Question

1) Ribosmal RNAs (rRNAs) are transcribed from rRNA genes, and eukaryotic ribosome subunits are assembled in the nucleolus. How many times must a eukaryotic rRNA molecule cross the nuclear envelope before functioning in translation inside a functional ribosome in the cytosol of the cell?

a) never

b) once

c) twice

d) three times

2)  Polymerization of amino acids into a polypeptide requires energy. In terms of chemical thermodynamics, the chemical energy for peptide bond formation in translation technically comes from:

a) hydrolysis of GTP

b) hydrolysis of ATP

c) translocation of the ribosome as it moves along the mRNA

d) ribosomal RNA (rRNA) secondary structure

e) transcription of the mRNA that is being translated

Answer 1

1) Assembly of ribosomal proteins and rRNA occurs in nucleolus, the site of rRNA biosynthesis of RNA pol I. Ribosomal proteins are synthesized in cytoplasm and are transported into nucleolus to be assembled with rRNA to form ribosoma subunits. These ribosomal subunits are then trasnported into cytoplasm for their assembly into a mature ribosome. Hence, rRNA cross the nuclear envelop just once before functionning in transation. Correct answer is B.

2) Peptide bond formation during elongation step of translation is facilitated by binding of incoming aminoacyl-tRNA to GTP- EF-Tu complex. This drives binding of resulting aminoacyltRNA–EF-Tu–GTP complex to the A site of the 70S initiation complex. This is followed by GTP hydrolysis and release of EF-Tu–GDP complex from the 70S ribosome. Transfer of initiating N-formylmethionyl group from its tRNA to the amino group of the second amino acid (present in A site) faciliates peptide bond formatation between two amino acids present in A and P site. This makes option A correct answer.