Out of Syllabus (OFS) Questions

nO.1
R/S between tRNA, mRNA and rRNA in translation.
we had discussed extensively on the roles of tRNA and mRNA in translation but often skip rRNA. We only know rRNA as a component of ribosomes but what are they doing in ribosomes?

Let's appreciate the fact that different rRNAs are found in the 2 ribosomal subunits.


So what are their roles?
1) Peptidyl transferase is an enzyme found in the large ribosomal subunit that catalyse the formation of a peptide bond between the polypeptide chain (peptidyl-tRNA) in P site and the adjacent aa (aminoacyl-tRNA) in the A site of the ribosome. Now the elongated chain is now attached to the tRNA at the A site.
This peptidyl transferase is rRNA in action = it is a ribozyme, an RNA enzyme.

2) They are known to interact with
a) mRNA (rRNA in small subunit - translation step 1: small subunit binds to the mRNA..=)),
b) anti-codon regions and 3'CCA ends of the tRNAs at the P and A sites (rRNA in both subunits - no prize for guessing which for which).


3) also the subunit are able to interact because of interaction between the rRNAs in the 2 subunits!

COol yeah?!
Actually rRNA is not as well-studied so I will stop here.
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nO.2
Proofreading mechanism of DNA polymerase in details

DNA polymerase has intrinsic 3'-> 5' proofreading exonuclease activity
(exo=outside; nuclease=enzyme that cleave nucleotides; exonuclease = enzyme that remove/cleave nucleotides from the terminal)

From Wiki:
When an incorrect base pair is recognized, DNA polymerase reverses its direction by one base pair of DNA. The 3'-5' exonuclease activity of the enzyme allows the incorrect base pair to be excised (this activity is known as proofreading).



How does the DNA polymerase know when there has been a mistake?

anticodon and codon pairing is very impt btw the tRNA (AC) and mRNA (C).
Incorrect base pairing --> conformation is altered/distortion in shape of the regular double helix which is recognised as a mismatch of base pair to initiate the exonuclease activity.


NOTE: not all DNA polymerase have proofreading capability (there are many types of DNA polymerase but the one we are interested in for replication has such capability)

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