Question 1

Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator

in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific

NOS enzymes which exist in at least three different isoforms – bNOS, eNOS, and macNOS. Each of these

isoforms differ in location and function and serve to mediate different physiological responses to NO. Some

physiological roles of NO have been demonstrated as follows:

I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200-400

nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on

brain function although its specific roles are not well established. bNOS inhibitors have been found to block the

release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to

neurodegenerative disorders such as Alzheimer’s disease.

II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO

acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to

muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO

regulates the vascular system by inhibiting platelet aggregation and adhesion.

III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+

dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- γ, leads to synthesis of large

amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a

definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the

mechanism of viral replication.

A “knock out” mouse with a mutant bNOS protein was generated by recombination techniques. The mutant

protein was identical to the wild-type protein except for the identity of amino acid 675; the mutated bNOS has

Tryptophan instead of Cysteine at position 675. Which of the following is responsible for the mutant protein?

Section: Biological Sciences

Options :

A :

Frame shift mutation

B :

Single base-pair deletion

C : Point mutation

D : Nonsense mutation

Answer: C

Question 2

If the distance between two bodies each 1 Kg is 1 meter, then the force of attraction between these two bodies

is:

Section: Physical Sciences

Options :

A :

B :

C :

D :

Answer: C

Question 3

Before birth, the rodent brain is sexually undifferentiated. It is only in the first few days following birth, during a

period referred to as the critical period, that the rodent brain differentiates along male or female lines. The

hormone testosterone plays a critical role in this development. Specifically, sexual differentiation is determined

by the presence of estradiol, an estrogen derivative of testosterone, in certain areas of the brain. Testosterone

is converted to estradiol in critical brain cells that contain the enzyme aromatase.

To study the effects of testosterone on the neonatal rodent brain, the following experiments were conducted:

The above research, combined with additional studies, concluded that testosterone has two “organizational”

effects on the male rodent brain:

Defeminization

Moderate levels of testosterone-derived estradiol during the critical period are sufficient for defeminization of

the brain. Defeminization of the rodent brain results in loss of estrogen positive feedback on LH and FSH

secretion and the ensuing loss of cyclicity, as well as loss of female sex behavior.

Masculinization

High levels of estradiol due to high levels of testosterone during the critical period results in masculinization of

the brain. Masculinization leads to the induction of male sex behavior including antagonism towards other

males and the mounting of females.

The conversion of testosterone to estradiol is what type of reaction?

Section: Biological Sciences

Options :

A : Reduction

B : Aromatization

C : Electrophilic aromatic substitution

D : Methylation

Answer: B

Question 4

Elephants have large flat feet. It helps them, having heavy weight, in walking properly on ground and

maintaining proper body posture by:

Section: Physical Sciences

Options :

A :

reducing ground friction on their feet.

B : reducing their pressure on ground.

C : increasing feet penetration into ground.

D :

none of the above.

Answer: B

Question 5

Several techniques have been developed to determine the order of a reaction. The rate of a reaction cannot be

predicted on the basis of the overall equation, but can be predicted on the basis of the rate-determining step.

For instance, the following reaction can be broken down into three steps.

A + D → F + G

Step 1

A → B + C

(slow)

Step 2

B + D → E + F

(fast)

Step 3

E + C → G

(fast)

Reaction 1

In this case, the first step in the reaction pathway is the rate-determining step. Therefore, the overall rate of the

reaction must equal the rate of the first step, k1 [A] where k1 is the rate constant for the first step. (Rate

constants of the different steps are denoted by kx

, where x is the step number.)

In some cases, it is desirable to measure the rate of a reaction in relation to only one species. In a secondorder reaction, for instance, a large excess of one species is included in the reaction vessel. Since a relatively

small amount of this large concentration is reacted, we assume that the concentration essentially remains

unchanged. Such a reaction is called a pseudo first-order reaction. A new rate constant, k', is established, equal

to the product of the rate constant of the original reaction, k, and the concentration of the species in excess.

This approach is often used to analyze enzyme activity.

In some cases, the reaction rate may be dependent on the concentration of a short-lived intermediate. This can

happen if the rate-determining step is not the first step. In this case, the concentration of the intermediate must

be derived from the equilibrium constant of the preceding step. For redox reactions, the equilibrium can be

correlated with the voltage produced by two half-cells by means of the Nernst equation. This equation states

that at any given moment:

Equation 1

When

a A + b B → c C + d D

Reaction 2

Note: R = 8.314 J/K·mol; F = 9.6485 × 104 C/mol.)

If Step 2 above were the rate-determining step of Reaction 1, which of the following equations would correctly

define the rate?

Section: Physical Sciences

Options :

A :

B :

C :

D :

Answer: C