Translation

Translation is appropriately named because it converts the nucleotide sequence into a different language- that of amino acids!

To understand this mechanism, we have to understand a couple things first: tRNA and Ribosomes.

Transfer RNA (tRNA) a small strand of RNA (~70-90 base pairs) that has multiple loops and functions by ferrying amino acids in the cytoplasm to the ribosomes. The bottom loop is called an Anticodon, and has the complimentary sequence of base pairs that correspond with a codon on an mRNA. The other end of the molecule is a region that caarries the amino acid that corresponds to the codon.

Ribosomes are made up of two different sized parts, called the large and small ribosomal subunits. Each subunit is made up of Ribosomal RNA (rRNA) and proteins. The ribosome has 3 sites where tRNA passes through as translation proceeds. These sites are named E, P and A sites.

A tRNA adding an amino acid to polypeptide chain is called Aminoacylation, and the bonds between amino acids are Peptide Bonds. The A site is short for the Aminoacyl site, where a the tRNA first enters. The P site is the Peptidyle site, where peptide bonds are formed between the amino acids. The E site is the Exit site, where the tRNA leaves without an amino acid.

Translation begins with a familiar stage:

Initiation

The two ribosome subunits bind to the 5’ cap of the mRNA transcript, sandwiching the mRNA between them. The ribosome moves along the 5’cap until it reaches the Start Codon (AUG). The 5’ cap prevents initiation of translation in the middle of genes having an internal AUG codon.

Elongation

When the start codon is in the P site, a tRNA delivers the amino acid Methionine (Met). The tRNA recognizes the codon because of its complimentary anticodon. The second tRNA, with the appropriate amino acid and anticodon binds to the codon in A site. The Met in the P site is cut off its tRNA, and a peptide bond forms between Met and the second amino acid in the A site (using a special enzyme called peptidyl transferase). The ribosome moves down the mRNA one codon and the two amino acids shift over to the P site. The now empty first tRNA, which brought Met, is released at the E site.

Termination

Elongation continues until a Stop Codon (UAA, UAG or UGA) is read in the A site. The ribosome stalls and a release factor protein binds into the A site and causes the ribosome to dissociate (break apart) into its subunits, releasing the newly formed polypeptide chain and the mRNA.

The polypeptide chains are combined with other polypeptide chains, and are then folded and modified into its final form: a fully functioning protein!

Textbook Practice

p. 325-331# 2, 3, 6-8

Summary Videos

Page updated

Report abuse