CSIR NET June 2012 Part C solved Question

This time CSIR does not allow candidates to carry questions with them. We have collected maximum questions from our candidates (memory based).
1.     A night blindness patient was advised to take Vit.A but found no difference. Then the doctor thinks that this may be the defect of the enzyme

1) Retinal synthase 2. retinal isomerase 3)retinol dehydrogenase 4).retinal reductase

Ans.   2

Expln:- The action of rhodopsin in rods may be considered as a typical mechanism in the photoreceptor cells. The retinal in rhodopsin can exist in two isomeric forms, cis-retinal and trans-retinal. In rhodopsin, it exists in cis-form. Light excites the electrons in the cis-retinal and converts it to trans-retinal. This causes the dissociation of scotopsin from retinal. Thus the purple coloured pigment gets decolourized. The dissociation of rhodopsin results in the generation of an impulse that is transmitted to the associated neurons. For the continued functioning of the rods, the rhodopsin has to be resynthesized. This takes place by two side pathways. First, the trans-retinal is reconverted to the cis-retinal by the enzyme retinal isomerase. Since the free scotopsin is present (having been removed from the rhodopsin), it immediately combines with the cis-retinal to regenerate new rhodopsin.

2.     Transcript of a transgenic peptide is made of AU repeating units. The product of this mRNA has

1. Polypeptide having many reading frames 2. single peptide having single amino acid 3.single peptide having two alternating amino acids 4.single peptide having three alternating amino acids

Ans.   3

Expln:- An mRNA with repeating unit of AU means, it has sequence of codon like AUAUAUAUAU…………. and so on. Then the codon AUA can specify the amino acid isoleucine and the codon UAU can specify the amino acid tyrosine.

3.     Phenyl ammonia lyase (PAL) and chalcone synthase (CHS) involved in biosynthesis of phenolic compound in plants. Following are some statements regarding the action of PAL and CHS:

A. Substrate for PAL is phenyl alanine and for CHS is chalcone
B. PAL catalyze conversion of phenyl alanine to trans-cinnamic acid
C.PAL catalyse conversion of phenyl alanine to p-coumaric acid
D.Coumaryl CoA is converted to chalcone by chalcone synthase
Which of the following combinations of above statement is true ?

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

Ans.   4

Expln:- Secondary metabolites in plants can be broadly divided into three : terpenes, alkaloids and phenolics.

The natural phenolic compounds can be broadly separated into two categories – flavonoids and non-flavonoids. Flavonoids include the anthocyanins, tannins etc. The non-flavonoids include resveratrol. Stilbenoids etc. Except flavonoids, all other plant phenolics are biosynthesized in plants from phenylalanine or its precursor shikimic acid through shikimic acid pathway. (Refer instant notes).

In the case of flavonoids, which have C6 – C3 – C6 carbon skeleton, one aromatic ring and its side chain arises from phenyl alanine while the other aromatic ring arises from acetyl-CoA via malonic acid pathway.

The first step in the synthesis of phenolic compounds from phenylalanine in plants is deamination of phenylalanine by the enzyme phenyl alanine ammonia lyase (PAL). PAL is an important plant enzyme that converts phenylalanine into cinnamic acid. Cinnamic acid is the precursor of various phenyl propanoids such as lignins, chalcone, flavonoids, coumarins etc.

Flavonoids are synthesized via phenyl propanoid pathway as follows. (Phenyl propanoids includes a family of organic compounds that are synthesized by plants from the amino acid phenylalanine.).
4.     Upon absorption of a photon, a chlorophyll molecule get converted to its excited state when the energy of photon is

1) More than that of ground state of pigment molecule
2) Equal to that of pigment molecule’s excited state
3) More than that of ground state but lesser than excited state of the pigment molecule
4) Equal to energy gap between ground state and excited state energy

Ans.   4

Expln:- Excitation of chlorophyll by light:- What happens when chlorophyll and other pigments absorb light ? The colours corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy can not disappear. When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more potential energy. When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption of a photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and the energy difference varies from one kind of atom or molecule to another. Thus, a particular compound absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum.
5.     The intestinal absorption of glucose is impaired by use of ovabain, an inhibitor of Na+-K+ ATPase. Indicate the correct explanation.

1) The inhibitor has blocked the transport of Na+ from intestinal lumen to epithelial cells
2) The inhibitor has blocked the transport of Na+ from epithelial cells to intestinal lumen
3) The inhibitor has blocked the transport of Na+ from interstial lumen to intestinal cells
4) The inhibitor has blocked the transport of Na+ from interstial cell to intestinal lumen

Ans.   2
6.     Which of the following statement regarding aquaporin or water channels is NOT correct ?

1) Aquaporins are found in both plants and animals membrane
2) Aquaporins cannot transport uncharged molecules like ammonia
3) Phosphorylation and calcium concentration regulates aquaporin activity
4) Activity of aquaporin is regulated by pH and reactive oxygen species


Expln:- Aquaporins are the membrane water channels of the biological world. Protein conformation analysis of the amino acid sequence of the aquaporin family suggests a six-transmembrane-spanning topology for each AQP molecule. There are cytosolic amino and carboxy termini, extracellular loops A, C and E and intracellular loops B and D. A highly conserved three amino acid motif, Aspargine-Proline-Alanine (NPA) is present in the B and E loops of nearly all AQPs.

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Aquaporins can be divided into two classes, those that only allow water to flow through them, and those that will also transport glycerol and a few other small uncharged molecules.

The factors affecting the gating behaviour of aquaporins involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradient, temperature etc. The regulation of aquaporin permeability by Ca2+ and/or protons concentrations has been reported for mammalian AQP0, AQP3 and AQP6 and for plant PIPs.
7.     The lac operon in E.coli, is controlled by both the lac repressor and the catabolite activation protein CAP. In an in vivo experiment with lac operon, the following observations are made:
(A) cAMP levels are high (B) Repressor is bound with allolactose
© CAP is interacting with RNA polymerase
Which of the following conclusions is most appropriate based on the above observations ?

1). Glucose and lactose are present
2) Glucose is present and lactose is absent
3)Both are absent
4) Glucose is absent and lactose is present

Ans.   4

Expln:- Lac operon is an example for both negative and positive control. Interaction of DNA (operator gene) with repressor protein inhibits gene expression. That is operon is normally ‘on’ but is kept ‘off’ by repressor produced by regulator gene. This is negative control.

However, lac operon is also under positive control by a regulatory protein called CAP (catabolite activator protein) and a small effector molecule called cyclic AMP. (CAP is also sometimes called CRP or cyclic AMP receptor protein). The lac promotor contain two separate binding sites: (1) one for RNA polymerase and (2) one for the CAP-cAMP complex. The lac promotor on its own is a weak one. So RNA polymerase does not readily bind to it and it needs extra help to initiate transcription. The extra help in polymerase binding is given by the attachment of the CAP. However, CAP does not attach unless cyclic AMP is bound to it. The CAP-cAMP complex must be bound to its binding site in the lac operon in order for the operon to be induced. The CAP-cAMP complex thus exerts positive control over the transcription of the lac operon. cAMP is produced by thecell only when glucose levels are low. When glucose level is high, cAMP is rare and CAP does not bind and hence RNA polymerase dose not bind effectively and lac operon transcription is minimal (glucose effect).

In the presence of lactose and allolactose (Allolactose is also an inducer of lac operon. It is a combination of glucose and galactose with an a-1, 6 rather than an a-1, 4 linkage does), repressor binds with it and the operator become free to allow the RNA polymerase to bind with the promotor.
8.     Cystic fibrosis (CF) trans membrane conductance regulator (CFTR) protein is known to be a cAMP – dependent Cl- channel. CF patients (with mutant CFTR proteins) show reduced Cl- permeability and as a result exhibit elevated Cl- level in sweat. To prove this, CFTR proteins (both wild type and mutant) are inserted in a model membrane (liposome) and Cl- transport is followed with radioactive Cl- . It is known that topology of CFTR in membrane is important for its function. Despite no proteolytic degradation of denaturation of CFTR proteins, wild type CFTR failed to transport Cl- in liposome.
Which of the following is the correct explanation of this ?


1). CFTR protein gets mutated during insertion in liposomes
2) CFTR protein loses affinity with Cl- ions
3) CFTR protein gets wrongly inserted in liposomes
4) CFTR protein loses channel forming property in liposomes

Ans.   3