Question

3.  What are the “translator” molecules that recognize a codon in the mRNA and deliver the correct amino acid?

6. If each amino acid was encoded by a single codon, what is the minimum number of amino-acyl tRNA synthetases required for translation?

7. Looking at the codon table, if there was a unique aminoacyl-tRNA synthetase required for each anticodon, what is the minimum required?

9. If an aminoacyl-tRNA synthetase recognized any nucleotide (purine or pyrimidine) in the 5’end of the anticodon, give the codons (denote the 3^rd position with X) and the amino acid that could be loaded unambiguously. Is that sufficient to load all?

10. If some aminoacyl tRNA synthetases recognize if a nucleotide is a purine (Pu)or a pyrimidine (Py) in the 5’end of the anticodon, give the codons (denote the 3^rd position with Pu or Py) and the amino acid that could be loaded unambiguously.

11. From the answers to Questions 9,10 and 11 what is the minimum number of aminoacyl-tRNA synthetases required to load all the tRNA molecules with the correct amino acid?

12. In what compartment of the cell does polypeptide synthesis originate for translation of mRNA transcribed from nuclear genes?

13. What two macromolecular groups comprise components of the large and small ribosomal subunits?

14. What two macromolecular groups comprise components of the large and small ribosomal subunits?

15. Where are the macromolecular components of the ribosome synthesized in a eukaryotic cell? Where are they assembled into large and small subunits?

16. What are the A, P and E sites on a ribosome and what is their roles in polypeptide synthesis?

17. Detail the steps required for the initiation of translation of mRNA in eukaryotic cells.

18. The codon for the start of polypeptide synthesis is 5’-AUG-3’. What is the nucleotide sequence of the anticodon found in the tRNA? Indicate the orientation.

19. What nucleotide triphosphate is responsible for the ribosome subunit assembly on the mRNA, specificity of amino acyl-tRNA selection and movement of the ribosome in relation to the mRNA sequence?

20. The energy for peptide bond formation is directly derived from what source?

To what category of macromolecules do releasing factors belong?

Answer 1

3. The translator molecules are called tRNA which deliver the anticodons for the right codon present on the mRNA. These tRNA molecules are specific are each amino acid and also there exists wobble base pairing among the anti-codons.

6. The answer is 20, one for each amino acid.

7. There would total of 61 unique aminoacyl-tRNA synthetase for each amino acid. Total of 64 codons, out of which three are stop codons.

12. The mRNA originates from nucleus and moves into the cytoplasm where the translation happens.

13. The small and large ribosomal subunits are made of two macromolecular components namely rRNA and proteins

15. Ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus for subassembly in the nucleolus. The subunits are then returned to the cytoplasm for final assembly.

16. The three operational or binding SITES are A, P and E reading from the mRNA entry site. Sites A and P span both the ribosome sub-units with a larger part residing in the ribosome large sub-unit and a smaller part in the smaller sub-unit. Site E, the exit site, resides in the large ribosome sub-unit.

17. The initiation of protein synthesis occurs by combination of mRNA, tRNA for a specific amino acid binding to the ribosome. The mRNA strand remains continuous, but the true initiation point is the start codon, AUG.

The eukaryotic initiation is divided into 8 steps:

1. Eukaryotic initiation factor 2 (eIF2)–GTP–Met-tRNA^Met_i ternary complex formation.
2. Formation of a 43S preinitiation complex comprising a 40S subunit, eIF1, eIF1A, eIF3, eIF2–GTP–Met-tRNA^Met_i and probably eIF5.
3. mRNA activation, during which the mRNA cap-proximal region is unwound in an ATP-dependent manner by eIF4F with eIF4B.
4. Attachment of the 43S complex to this mRNA region.
5. Scanning of the 5′ UTR in a 5′ to 3′ direction by 43S complexes.
6. Recognition of the initiation codon and 48S initiation complex formation, which switches the scanning complex to a 'closed' conformation and leads to displacement of eIF1 to allow eIF5-mediated hydrolysis of eIF2-bound GTP and P_i release.
7. Joining of 60S subunits to 48S complexes and concomitant displacement of eIF2–GDP and other factors (eIF1, eIF3, eIF4B, eIF4F and eIF5) mediated by eIF5B.
8. GTP hydrolysis by eIF5B and release of eIF1A and GDP-bound eIF5B from assembled elongation-competent 80S ribosomes.

18. The anticodon for the start codon will be 5'---CAU---3'

19. The energy for the process during ribosome assembly, specificity, and movement of ribosome is from hydrolysis of GTP.

20. The energy for the peptide bond formation is derived from the condensation reaction between NH2 and COOH group of respective amino acids.

Releasing factors are made up of proteins.