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1.5 DNA Structure β Test 1
Q1. In a DNA double helix, guanine and cytosine are held together by:β Hydrogen bonds (three)
Q2. Which statement represents Chargaff's rule?β A = T and C = G (purines = pyrimidines)
Q3. An organism's DNA contains 20% adenine. Its G+C content is:β 60%
Q4. If G is 20% of a DNA sample, the molar amount of T is:β 30%
Q5. A DNA sample shows A=40, T=22, G=21, C=17 (mole ratios). The most appropriate conclusion is:β It is single-stranded
Q6. Which DNA does NOT obey Chargaff's rule (A=T, G=C)?β ΟX174 and M13 (single-stranded)
Q7. The WatsonβCrick model corresponds to which DNA conformation?β B-DNA
Q8. Which DNA conformation is left-handed?β Z-DNA
Q9. Short stretches of Z-DNA have approximately how many base pairs per helical turn?β 12
Q10. In Z-DNA, the glycosidic bond conformation of the bases is:β Anti for pyrimidines, syn for purines
Q11. Increasing temperature of double-stranded DNA produces a:β Hyperchromic shift (increased Aβββ)
Q12. Why is Tm of DNA monitored at 260 nm?β Single-stranded DNA absorbs more UV than double-stranded
Q13. Adding salt (e.g. NaCl) to a DNA solution changes its melting temperature by:β Increasing Tm
Q14. Which DNA duplex would have the highest melting temperature?β A sequence with the most Gβ‘C pairs
Q15. Two 1000-bp duplexes: molecule 1 has 20% (A+T), molecule 2 has 60% (A+T). Which has higher Tm, and how many C residues in molecule 2?β Molecule 1; 200 C
Q16. DNA is NOT hydrolysed by alkali whereas RNA is, because RNA has:β A 2'-OH group on its ribose
Q17. Hoogsteen hydrogen bonding is characteristically present in:β Triplex DNA
Q18. The spontaneous loss of purine bases from DNA (depurination) results from breaking the:β N-glycosidic bond
Q19. A tautomeric shift causing substitution of one purine for another purine (or pyrimidine for pyrimidine) is called a:β Transition
Q20. DNA fingerprinting (using minisatellite VNTRs) was developed by:β Alec Jeffreys
Q21. Rosalind Franklin's X-ray diffraction image crucial to the DNA model was:β Photo 51
Q22. The triple-helix model of DNA (later shown incorrect) was proposed by:β Linus Pauling
Q23. Microsatellites are:β Short tandem repeats varying in copy number among individuals
Q24. DNA is separated in cesium chloride (CsCl) ultracentrifugation on the basis of its:β Buoyant density
Q25. Adding alcohol (e.g. ethanol) to an aqueous DNA solution destabilises the duplex because it interferes with:β Hydrophobic/van der Waals (base-stacking) interactions
Q26. Most stable: heating two complementary single strands to 368 K then slowly cooling to 278 K yields:β Double-stranded DNA (reannealed)
Q27. Maximal UV absorbance (260 nm) is shown by:β dsDNA heated to 90Β°C (denatured)
Q28. Increasing the pH well above neutrality (alkaline) denatures DNA most readily because it removes susceptible protons. Denaturation probability is highest at:β pH 9.0 (and above)
Q29. In terms of thermal stability, which order is correct?β dsDNA > dsRNA > DNAβRNA hybrid > ssDNA
Q30. A common triplet-repeat genetic disorder with >1000 CTG repeats is:β Myotonic dystrophy
1.5 DNA Structure β Test 2
Q31. If a single phosphodiester bond in one strand of a double-stranded circular DNA is broken, the DNA is:β Nicked
Q32. On an agarose gel, which form of a plasmid migrates FASTEST?β Supercoiled DNA
Q33. Which enzyme can change the linking number of a covalently closed circular DNA?β Gyrase (a topoisomerase)
Q34. Which topoisomerase type can INTRODUCE negative supercoils (using ATP)?β Type II (DNA gyrase)
Q35. A relaxed circular DNA of 10 turns at 10.5 bp/turn (105 bp, sealed) has a linking number of about:β 10
Q36. For supercoiled DNA, the relationship between linking number (Lk), twist (Tw) and writhe (Wr) is:β Lk = Tw + Wr
Q37. A plasmid with Lk 200, Tw 200, Wr 0 is re-isolated with Wr = β5 (same molecular weight). Its new Lk and Tw are:β Lk 195, Tw 200
Q38. The number of base pairs per helical turn in B-DNA is approximately:β 10
Q39. A typical gene of DNA molecular weight ~1 million represents about how many helical turns? (MW per bp β 660; 10 bp/turn)β ~150
Q40. A protein of 500 amino acids needs a coding sequence of at least how many base pairs?β 1500
Q41. In a 238 nm length of B-DNA (pitch 3.4 nm = 10 bp), the number of base pairs is:β 140
Q42. Given MW per nucleotide pair = 650 and rise per bp = 3.4 Γ
, a DNA of MW 120Γ10βΆ has length about:β 6.3Γ10β΅ Γ
Q43. E. coli circular DNA (4.6Γ10βΆ bp) cut by NotI (8-bp recognition site, GCGGCCGC) yields approximately how many fragments? (random sequence, equal bases)β ~70
Q44. Electrophoretic mobility shift assay (EMSA / gel-shift) detects:β DNAβprotein (nucleoprotein) interactions
Q45. Far-western blotting is used to detect:β Proteinβprotein interactions
Q46. In a nitrocellulose filter-binding assay, which DNA form is NOT retained on the filter?β Double-stranded DNA with blunt ends
Q47. Ethidium bromide visualises DNA because it:β Intercalates between base pairs and fluoresces under UV
Q48. Which molecule binds in the MAJOR groove of DNA?β Methyl green
Q49. The most common mechanism by which a protein recognises a specific DNA sequence is:β Insertion of an Ξ±-helix into the major groove
Q50. The lac repressor binds DNA using which structural motif?β Helix-turn-helix
Q51. MgClβ is included in a PCR because MgΒ²βΊ:β Is an essential cofactor that promotes Taq polymerase activity
Q52. Amplifying an 8-kb gene from 10 ng template for 10 PCR cycles gives roughly:β ~10 ng
Q53. Which enzyme hydrolyses INTERNAL phosphodiester bonds within a polynucleotide chain?β Endonuclease
Q54. DNA ligase forms a phosphodiester bond between nucleotides bearing:β 5'-phosphate and 3'-OH
Q55. The Holliday junction is:β A four-stranded crossover intermediate of recombination
Q56. Which solvent treatment selectively separates supercoiled plasmid DNA from nicked/linear/ssDNA?β Buffer-saturated (alkaline) phenol
Q57. Which statement about Z-DNA is INCORRECT?β Once formed it is always more stable and permanent than B-DNA
Q58. The number of major and minor grooves in 100 bp of B-DNA (10 bp/turn) is approximately:β 10 and 10
Q59. ΟX174 has base composition 25% A, 33% T, 24% G, 18% C. The best explanation is that its genome is:β Single-stranded DNA