Part C-Dec 2012

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.     Mutants of lac Y(Y’) gene of E.coli do not synthesize the lactose permease protein. The following statements refer to the behaviour of lac Y’ mutants under different experimental conditions.

A.   No synthesis of β – galactosidase when Y’cells are induced with lactose.
B.   Synthesis of β – galactosidase when cells are induced with lactose.
C.   No synthesis of β – galactosidase when cells are induced with IPTG.
D.   Synthesis of β – galactosidase when cells are induced with IPTG
E.   The cells induced with IPTG cannot grow in the presence of TONPG (TONPG is a compound, whose uptake is mediated by lactose permease and cleaved by β – galactosidase to release a toxic compound).
F.   Cells induced with IPTG can grow in the presence of TONPG

Which combination of the above statements is correct ?

1.   A, D and F   2.   B, C and E   3.   A, C and F   4.   A, C and E

Ans: 1
Expln:– The three structural genes are: lacZ, lacY, and lacA.
• lacZ encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose.
• lacY encodes β -galactoside permease (LacY), a membrane-bound transport protein that pumps lactose into the cell.
• lacA encodes β-galactoside transacetylase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.
Only lacZ and lacy appear to be necessary for lactose catabolism.
A mutant of E.coli (lacY mutant) is described that they can be grown only in the presence of isopropylthiogalactoside (IPTG).

This compound is used as a molecular mimic of allolactose, a lactose metabolite that triggers transcription of the lac operon. (Allolactose – an inducer of lac operon is a combination of galactose and glucose with an α-1,6 rather than an α-1,4 linkage does.] Unlike allolactose, the sulfur (S) atom creates a chemical bond which is non—hydrolyzable by the cell, preventing the cell from metabolizing or degrading the inductant; therefore the IPTG concentration remains constant.

IPTG binds to the lac repressor and releases the tetrameric repressor from the lac operator in an allosteric manner, thereby allowing the transcription of genes in the lac operon, such as the gene coding for β-galactosidase, a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides.

2.     Some important events in the history of life on Earth are given below :

A.   First vertebrates (jawless fishes); first plants.
B.   Forest of ferns and conifers; amphibians arise; insects radiate.
C.   Conifers dominant; dinosaurs arise; insects radiate.
D.   Flowering plants appear; climax of dinosaurs followed by extinction.
E.   Radiation of flowering plants, most modern mammalian orders represented.
F.   Ice Ages, Modern humans appear.

Match the above with the geological time periods and choose the correct combination.

1.   A – Silurian, B-Permian; C-Triassic; D-Jurassic; E-Cretaceous; F- Tertiary
2.   A – Ordovician; B- Carboniferous; C – Triassic; D-Cretaceous; E- Tertiary; F-Quaternary
3.   A – Cambrian; B – Ordovician; C-Silurian; D-Devonian; E-Permian ; F – Tertiary
4.   A –Devonian; B-Permian; C-Triassic; D – Cretaceous; E- Tertiary; F – Quaternary

Ans:- 2

Expln:- Two of the greatest evolutionary events in the history on Earth occured during early paleozoic time. The first was the cambrian explosions of skeletonized marine animals about 540 million years ago. The second was the great ordovician biodiversification event. Thus during Ordovician period, both animal diversification (jawless vertebrates) and plant diversification (mosses) occured.

3.     A boy eats a large serving of cheese having high amount of sodium. He hardly drinks any fluid. Inspite of this, the water and electrolyte balance was maintained. Which one of the following explanation is correct ?

1.   His aldosterone was decreased and alcohol dehydrogenase (ADH) was increased
2.   His aldosterone was increased and ADH was decreased
3.   There was no change in either of the hormones
4.   His sympathoadrenal system was stimulated

Ans:- 1

Expln:- Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. Thus aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressure. Antidiuretic hormone (ADH) or vasopressin acts mainly at the kidney and stimulates reabsorption of water and thereby reduces loss of water through urine (diuresis).


4.     During transgenesis, the location of the genes and their number integrated into the genome of the transgenic animal are random. It is often necessary to determine the copy number of genes and their tissue-specific transcription. The following are the possible methods used for the determination.

A.   Polymerase Chain Reaction (PCR)
C.   Reverse Transcriptase PCR
B.   Southern blot hybridization
D.   Western blot
Choose the correct set of combinations.

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

Ans:- 2

Expln:- Transgene copy number is defined as the number of exogenous DNA insert(s) in the genome. For example, if the exogenous DNA fragment inserts only once at a single locus of the genome, it is a single copy transgenic event. Multiple transgene copies could lead to extremely high expression of the gene, and sometimes result in transgene silencing.
The conventional method for transgene copy number determination is Southern blot hybridization. Southern blot hybridization based transgene copy number determination is both costly, time consuming, and requires tens of microgram quantities of high-quality DNA. Real-time PCR has emerged the method of choice for fast, affordable, and efficient estimation of copy number.

5.    From the following statements:

A.   Triose phosphate is utilized for the synthesis of both starch and sucrose
B.   Triose phosphate is translocated to cytosol from chloroplast
C.   Triose phosphate is confined to chloroplast and is utilized for synthesis of starch only
D.   Triose phosphate is translocated from cytosol to chloroplast
Which of the following combinations is correct regarding starch and sucrose synthesis during day time ?

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

Ans:- 1 Expln:- Most of the triose phosphate synthesized in chloroplast is converted to either sucrose or starch. During the dark period, chloroplast starch is rapidly brocken down and its products are exported into the cytoplasm, where a major portion of them is converted into sucrose. Generally, an active flow and exchange of carbon between starch and sucrose exist at all times.
During the dark period, starch is degraded providing more triose phosphate which is exported from the chloroplast to secure the continuous synthesis of sucrose in the cytoplasm. The 3PGA/Pi translocator in the plastid membrane is the vehicle which maintains the regulated flow of organic carbon between the chloroplast (starch) and cytoplasm (sucrose). The transfer of triose phosphate molecule that is facilitated by this translocator explains the rapid exchange of carbon that occurs between starch and sucrose at all times when there is a net synthesis and accumulation of starch and also during periods of starch depletion.

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