Mock Test 7

Mock Test 07: Advanced Biochemistry & Mol Bio

60:00

1. A circular double-stranded DNA has 4200 base pairs. If it is relaxed, what is its Linking Number (Lk)? (Assume 10.5 bp/turn) A) 400 B) 420 C) 4200 D) 0

Solution: A. Lk = Total base pairs / bp per turn = 4200 / 10.5 = 400.

2. Cyanogen bromide (CNBr) specifically cleaves peptide bonds at the carboxyl side of which amino acid? A) Tryptophan B) Arginine C) Methionine D) Lysine

Solution: C. CNBr specifically cleaves after Methionine residues.

3. In a Dixon plot (1/v vs [I]) for a competitive inhibitor, the lines for different substrate concentrations intersect at: A) The x-axis (-Ki) B) The y-axis (1/Vmax) C) Above the x-axis D) Below the x-axis

Solution: C. In competitive inhibition, the lines intersect above the x-axis at [I] = -Ki and 1/v = 1/Vmax.

4. The MWC (Monod-Wyman-Changeux) model of allostery assumes that: A) Subunits undergo sequential conformational changes B) All subunits are either in the T state or the R state simultaneously C) Binding of substrate decreases affinity for the next substrate D) The protein exists as a monomer

Solution: B. The MWC (Symmetry) model postulates that all subunits change conformation simultaneously (no hybrid T/R states).

5. In the Tryptophan operon attenuation mechanism, which regions of the mRNA form the terminator hairpin? A) Regions 1 and 2 B) Regions 2 and 3 C) Regions 3 and 4 D) Regions 1 and 4

Solution: C. High Trp levels cause the ribosome to cover region 2, allowing regions 3 and 4 to form a Rho-independent terminator hairpin.

6. Which ionophore acts as a mobile carrier specifically for Potassium (K+) ions, collapsing the membrane potential? A) Gramicidin B) Valinomycin C) Nigericin D) Dinitrophenol (DNP)

Solution: B. Valinomycin is a highly specific K+ mobile carrier.

7. During apoptosis, the release of Cytochrome c from mitochondria leads to the formation of the apoptosome by binding to: A) Caspase-8 B) Apaf-1 C) Bcl-2 D) Bax

Solution: B. Cytochrome c binds to Apaf-1, which oligomerizes to form the apoptosome, recruiting Caspase-9.

8. The unique step in gluconeogenesis that bypasses Pyruvate Kinase requires two enzymes. These are: A) Pyruvate carboxylase and PEP carboxykinase B) Pyruvate dehydrogenase and Pyruvate carboxylase C) PEP carboxykinase and Enolase D) Malic enzyme and Pyruvate kinase

Solution: A. Pyruvate is first converted to Oxaloacetate (by Pyruvate carboxylase) and then to PEP (by PEPCK).

9. In eukaryotic DNA replication, which polymerase is primarily responsible for synthesizing the leading strand? A) Polymerase Alpha B) Polymerase Delta C) Polymerase Epsilon D) Polymerase Gamma

Solution: C. Pol Epsilon primarily synthesizes the leading strand, while Pol Delta synthesizes the lagging strand.

10. The 'Levinthal Paradox' in protein chemistry highlights that: A) Proteins cannot fold without chaperones B) Random search for the native conformation would take longer than the age of the universe C) Hydrophobic collapse is the only folding mechanism D) All proteins form amyloid fibrils

Solution: B. Levinthal pointed out that proteins must fold via directed pathways, not random conformational searches.

11. Neoschizomers are restriction enzymes that: A) Recognize the same sequence and cut at the same position B) Recognize the same sequence but cut at different positions C) Recognize different sequences but produce the same sticky ends D) Only cut single-stranded DNA

Solution: B. Isoschizomers recognize and cut the same sequence identically. Neoschizomers recognize the same sequence but cleave differently.

12. The committed, rate-limiting step in de novo Purine biosynthesis is catalyzed by: A) Ribonucleotide reductase B) PRPP synthetase C) Glutamine-PRPP amidotransferase D) Adenylosuccinate synthetase

Solution: C. This enzyme adds the first nitrogen to PRPP, committing it to purine synthesis.

13. In the Ramachandran plot, a polypeptide strictly composed of D-amino acids would show allowed regions in: A) The exact same regions as L-amino acids B) The completely inverted (mirror image) regions compared to L-amino acids C) Only the top right quadrant D) No regions, as D-amino acids cannot form secondary structures

Solution: B. Because D-amino acids are enantiomers of L-amino acids, their allowed phi/psi angles are inverted.

14. N-acetylglutamate (NAG) is an essential allosteric activator for which Urea cycle enzyme? A) Ornithine transcarbamylase B) Carbamoyl phosphate synthetase I (CPS I) C) Argininosuccinate synthetase D) Arginase

Solution: B. CPS I requires NAG for its activity, serving as the primary regulatory mechanism for the urea cycle.

15. In GPCR signaling, the Gq alpha subunit specifically activates: A) Adenylyl cyclase B) Phosphodiesterase C) Phospholipase C (PLC) D) Protein Kinase A (PKA)

Solution: C. Gq activates PLC, which cleaves PIP2 into IP3 and DAG.

16. Nitrogen fixation by the Nitrogenase complex requires the hydrolysis of how many ATP molecules per mole of N2 reduced? A) 8 B) 12 C) 16 D) 32

Solution: C. The biological reduction of one molecule of N2 to 2 NH3 requires 8 electrons and 16 ATP.

17. Group II Introns splice via a mechanism most similar to: A) Spliceosome-mediated splicing of nuclear mRNA B) tRNA splicing by endonucleases C) Group I intron autocatalytic splicing D) Protein splicing (Inteins)

Solution: A. Both Group II introns and nuclear spliceosomes use an internal branch-point Adenine to form a lariat structure.

18. The Retinoblastoma (Rb) protein acts as a tumor suppressor by binding to and inhibiting which transcription factor? A) p53 B) E2F C) Myc D) AP-1

Solution: B. Unphosphorylated Rb binds E2F, halting the cell cycle at G1. Phosphorylation by CDKs releases E2F.

19. In V(D)J recombination of the immune system, the RAG1 and RAG2 proteins recognize: A) Promoter regions B) Recombination Signal Sequences (RSS) C) Centromeres D) Telomeres

Solution: B. RAG complexes recognize the specific heptamer-spacer-nonamer RSS flanking the V, D, and J segments.

20. Which of the following statements about FRET (Förster Resonance Energy Transfer) is true? A) Efficiency is inversely proportional to the 2nd power of the distance B) Efficiency is inversely proportional to the 6th power of the distance C) It requires emission spectra of both fluorophores to be identical D) It operates via emission and re-absorption of a photon

Solution: B. FRET efficiency drops off at 1/R^6, making it an excellent "molecular ruler" for 1-10 nm distances.

21. In Real-Time PCR, a TaqMan probe produces a fluorescent signal due to the 5'-3' exonuclease activity of: A) Reverse Transcriptase B) RNase H C) Taq Polymerase D) DNA Ligase

Solution: C. Taq Polymerase degrades the probe as it extends the primer, separating the fluorophore from the quencher.

22. The "Wobble" base Inosine (I) in the tRNA anticodon can pair with which bases in the mRNA codon? A) U, C, and A B) A and G C) U and G D) C and G

Solution: A. Inosine at the 1st position of the anticodon can form stable base pairs with U, C, or A at the 3rd position of the codon.

23. In MALDI-TOF Mass Spectrometry, "TOF" stands for Time-Of-Flight. The time taken for an ion to reach the detector is proportional to: A) Charge-to-mass ratio (z/m) B) Square root of mass-to-charge ratio (m/z)^(1/2) C) Square of mass-to-charge ratio (m/z)^2 D) Its absolute molecular mass only

Solution: B. Time of flight is directly proportional to the square root of m/z.

24. What is the major role of the snRNP U1 in eukaryotic mRNA splicing? A) Binds to the branch point sequence B) Binds to the 5' splice site C) Binds to the 3' splice site D) Catalyzes the nucleophilic attack

Solution: B. U1 recognizes and binds the 5' splice site, initiating spliceosome assembly.

25. A population is in Hardy-Weinberg equilibrium. If the frequency of a recessive allele (q) is 0.4, what is the frequency of heterozygotes? A) 0.16 B) 0.36 C) 0.48 D) 0.84

Solution: C. If q = 0.4, then p = 0.6. Heterozygotes (2pq) = 2 * 0.6 * 0.4 = 0.48.

26. Which of the following components is highly enriched in "Lipid Rafts" of plasma membranes? A) Phosphatidylserine B) Cholesterol and Sphingolipids C) Cardiolipin D) Unsaturated fatty acids

Solution: B. Lipid rafts are microdomains enriched in cholesterol, sphingolipids, and GPI-anchored proteins.

27. Which of the following RNA species is transcribed by RNA Polymerase III in eukaryotes? A) 28S rRNA B) mRNA C) tRNA and 5S rRNA D) 18S rRNA

Solution: C. Pol I transcribes large rRNAs (28S, 18S, 5.8S), Pol II transcribes mRNA, and Pol III transcribes tRNA and 5S rRNA.

28. Which modification targets a protein for degradation by the 26S Proteasome? A) Phosphorylation B) Poly-ubiquitination C) Sumoylation D) Acetylation

Solution: B. The addition of a poly-ubiquitin chain (usually linked via Lys-48) acts as the "kiss of death" for proteins.

29. Assuming Michaelis-Menten kinetics, what is the catalytic efficiency of an enzyme? A) Vmax / Km B) kcat * Km C) kcat / Km D) Km / Vmax

Solution: C. The ratio kcat/Km is a measure of catalytic efficiency. An enzyme is said to achieve catalytic perfection when this value approaches the diffusion limit (10^8 - 10^9 M-1 s-1).

30. During translation, the energy for translocation of the ribosome by one codon is provided by the hydrolysis of: A) ATP by EF-Tu B) GTP by EF-G C) GTP by IF-2 D) ATP by Peptidyl transferase

Solution: B. Elongation Factor G (EF-G in bacteria, eEF2 in eukaryotes) uses GTP hydrolysis to translocate the ribosome.