Part C-June 2011

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

Previous Solved Questions – 2011 June CSIR-NET Exam

1. During receptor-mediated endocytosis, apolipoprotein B on the surface of a LDL particle binds to the LDL-receptor present in coated pits containing clathrin. The receptor-LDL complex is internalized by endocytosis, trafficked to lysosomes and the LDL-receptor is finally recycled. A patient reports with familial hypercholesterolemia. This could be due to

    1. mutation in the LDL molecule
    2. defect in LDL-receptor recycling
    3. mutation in the LDL-receptor
    4. defect in cholesterol binding with its receptor

Ans:- 3     Expln:- Lipoproteins are vehicles for the transport of triglycerides and cholesterol in the body. On the basis of their functional and physical properties, plasma lipoproteins have been classified into five broad categories. They are 1) Chylomicrons 2) Very low density lipoproteins (VLDL) 3) Intermediate density lipoproteins (IDL) 4) Low density lipoproteins (LDL) 5) High density lipoprotein (HDL).

Chylomicrons, the largest of the lipoproteins, are synthesized in the intestine. They transport ingested triacylglycerols (triglycerides) to other tissues, mainly skeletal muscle and adipose tissue, and transport ingested cholesterol to the liver. At the target tissues the triacylglycerols are hydrolyzed by the action of lipoprotein lipase. The released fatty acids and monoacylglycerides are taken up by the tissues, and either used for energy production or re-esterified to triacylglycerol for storage. As their triacylglyceride content is depleted, the chylomicrons shrink and form cholesterol-rich chylomicron remnants which are transported in the blood to the liver. VLDLs are synthesized in the liver and transport a variety of lipids to other tissues, again mainly adipose tissue and skeletal muscle. As with chylomicrons, the tricylglycerols in VLDLs are acted on the lipoprotein lipase and the released fatty acids taken up by the tissues. The VLDL remnants remain in the blood, first as IDLs and then as LDLs. LDLs are then taken up by target cells through receptor-mediated endocytosis. Once in the lysosomes, the LDLs are digested by lysosomal enzymes.

LDL receptors clearly play an important role in the maintenance of plasma LDL-cholesterol levels. In normal individuals, about one half of the IDL formed from the VLDL reenters the liver through LDL receptor-mediated endocytosis.

2. Precursors of the atoms in the purine skeleton are

partc

    1. N1, Asp, C2 and C8, formate; N3 and N9, guanidine of Arg; C4, C5 and N7, Gly, C6, CO2
    2. N1, Asp, C2 and C8, citrate; N3 and N9, amide nitrogen of Gln; C4, C5 and N7; Gly; C6, CO2.
    3. N1, Asp; C2 and C8, formate; N3 and N9, amide nitrogen of Gln, C4, C5 and N7, Gly; C6, CO2.
    4. N1, Glu; C and C8, acetate; N3 and N9, amide nitrogen of Asn; C4, C5 and N7, Gly; C6, CO2

Ans:- 3     Expln:- By using isotopic labelling technique, the source for the synthesis of purine skeleton, has been traced. N1 of purines arises from the amine group of aspartate, C(2) and C(8) originate from formate, N(3) and N(9) are contributed by the amide group of glutamine, C(4), C(5), and N(7) are derived from glycine (strongly suggesting that this molecule is wholly incorporated into the purine ring), and C(6) comes from CO2. Isotopic labelling experiments have shown that atoms N(1), C(4), C(5), and C(6) of the pyrimidine ring are all derived from

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3. In eukaryotic chromatin, 30 nm fiber (solenoid) can open up to give rise to two kinds of chromatin. In one type (A), the promoter of a gene within the open chromatin is occupied by a nucleosome whereas in the other (B), the promoter is occupied by histone H1.

The following possibilities are suggested

A) The gene in (A) is repressed                       B)   The gene in (B) is repressed

C) The gene in (A) is active                                D) The gene in (B) is active

Which of the following sets is correct ?

    1. A and D
    2. A and B
    3. B and D
    4. C and D

Ans: 1

4. siRNAs and miRNAs are used for achieving gene silencing. Which of the following is/are correct ?

    1. Both siRNA and miRNA are processed by cytoplasmic endonuclease Dicer.
    2. “Drosha” is needed for processing miRNA only
    3. Both siRNA and miRNA show association with Argonaute protein
    4. All the following

Ans:- 4     Expln:- Distinguish between microRNA and siRNA – MicroRNA is a short (about 21 to 23 nucleotides) single stranded RNA molecules that is now recognized as playing an important role in gene regulation. It is similar to, but distinct from, another type of short RNA, known as small interfering RNA (siRNA).

miRNA are slightly shorter than siRNA (upto 25 nucleotides). siRNA is formed from two complementary strands. The two kinds of RNA are encoded slightly differently in the genome. The mechanism by which they regulate genes is slightly different.

miRNA attaches to a piece of messenger RNA (mRNA) – which is the master template for building a protein-in a non-coding part at one end of the molecule. This acts as a signal to prevent translation of the mRNA into a protein. siRNA, on the other hand, attaches to a coding region of mRNA, and so it physically blocks translation.

Two classes of small RNAs, small-interfering (si) RNAs and miRNAs, function as sequence-specific regulators of gene expression. miRNA precursors that are encoded by endogenous genes are folded into double-stranded (ds) RNA-like hairpins. The precursor molecules (pre-miRNAs) are first processed from primary miRNA transcripts (pri-miRNAs) by the nuclear enzyme Drosha and the pre-miRNAs are then transported to the cytoplasm. siRNAs are generated from longer dsRNA precursors formed in cells by DNA-or RNA-dependent synthesis of complementary strands, or introduced into cells by viral infection or artificial expression. Both siRNA and miRNA precursors are processed to

5. Cancer causing genes can be functionally classified into mainly three types: 1) genes that induce cellular proliferation, 2) tumor suppressor genes, 3) genes that regulate apoptotic pathway.

Epstein-Barr virus that causes cancer by modulating apoptotic pathway, contains a gene having sequence homology with which of the following genes ?

    1. bax
    2. bcl-2
    3. p53
    4. caspase-3

Ans: 2

More Questions and Explanations Refer Simple Instant Notes