ICAR AIEEA PG Animal Biotechnology 09 July 2023 (Shift 1) Paper

Allosteric control of bacterial aspartate transcarbamoylase (ATCase):

• Adenosine triphosphate (ATP) is a purine nucleotide that signals an abundance of purines and energy; it binds to the regulatory subunits of ATCase, stabilizing the high-affinity, catalytically active relaxed (R) state, thereby acting as a potent allosteric activator to coordinate balanced purine-to-pyrimidine pools.
• Cytidine triphosphate (CTP) is the end-product of pyrimidine synthesis that acts as a feedback allosteric inhibitor by stabilizing the low-affinity tense (T) state.
• Cytidine monophosphate (CMP) does not serve as the primary allosteric effector.
• Thiamine triphosphate (TTP) is a coenzyme form of vitamin B1 unrelated to ATCase regulation.

Enzymatic inhibition of in vitro glycolysis for blood glucose determination:

• Potassium oxalate functions solely as a calcium-chelating anticoagulant, usually combined with fluoride.
• Heparin activates antithrombin III to inhibit thrombin but does not block erythrocyte glycolysis.
• Sodium fluoride ($ ext{NaF}$) specifically acts as an enolase inhibitor by forming a fluorophosphate-magnesium complex that blocks the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate, preventing RBCs from consuming plasma glucose post-phlebotomy.
• EDTA chelates divalent calcium to prevent clotting in hematology profiles but does not halt glycolysis.

Hyperchromicity of nucleic acids upon thermal denaturation:

• Assertion (A) is true: Thermal denaturation (strand unwinding/melting) of double-stranded DNA produces an increase in absorbance at $260\text{ nm}$ by approximately $30\text{--}40\%$, termed the hyperchromic effect.
• Reason (R) is true: In native dsDNA, aromatic purine and pyrimidine ring $\pi$-electron systems are closely stacked parallel to each other inside the double helix, which quenches their UV dipole absorption. Upon denaturation, the complementary strands separate and the stacked bases unstack and become fully solvent-exposed, unshielding the transition dipoles.
• Reason (R) provides the direct physical mechanism and explanation for Assertion (A).

DNA thermal stability and base composition kinetics:

• Statement (I) is correct as a valid experimental observation where duplex D2 exhibits higher thermal resistance than D1.
• Statement (II) is correct: The thermal denaturation transition temperature (melting temperature, $T_m$) of a DNA duplex is directly proportional to its GC content (ratio of Guanine-Cytosine vs Adenine-Thymine pairs) because G-C pairs are joined by three hydrogen bonds and stronger base-stacking interactions compared to two hydrogen bonds in A-T pairs. Therefore, a higher denaturation temperature for D2 ($65^\circ\text{C}$ vs $60^\circ\text{C}$) directly indicates that D2 possesses a higher percentage of guanine (GC content) than D1.

Biochemical composition of deoxyribonucleosides:

• Adenine combines with 2'-deoxyribose to form the purine deoxyribonucleoside deoxyadenosine.
• Ribose sugar is the pentose component of ribonucleosides (RNA), not deoxyribonucleosides (which contain 2'-deoxy-D-ribose).
• Thymine combines with 2'-deoxyribose to form the canonical pyrimidine deoxyribonucleoside deoxythymidine.
• Uracil is standardly found in ribonucleosides (uridine of RNA), not standard deoxyribonucleosides of DNA.