Question

Why does mRNA have to be processed in eukaryotes and what does involve?

What translates the DNA message and how? Explain what tRNA does. How is it possible to have 45 tRNA but 61 mRNA to code for the same amino acids?

Explain how a ribosome works to generate a protein. Be sure to start with 2 ribosomal subunits coming together, and include the A,P,E sites in your explanation.

Answer 1

1. The mRNA in eukaryotes are important for transferring the information from DNA into protein. Eukaryotes have both introns and exons in their genome. Exons code for protein while introns are non-coding part of the genome. Before mRNA is translated into protein, the mRNA has to be processed to remove all the intron regions and this done by the mRNA processing. This process involves Heterogenous ribonucleoprotein particles (hnRNP) which assist in the mRNA processing and transport of mRNAs.

2. The DNA message is translated into protein by a process called translation done by the ribosomes. First DNA is transcribed by RNA polymerase into mRNA and the information on the mRNA is read by ribosomes with the help of tRNAs to form a string of amino acids ultimately forming a protein.

There is one tRNA for every amino acid to match its codon. There are 61codons for 20 amino acids but only 45tRNA are present due to the presence of wobble base pairing in which the third base in the tRNA anticodon can recognize two or more different codons on an mRNA.

3. The 40S subunit binds to the mRNA at the ribosome binding site and scans for the initiator codon AUG. When it encounters the AUG, it binds recruits the 60S larger subunit to form the pre-initiation complex.

A site (aminoacyl-tRNA site) - the site where a tRNA can bind to deliver the next amino acid to be added to a polypeptide chain that is being synthesised.

P site (peptidyl-tRNA site) - the site where the tRNA holding the growing polypeptide chain is located. The chain will be transferred to the tRNA in the A site to add on the amino acid being held there. The first tRNA starts in the P site.

E site (exit site) - the site where the tRNA that was in the P site moves to before leaving the ribosome.