Part C-June 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. Glycolipids and sphingomyelin are produced by the addition of sugars or phosphorylcholine to ceramide on cytosolic and luminal surfaces, respectively, of the Golgi apparatus. Finally, after such modifications, these molecules are located on the outer half of the plasma membrane. What key events are responsible for such localization ?

1.   Membrane fusion only
2.   Action of Flippase and membrane fusion
3.   Action of only Flippase
4.   Flip flop of these molecules in the golgi membrane catalyzed by proton pump.

Ans: 1
Expln:- Glycolipids are derivatives of the lipid sphingosine. These sphingolipids include the cerebrosides and gangliosides. They containceramide with carbohydrate moieties attached to its hydroxymethyl group.
Glycolipids are involved in intercellular communication. These are synthesized from ceramide and UDP-glucose or UDP-galactose. Synthesis of sphingolipids (glycolipids) takes place in Golgi complex. Their lipid component becomes part of the membrane of the secretory vesicle that buds from the trans face of the Golgi. Then the vesicle membrane fuses with the cell membrane. The lipid component of the glycolipid remains in the outer layer of the cell membrane, and the carbohydrate component extends into the extracellular space.
Sphingomyelin is synthesized on the lumenal side of the Golgi complex. [For more details refer Simple Instant Notes].
2. Sting of a bee causes pain, redness and swelling. Melittin is a major peptide in bee venom. Melittin is a membrane binding peptide that is involved in activating phospholipases in the membrane. The possible target phospholipase that is activated by melittin is

1. Phospholipase C to generate inositol phosphates
2. Phospholipase A2 to generate arachidonic acid.
3. Phospholipase D to generate 1`-3`-inositol.
4. Phospholipase A1 to generate palmitic acid.

Expln:- Melittin is a stimulator of phospholipase A2, which can recognize the 2 acyl bond of phospholipids and hydrolize to releasearachidonic acid. It is converted into active compounds like prostaglandins and their related compounds, the prostacyclins, leukotrienes etc. by the enzyme cyclooxygenase (cox). These are collectively called eicosanoids, since they are all C20 compounds (Greek: eikos means twenty). Eicosanoids are categorized as anti-inflammatory and inflammatory mediators. [For more details refer Simple Instant Notes].
3.     n order to study the transcription factor TFIIH, it was cloned from a large number of human subjects. Surprisingly, the subjects having mutation in TFIIH, also showed defects in their DNA repair system. Given below are the explanations:

A.   DNA damage is always associated with transcription inhibition.
B.   TFIIH has no role in DNA repair.
C.   In mammalian system, TFIIH plays an active role in transcription coupled DNA repair process
D.   Because of mutation in TFIIH, transcription initiation is inhibited and incompletely synthesized mRNAs remain attached to the template DNA leading to DNA damage.

Choose the correct answer.    1.    A and B     2.   C only    3.   B and D    4.   D only

Ans:- 2

Expln:- TFIIH, a nine-subunit protein complex, plays a dual role in both DNA excision repair and in transcription. Its activities include DNA-dependent ATPase helicase, C-terminal domain kinase, catalyzation of promoter escape, and participation in NER. During the transcription, TFIIH enters the RNA polymerase II preinitiation complex and activates it prior to initiation. It may act in open complex formation and/or promoter clearance.
In nucleotide excision repair (NER), TFIIH is recruited by xeroderma pigmentosum group B protein (XPA) to the damaged sites. At least two excision-repair proteins, encoded by the xeroderma pigmentosum XPB/ERRC3 and XPD/ERR2 subunits, were found in TFIIH. This suggested that the TFIIH complex might involve in excision repair. [For more details refer Simple Instant Notes].

4. In high altitude, hypoxia induces increased number of circulating red blood cells, which can be explained by the following changes:
A.   The transcription factors, HIFs, are produced.
B.   Erythropoietin secretion is increased
C.   Myoglobin content is decreased
D.   Cytochrome oxidase is decreased
Which one of the following is NOT true ?
1.   Only A
2.   A and B
3.   B and C
4.   C and D
Ans:- 4

Expln:- The body compensates low oxygen availability by increasing red blood cell production, decreasing the binding capacity of hemoglobin and by increasing breathing rate. The respiratory alkalosis produced by the hyperventilation shifts the oxygen hemoglobin dissociation curve to the left, but there is a concomitant increase in red blood cell 2,3-DPG, which tends to decrease the O2 affinity of hemoglobin. Erythropoietin secretion increases promptly on ascent to high altitude and then falls somewhat over the following 4 days as the ventilatory response increases and the arterial pO2 rises. The increase in circulating red blood cells triggered by the erythropoietin begins in 2-3 days and is sustained as long as the individual remains at high altitude. There are also compensatory changes in the tissues. Themitochondria, which are the site of oxidative reactions, increase in number, and there is an increase in myoglobin that facilitates the movement of O2 in the tissues. There is also an increase in the tissue content of cytochrome oxidase.

More Questions & Answers Refer Simple Instant Notes