating tRNA: Met (AUG)

This formyl group is removed after exit of the polypeptide from the large subunit of ribosome.

CAT TTC TAC CGA TGA CCC ATT AAT Template strand

GUA AAG AUG GCU ACU GGG UAA UUA mRNA

Val Lys Met Ala Thr Gly Stop Phe Protein

gladiator answered 1 year ago

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