Question

1. Which type of bonding is catalyzed by aminoacyl-tRNA-synthetases to form between amino acids and tRNAs?

Select one:

A.
Hydrogen bond

B.
Salt bridge

C.
Peptide bond

D.
Ester bond

E.
Glycosidic bond

2.What is added to methionine when it is used as the start codon?

Select one:

A. Phosphate

B. Formyl

C. N-acetyl

D. Sulphate

E. rRNA

3.

In translation (initiation, elongation and termination), the energy source is provided by ______.

Select one:

A. ATP

B. TTP

C. GTP

D. CTP

E. UTP

4.

Which of the following statements is correct regarding transcription and translation in eukaryotes?

Select one:

A.
During transcription, an mRNA molecule is synthesized from the DNA molecule.

B.
Translation takes place in the nucleus, while transcription takes place in the cytoplasm.

C.
During protein synthesis, translation occurs prior to transcription.

D.
During translation, a DNA sequence is used to synthesize an amino acid sequence.

E.
Transcription and translation occur in nucleus.

Answer 1

1. D. Ester bond

During the first stage of protein synthesis, aminoacyl -tRNA synthetases catalyse the addition of the correct amino acid to the corresponding tRNA molecule. This is a 2-step process. In the first step, carboxyl group of the amino acid interacts with phosphoryl group of ATP, resulting in the formation of aminoacyl adenylate. In the second step, the aminoacyl group from the intermediate is transferred to the tRNA forming an ester link between amino acid and tRNA.

2. B. Formyl

In prokaryotic cells, N-formyl group is added to the amino group of the 5’ methionine or the initial methionine to differentiate it from the internal methionine. The 2 enzymes involved in the addition of the N-formyl group to the initial methionine are Met-tRNA synthetase and transformylase.

3. C. GTP

GTP is the energy source in all 3 steps of protein translation. During the initiation stage, GTP is bound to initiation factor 2 (IF-2) and is hydrolysed to GDP. During elongation GTP is bound to the soluble elongation factor EF-Tu. During the translocation step, the energy required to move the ribosome along the mRNA is provided by the hydrolysis of GTP molecule.

4. A. During transcription, an mRNA molecule is synthesized from the DNA molecule.

The central dogma states that DNA is transcribed into mRNA in the nucleus. The mRNA travels from the nucleus to the cytoplasm where it serves as the template for the production of the polypeptide via translation.

The option B is wrong because translation takes place in the cytoplasm, while transcription takes place in the nucleus.

The option C is wrong because, transcription occurs first where DNA acts as template for production of mRNA. Translation occurs later where the mRNA acts as the template for protein synthesis

The option C is wrong because, during translation, a mRNA sequence is used to synthesize an amino acid sequence.

The option D is wrong because only transcription occurs in the nucleus. Translation occurs in the cytoplasm.