AIIMS PG 2011 BIO.pdf

7/28/2019 AIIMS PG 2011 BIO.pdf

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AIIMS PG 2011: SUBJECT: BIOCHEMISTRY.

1. Which of the following is not degraded by colonic flora?a. Pectinb. Ligninc. Starchd. Glucose

2. CAP in LAC operon is:a. Positive regulatorb. Negative regulatorc. Attenuationd. Constitutive expression

3. Following modification occurs in Gs subunit which leads to:a. ADP ribosylationb. ATP-ADP transferc. Phosphorylationd. Dephosphorylation

4. Apo-B48 AND Apo-B100 is synthesized from the same Mrna,the difference between them is due to:

a. RNA Splicingb. Allelic exclusionc. Deamination of cytidine to uridined. Upstream repression

5. Among the following all are hydrophilic hormones that act oncytosolic receptors except one which is a lipophilic hormonethat acts on nuclear receptors:

a. Thyroxineb. Epinephrinec. GHd. ACTH

6. Gene duplication plays an important role in the evolution of:a. M RNAb. R RNAc. T RNAd. HN RNA


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7. Movement of protein from nucleus to cytoplasm can be seenby:

a. FISHb. FRAPc. Confocal microscopyd. Electron microscopy

8. Histone Acetylation causes:a. Increased heterochromatin formationb. Increased euchromatin formationc. Methylation of cysteined. DNA replication

9. Triplex DNA is due to:a. Hoogsteen pairingb. Palindromic sequencesc. Large number of guanosine repeatsd. Polypyramidine tracts

10. Acetyl CoA can be directly converted to all except:a. Glucoseb. Fatty acidsc. Cholesterold. Ketone bodies

11. After digation by restriction endonucleases DNA strands can

be joined again by:

a. DNA polymeraseb. DNA ligase

c. DNA topoisomerase

d. DNA gyrase

12. True about sickle cell disease are all except:a. Single nucleotide change results in change of glutamine

to valine.

b. Sticky patch is generated as a result of replacement of anon polar residue with a polar residue

c. HbS confers resistance against malaria in heterozygotes

d. RFLP results from a single base change


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ANSWERS AND EXPLANATIONS

1.B, Explanation: Lignin is a dietary fibre that cant be digested

by the G.I. enzymes or cant be fermented by the colonic flora.

But starch and pectin are not digested by the G.I enzymes but are

fermented by colonic flora. And glucose is destroyed by both ofthem easily.

2. A, Explanation: The genes of LAC operon system are Z,Y and

A that codes for subsequentially the enzymes galactosidase

(hydrolyses lactose into glucose and lactose), permease(facilitates the entry of lactose into the cell) and thiogalactoside

transacetylase (physiologic function is unknown). In the absence

of glucose, adenyl cyclase is active, so sufficient of cyclic AMP

is formed. The Cyclic AMP-CAP complex is formed that binds to

the CAP binding site, causing RNA polymerase to initiatetranscription at the promoter site. This is called Positive

regulation.

3. A, Explanation: Cholera toxin is a potent entreotoxin, that isresponsible for the virulence of Vibrio cholera. Effect of cholera

toxin leads to watery diarrhoea, the characteristic feature of

cholera.Mechanism of action of cholera toxin:

1. Cholera toxin consists of two subunits: A subunit: monomeric enzymatic activity B subunit: pentameric binding activity

2. The B subunit binds to the GM1 ganglioside, a glycolipidson the surface on the epithelial cells of the small intestine.

3. The A subunit enters into the cell.4. This leads to activation of A1 subunit of the cholera toxin.

Activated A1 subunit interacts with ADP ribosylation unit

(ARF).

5. The ARF-A1 complex is formed.6. The ARF-A1 complex catalyses ADP-ribosylation of GS

, a G protein.

7. The ADP-ribosylated GS stimulates adenyl cyclaseenzyme and high level of cyclic AMP is generated.

8. The Cyclic AMP inhibits the sodium transport system(decreases Na and Cl absorption) and activates the


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Chloride transport ( increases Cl and HCO3

secretion).9. This leads to accumulation of NaCl inside the intestinal

lumen which attracts water into the lumen causing Watery

Diarrhoea.

4. c, Explanation: Apo B-48 and Apo B-100 are synthesized

from the same m-RNA. Apo-B100 is the full length

translation of the corresponding m-RNA. While apo-B48 is

synthesized only by 48% translation of the full RNA. Thestoppage of translation occurs due to a post-transcriptional m-

RNA editing in which 2153rd codon is edited. The change is

codon CAA (cytidine) is deaminated to UAA (uridine).5.a, Explanation:

Mechanism of cytosolic recepters:

Hormones penetrate cell membrane

Combines with cytosolic receptors

Passes through nuclear membrane

Binds to specific genes on DNA

Specific m-RNA synthesis

Synthesis of specific functional proteins

Ex.- Androgens, Calcitriol, Estrogens, Glucocorticoids,Mineralocorticoids, Progesterons, Retinoic acid etc.

Mechanism of nuclear recepters:

Hormone penetrates nuclear membrane

Combines with its receptor

DNA mediated specific m-RNA synthesis

Synthesis of functional proteins


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Ex.- Thyroxine

6. a. Explanation: Gene duplication is any duplication of any

region of DNA. The newly synthesized DNA is often free

from selective pressure- that is the mutation often does not

have any deleterious effect on the host organism. So it is avery important mechanism of evolutionary events.

7. b, Explanation: FRAP (Flurorescence recovery after

photobleaching). This method is to tag a specific cellcomponent (protein, fat, carbohydrate etc) with a fluorescent

molecule, imaging the cell, photobleach a asmall portion of

the cell, then imaging the recovery of fluorescence over time.

8.b, Explanation: Histone Acetylation of the DNA promotesthe transcriptional activation the DNA that si the formation of

the euchromatin.

More details: Acetylation of the lysine residue of N-terminalof histone protein removes the positive charge on the lysine

and thereby decreases the interaction of the histone with the

negatively charged DNA. So the complexed chromatinstructure is transformed into a more relaxed structure. So

transcription factors get successfully attached to the specific

regions of DNA.

10.a, Explanation: Hoogsteen paiing allows the formation of

the triplex DNAs.- Lehninger.

Normal duplex DNAs contain Watson-Crick base pairing,

according to which Adenine pairs with Guanine by 2hydrogen bonds and Thymine pairs with Cytosine by 3

hydrogen bonds.

Hoogsteen pairing is an alternate base pairing pattern, more

commonly seen in RNA. Here also there is same pairing as

seen in Watson crick Duplex DNA model (AG and T---C).But the difference lies in the geometry that leads to formation

of a third strand giving rise ro Triplex DNA model.

11.a, Explanation: The PDH complex catabolises the

convertion of Pyruvate to Acetyl CoA. The reaction is an


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irreversible reaction. So acetyl CoA cant be converted to

glucose.

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AIIMS PG 2011 BIO.pdf