Part C-Dec 2014

CSIR-UGC National Eligibility Test (NET) for Junior Research Fellowship and Lecturer-ship

Part C

PREVIOUS SOLVED QUESTIONS – 2014 December CSIR – NET EXAM

This time CSIR does not allow candidates to carry questions with them. We have collected maximum questions from our candidates (memory based).

1.The lifetime of a peptide bond in proteins is very large (~1000 years). Which statement below is INCORRECT with respect to stability of the peptide bond ?
1.   The free energy of hydrolysis is negative
2.   The free energy of hydrolysis is positive and large
3.   The energy barrier to be crossed to go to the hydrolyzed state is large
4.   The peptide bond can be hydrolyzed by 6N HCl at 100°C

Ans: 2
Expln:- A peptide bond can be broken down by hydrolysis. The peptide bonds that are formed within proteins have a tendency to break spontaneously when subjected to the presence of water releasing about 10 kj/mol of free energy. Hence the free energy of hydrolysis of a peptide is negative.

2. Acetyl-(Ala)18-CONH2 exists in α-helical conformation in solution. Most of the backbone dihedral angles (Φ, ψ) will be
1.-60°, -30°
2. 60°, 30°
3.-60°, -30°(50 %) and 60°, 30° (50 %)
4. -80°, 120°

 

Ans:-1
Expln:- Dihedral angle (torsion angle) is Φ and ψ angles between N ‘ Cα and C α -C that have a certain range of allowable values. Then how much rotation is possible between Φ and ψ ? Based on this rotation, different forms of proteins like α , β structure are formed. The average phi and psi values for alpha-helices and beta-sheets is around -57, -47 and -80, +150 respectively. [For more details refer Simple Instant Notes].

3. DNA is not hydrolyzed by alkali whereas RNA is readily hydrolyzed. This is due to

1.   The double helical structure of DNA
2.   The presence of uridine in RNA
3.   Due to features observed in RNA such as stem-loop structures
4.   The presence of 2-OH group in RNA

Ans:- 4

Expln:- Ribose of RNA has OH group in second carbon atom. Hence RNA is readily hydrolyzed by alkali treatment.

4.Two homologous proteins were isolated from a psychrophile (P) and a thermophile (T). The purified proteins were subjected to denaturation, protease digestion and circular dichroism (CD). Following observations were made :

A.   The CD spectra of P and T proteins are identical.
B.   Their amino acid composition is 95 % identical
C.   T and P are equally susceptible to proteolysis in the presence or absence of reducing agent
D.   T has higher midpoint of thermal denaturation than P.

The reason for enhanced stability in T is due to
1.   Altered secondary structure
2.   Increased number of disulfides in T
3.   Increase in water of hydration
4.   Increase in number of salt bridges

Ans:- 4

Expln:- The enhanced stability of proteins isolated from thermophile is due to increase in number of salt bridges and increase in number of side chain-side chain hydrogen interactions. The association of two ionic protein groups of opposite charge is known as an ion pair or salt bridge. For example, salt bridge most often arises from the anionic carboxylate of either aspartic acid or glutamic acid and the cationic ammonium from lysine or the guanidium of arginine.

More Questions & Answers Refer Simple Instant Notes