Part C-Dec 2015

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

Part C


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

1.     Lipid rafts are rich in both sphingolipids and cholesterol. Cholesterol plays a central role in raft formation since lipid rafts apparently do not form in its absence. Why do you think cholesterol is essential for the formation of lipid rafts?

1.   Cholesterol decreases the mobility of sphingolipids in the lipid bilayer.
2.   Large head groups of sphingolipids repel each other in presence of cholesterol.
3.   Cholesterol interacts with fatty acid tails in the membrane.
4.   The planar cholesterol molecules are postulated to fill the voids that form underneath the large head groups of the sphingolipids.

Ans: 1
Lipids rafts are localized regions of elevated cholesterol and glycosphingolipid content within cell membranes. The fatty-acid side chains of the phospholipids present in lipid rafts tend to be more highly saturated than those in the surrounding membrane. This allows close packing with the saturated acyl chains of sphingolipids, and probably leads to phase separation. Due to the presence of cholesterol, a liquid ordered domain is formed that exhibits less fluidity than the surrounding plasma membrane. This tight packing of lipids and phase separation is probably responsible for the signature property of lipid rafts : their insolubility in nonionic detergents.

Lipid rafts have been subdivided into caveolar and non-caveolar rafts that coexist in distinct membrane areas. Caveolar domains contain caveolae, stable flask shaped invaginations of the plasma membrane. Caveolae are enriched in cholesterol and coated by the hairpin-like plamitoylated structural protein caveolin 1, which is thought to stabilize the invaginated caveolar structure. Non-caveolar lipid rafts are flat membrane microdomains that are also enriched in cholesterol and resistant to detergent extraction but have less caveolin 1 than caveolae. Unlike caveolar rafts, non-caveolar lipid rafts are heterogeneous in lipid and protein composition.[For more details refer Simple Instant Notes].

2.    The following are the statements about pyruvate kinase (PK).

1. ATP is an allosteric inhibitor of PK
2. Fructose 1, 6 biphosphate is an activator of PK

3. ADP is an allosteric inhibitor of PK
4. Alanine is an allosteric modulator of PK

Which one of the above statements are true ?
1.   A, B, C       2.   A, B, D       3.   B, C, D       4.   Only A

Pyruvate kinase catalyzes the third step in glycolysis. It is activated by fructose 1, 6 biphosphate. ATP and pyruvate has been found to be a negative allosteric inhibitor. Alanine has also been found to be a negative allosteric modulator.  [For more details refer Simple Instant Notes].
3.     G-Protein coupled receptors (GPCR) consist of three protein subunits and . In unstimulated state, subunit is GDP bound and GPCR is inactive. When GPCR gets activated, it acts like guanine nucleotide exchange (GEF) factor and induces subunit to release its bound GDP allowing GTP to bind in its place. In order to regulate G-protein activity by regulating GDP/GTP concentration, subunit acts as
1.   GTPase
2.   GDP kinase
3.   cGMP , specific phosphodiesterase
4.   cAMP , specific phosphodiesterase
Ans:- 1
Expln:- G-proteins are heterotrimeric, with three subunits designated , and . The G-protein subunit has bound GDP and hence the name G protein. When the hormone (or any appropriate ligand) finds to the GPCR (G protein coupled receptor), it undergoes a conformational change. It stimulates the G-protein. Then the G releases GDP and binds GTP. When the signal is to be turned off, G acts as GTPase and G hydrolyses GTP to GDP.[For more details refer Simple Instant Notes].
4.    The “Red Queen Hypothesis” is related to
1.   the mating order in the harem of a polygamous male.
2.   the elimination by deleterious mutations by sexual reproduction.
3.   mate selection process by a female in a lek,
4.   the evolutionary arms race between the host and the parasite.
Ans:- 4
The Red Queen hypothesis is an evolutionary hypothesis which proposes that organisms must constantly adapt and evolve not only to gain reproductive advantage but also to survive. It is based on ideas based on coevolution.
The basis for the entire theory is down to the evolutionary arms race, where prey and predator constantly evolve together to reach some sort of uneasy balance.
An example of the Red Queen Hypothesis might be one of the plants that evolve toxins to kill off predators such as caterpillars.
If the plant, under predation selection pressure, evolved a new type of toxin to which the caterpillar had no immunity, most of the caterpillars would die off and the tree would flourish. This victory would be short lived, as only the caterpillars immune to the toxin, even if only a tiny percentage, would breed rapidly, and once again the tree would be under attack.
Because parasites harm their hosts, natural selection favours host genes that make them resistant to parasites. Unfortunately, however, parasites are also under selection to overcome this resistance. Thus, hosts and parasites may co-evolve in continuous arms races of defence and counter-defence. Based on a character in Lewis Carrol’s Through The Looking Glass (1871), such dynamics are often called “Red Queen dynamics” in evolutionary biology because hosts and parasites like Alice and the Red Queen in the book âhave to constantly be on the move without getting anywhere.

More Questions & Answers Refer Simple Instant Notes