Q-41. Xeroderma pigmentosum is produced as a result of a defect in:
a) DNA polymerase III
b) DNA polymerase I
c) DNA exo-nuclease
d) DNA ligase
Ans: b and d
Explanation:
Xeroderma pigmentosum, or XP, is an autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet (UV) light is deficient.
The most common defect in xeroderma pigmentosum is an autosomal recessive genetic defect in which nucleotide excision repair (NER) enzymes are mutated, leading to a reduction in or elimination of NER.
In a healthy, normal human being, the damage is first excised by endo-nucleases. DNA polymerase then repairs the missing sequence, and ligase “seals” the transaction. This process is known as nucleotide excision repair.
The enzymes affected in this condition are:
UV specific endo-nuclease (most common)
DNA polymerase I
DNA ligase
Q-42. Shine-Dalgarno sequence in bacterial m-RNA is near
a) AUG codon
b) UAA codon
c) UAG codon
d) UGA codon
Answer: AUG codon
Explanation:
Shine-Dalgarno sequence:
In E. coli, it is a purine rich sequence of nucleotide bases located six to ten bases upstream of the AUG codon on m-RNA molecule- that is, near its 5’ end.
16S ribosomal RNA component of 30S ribosomal subunit has a nucleotide sequence near its 3’ end that is complementary to all or part of Shine-Dalgarno sequence.
Shine-Dalgarno sequence is important for identification of the translation initiation site on the mRNA by the ribosome.
Eukaryotic message do not have Shine-Dalgarno sequence.
Q-43. RNA polymerase does not require
a) Template (ds-DNA)
b) Activated precursors (ATP, GTP, UTP, CTP)
c) Divalent metal ions (Mn2+, Mg2+)
d) Primer
Answer: Primer
Explanation:
Unlike DNA polymerase, RNA polymerase doesn’t require a primer and has no known endo-nuclease or exo-nuclease activity.
It, therefore has no ability to repair mistakes in the RNA, as dose DNA polymerase during DNA synthesis.
Q-44. A segment of a eukaryotic gene that is not represented in the mature messenger RNA is known as
a) Intron
b) Exon
c) Plasmid
d) TATA box
Answer: Intron
Explanation:
Maturation of eukaryotic m-RNA usually involves the removal of RNA sequence, which do not code for protein (Introns or intervening sequence) from primary transcript.
The remaining coding sequence, the exons are spliced together to form mature m-RNA.
The molecular machine that accomplishes these tasks is known as the spliceosome.
Q-45. An enzyme that makes a double stranded DNA copy from a single stranded RNA template molecule is known as
a) DNA Polymerase
b) RNA polymerase
c) Reverse transcriptase
d) Phospho-kinase
Answer: Reverse transcriptase
Explanation:
Messenger DNA can be used as a template to make a complementary double stranded DNA (c-DNA) molecule using the enzyme reverse transcriptase.
Resulting c-DNA is thus a double stranded copy of m-RNA.
Q-46. Which of the following can be a homologous substitution for valine in hemoglobin?
a) Iso-leucine
b) Glutamic acid
c) Phenylalanine
d) Lysine
Answer: Glutamic acid
Explanation:
The codon for valine at position 67 of the beta chain of hemoglobin is not identical in all persons who possess a normal functional beta chain of hemoglobin.
Hemoglobin Milwaukee has at position 67 a Glutamic acid. Hemoglobin Bristol contains aspartic acid at position 67. Hemoglobin Sydney contains alanine at position 67.
Homologous substitution for valine in hemoglobin:
Alanine
Aspartic acid
Glutamic acid
Q-47. The trans-membrane region of protein is likely to have
a) A stretch of hydrophilic amino acids
b) A stretch of hydrophobic amino acids
c) A disulphide loop
d) Alternating hydrophilic and hydrophobic amino acids
Answer: A stretch of hydrophobic amino acids
Explanation:
Proteins tend to fold with the R- groups of amino acids with hydrophobic side chins in the interior. Amino acids with charged or polar amino acids side chains (arginine, glutamine, and serine) generally are present on the surface.
Amino Acid with Aliphatic or Non-Polar Side Chains:
Alanine
Valine
Leucine
Iso-leucine
Glycine
Q-48. Which of the following group of proteins assist in the folding of other proteins?
a) Proteases
b) Proteosomes
c) Templates
d) Chaperones
Answer: Chaperones
Explanation:
Chaperone proteins participate in the folding of over half of mammalian proteins.
The hsp 70 (70-kDa heat shock protein) family of chaperones binds short sequences of hydrophobic amino acids in newly synthesized polypeptides, shielding them from solvent.
Chaperones prevent aggregation, thus providing an opportunity for the formation of appropriate secondary structural elements and their subsequent coalescence into a molten globule.
Hsp60 acts later in the folding process, often together with an hsp 70 chaperone.
Q-49. Proteins targeted for destruction in eukaryotes are covalently linked to
a) Clathrin
b) Pepsin
c) Laminin
d) Ubiquitin
Answer: Ubiquitin
Explanation:
Proteins that are defective or destined for rapid turnover are often marked for destruction by ubiquitination- the attachment of small, highly conserved protein, called Ubiquitin.
Proteins marked in this way are rapidly degraded by a cellular component known as the proteasome, which is complex, ATP-dependent, proteolytic system located in cytosol.
Important points:
Chaperone associate with nascent protein and ensures final proper conformation of the nascent protein
Signal peptide guides the trans-membrane proteins into endoplasmic reticulum.
Ubiquitin marks aged and abnormal proteins for degradation
Q-50. Which of the following is a membrane bound enzyme that catalyzes the formation of cyclic AMP from ATP?
a) Tyrosine kinase
b) Polymerase
c) ATP synthase
d) Adenylate cyclase
Answer: Adenylate cyclase
Explanation:
Adenylate cyclase is membrane bound enzyme that converts ATP to c-AMP.
Vibrio cholerae toxin activates the adenylate cyclase cascade with potentially disastrous consequences.