Part B-Dec 2014

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

Part B


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

1. Chirality of DNA is due to
1.   The bases   2.   Base stacking   3.   Hydrogen bonds between bases   4.   Deoxyribose

Ans: 4
Expln:- Explanation : Deoxyribose is a chiral molecule, meaning that its mirror image can be made as well. The two different molecules are called D- deoxyribose and L-deoxyribose; D-deoxyribose is the molecule appearing in DNA, whereas its mirror image L-deoxyribose does not occur naturally.

As nucleotide molecules come together to form the structure of DNA, they develop a twist that forms the double helix structure of DNA. DNA develops a twist in the chain because each component contains chirality or handedness. It is this handedness that gives DNA the spiral shaped helical structure. If one molecule in the DNA structure had the wrong chirality, DNA would not exist in the double helix form, and DNA would not function properly.

2. Proton motive force during oxidative phosphorylation is generated in mitochondria by
1. exchanging protons for sodium ions
2. Pumping protons out into intermembrane space
3. pumping hydroxyl ions into the mitochondria
4. hydrolysis of ATP

Ans:- 2
Expln:- Oxidative phosphorylation is the synthesis of energy rich ATP molecules with the help of energy liberated during oxidation of reduced co-enzymes (NADH2, FADH2) produced in respiration. The enzyme required for this synthesis is ATP synthetase. ATP-synthetase becomes active in ATP formation only where there is a proton gradient having higher concentration of protons on the F0 side as compared to F1 side. Increased proton concentration is produced in the outer surface of inner mitochondrial membrane by the pushing of protons with the help of energy liberated by passage of electrons from one carrier to another.

3.In proteins, hydrogen bonds form as follows: Donor (D)-H—Acceptor (A). Hydrogen bond is more favorable if the angle between D-H and A is
1.   <90°   2.   180°   3.   >180°   4.   120°

Ans:- 2

Expln:- Hydrogen bonds (D-H—A) are predominantly electrostatic interactions between a weakly acidic donor group (D-H) and an acceptor atom (A) that bears a lone pair of electrons. In biological systems the donor group is an oxygen or nitrogen atom that has a covalently attached hydrogen atom, and the acceptor is either oxygen or nitrogen. The angle between the D-H bond and the H—A hydrogen bond should be close to 180° for a strong hydrogen bond. However, usually the structural restrains in proteins will enforce minor deviations from linearly.

4.What will happen if histones are depleted from a metaphase chromosome and viewed under a transmission electron microscope ?
1. 30nm chromatin fibres will be observed
2. 10nm chromatin fibres will be observed
3. A scaffold and a huge number of loops of DNA fibres will be observed
4. A huge number of loops of DNA fibres without scaffold will be observed

Ans:- 3

Expln:- The molecular organization of chromatin is best explained by nucleosome model, proposed by Roger Konbery in 1974. According to this model, the chromatin occurs like a beaded string. The beaded units of the string are called nucleosomes. Each nucleosome consists of a spiral DNA wrapped around an octomer of histone molecules forming a core particle. The octomer of proteins consists of two molecules each of the four different histones (tetramers). These histones are H2A, H2B, H3, and H4. The core particles are linked by DNA which in turn is associated with only one type of histone (H1). It represents the first level of shortening of the very large strand of DNA into a beaded fibre of 10 nm width. The 10nm fibre of nucleosomes further condensed to produce a solenoid of a 30 nm diameter.
The packaging of chromatin fibre at higher level with non-histone chromosomal proteins (NHC proteins). This solenoid structure undergoes further coiling to produce a chromatin fibre of 200 nm and then a chromatid of 700 nm diameter which can be seen under the light microscope. All the folded loops of chromatin are held by a nuclear scaffold formed by non-histone proteins.