Question

Any t-RNA molecule is highly selective toward the amino acid to which it is linked in forming an aminoacyl t-RNA. There is one t-RNA for every amino acid and chances of obtaining t-RNA molecules linked to wrong amino acids are rare. For example, the incorporation rate of valine into Ile-tRNA is 1 in 200 but the actual mis-incorporation occurs only at about 1 in 3000. Explain the reason for this phenomenon.

A.

EF-Tu-GTP-aminoacyl-t-RNA halts at the GTP hydrolysis step and the wrong amino-acyl-t-RNA is expelled from the A site

B.

The aminoacyl transferase that transfers the amino acid to the cognate t-RNA harbors an RNAse activity that degrades the incorrect aminoacyl-tRNA molecule

C.

The wrongly incorporated amino acid stimulates a nucleoprotein degradation complex, which degrades the wrong aminoacyl-t-RNA

D.

The aminoacyl transferase that transfers the amino acid to the cognate t-RNA harbors a hydrolytic site that accommodates the wrongly incorporated amino acid but not the correct amino acid

E.

A second molecule of the aminoacyl-t-RNA synthetase joins the complex to provide a hydrolytic pocket that removes the wrongly incorporated amino acid

2. How does cytosine in our DNA undergo spontaneous transformation to Uracil? Through what mechanism is this mutation repaired?

A.

The amine group undergoes oxidation into a keto group; Nucleotide-excision repair

B.

The amine group undergoes Sandmeyer reaction to diazonium salt, which upon attack from a water molecule gets converted to a hydroxyl group; Base-excision repair.

C.

Conversion of the amine into a nitroso group, which rearranges into a keto group; Base-excision repair.

D.

Hydrolysis of Schiff base; Base-excision repair

E.

Direct attack of the amine group by a water molecule; Nucleotide-excision repair

3. Explain the mechanism of transcriptional regulation by a typical steroid receptor.

A.

The ligand-bound steroid receptor trigger signaling to open phosphorylation-dependent calcium channels, which activate the Ca/CaMKIV in the nucleus, thereby phosphorylating and activating transcription factors

B.

Two Zinc fingers on each steroid receptor of a dimer bind to two palindromic sequences separated by three nucleotides on promoters

C.

Steroid-mediated signaling through its receptor causes activation of protein kinase C, which in turn phosphorylates and activates transcription factors to cause transcriptional regulation

D.

The ligand-bound steroid receptor stimulates protein kinase A, which through Rap-1 stimulates B-Raf and the Erk pathway, thereby phosphorylating and activating the transcription factor Rsk.

Answer 1

1. When a wrong amino acid is bound to the tRNA, then the subsequent aminoacyl tRNA can be incorporated into the ribosomal A site but the EF-Tu protein halts the wrong aminoacyltRNA for wrong incorporated aminoacyl tRNA and eliminate them from the A site. Hence the answer to this question is option

A.

EF-Tu-GTP-aminoacyl-t-RNA halts at the GTP hydrolysis step and the wrong amino-acyl-t-RNA is expelled from the A site

2. The transformation of cytosine to uracil occurs through the deamination process with the intermediate formation if the Schiff's base and this forms an abasic site in the DNA thereby this is repaired by the AP endonucleases through the base excision repair mechanism and hence the correct option for this is

D.

Hydrolysis of Schiff base; Base-excision repair

3. The steroid hormones are lipophilic in nature and hence they can directly influence the signalling process by directly entering the nucleus and binding to the transcription factor or by forming dimeric protein complexes in the cytoplasm with the transcription factor and then entering the nucleus and initiating gene expression. Hence the correct option for this is

C.  

Steroid-mediated signaling through its receptor causes activation of protein kinase C, which in turn phosphorylates and activates transcription factors to cause transcriptional regulation

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