Molecular Biology MCQs Set 2
26. The first step in cloning a gene is to:
A) insert a plasmid into a bacterium B) isolate the DNA from the organism that contains the desired gene C) plate cells on agar D) treat plasmids with restriction enzymes
Correct Answer: B) isolate the DNA from the organism that contains the desired gene
Explanation: Before you can manipulate or move a gene, you must first isolate the genomic DNA from the donor organism. Once isolated, the DNA can then be digested with restriction enzymes to produce the specific fragment you wish to clone.
27. Plasmids are put into bacterial cells by:
A) restriction enzymes B) DNA ligase C) binding of cohesive sticky ends D) transformation
Correct Answer: D) transformation
Explanation: Transformation is the process by which a bacterial cell takes up foreign DNA (like a plasmid) from its environment. This is often induced in the lab using heat shock or electroporation to make the cells “competent.”
28. How many different amino acids are found in naturally-occurring proteins?
A) 20 B) 16 C) 24 D) 64
Correct Answer: A) 20
Explanation: There are 20 standard amino acids that are encoded by the universal genetic code. These building blocks are arranged in various sequences to form the vast array of proteins found in living organisms.
29. Which of the following correctly describes chromatin?
A) The complex of DNA and protein from which chromosomes are composed. B) The total genetic content of a cell. C) The proteins that give structural support to a chromosome. D) Unpacked DNA; the form in which DNA exists when it is not tightly packed into chromosomes.
Correct Answer: A) The complex of DNA and protein from which chromosomes are composed.
Explanation: Chromatin consists of DNA wound around specialized proteins called histones. This complex allows long DNA molecules to be efficiently packaged into the cell nucleus.
30. Which of the following is a common feature of all cells?
A) Intracellular organelles. B) A lipid cell membrane. C) A nuclear membrane. D) An RNA genome.
Correct Answer: B) A lipid cell membrane.
Explanation: All living cells, whether prokaryotic or eukaryotic, are enclosed by a plasma membrane composed of a lipid bilayer. This membrane acts as a selective barrier between the cell and its environment.
31. Which of the following is a typical feature of viruses?
A) 3,000 – 4,000 genes. B) A genome that may be single or double-stranded DNA or RNA. C) The ability to replicate independently. D) The ability to synthesize ATP.
Correct Answer: B) A genome that may be single or double-stranded DNA or RNA.
Explanation: Viruses are highly diverse in their genetic material; they can have genomes made of DNA or RNA, which can be single-stranded or double-stranded. They cannot replicate or produce energy (ATP) without a host cell.
32. What is the main function of the smooth endoplasmic reticulum (SER)?
A) It generates energy to drive other biochemical processes. B) It is the site of the modification of proteins following their translation from RNA. C) It synthesises proteins using RNA as a template. D) It is the site of lipid synthesis and detoxification.
Correct Answer: D) It is the site of lipid synthesis and detoxification.
Explanation: (Note: The provided option D in the text was “destruction,” but scientifically, the SER is primarily responsible for lipid synthesis, carbohydrate metabolism, and the detoxification of drugs and poisons.)
33. Which of the following is a purine?
A) Adenine B) Thymine C) Uracil D) Cytosine
Correct Answer: A) Adenine
Explanation: The nitrogenous bases are divided into two groups: Purines (Adenine and Guanine), which have a double-ring structure, and Pyrimidines (Cytosine, Thymine, and Uracil), which have a single-ring structure.
34. Which of the following replaces thymine in RNA?
A) Adenine B) Guanine C) Uracil D) Cytosine
Correct Answer: C) Uracil
Explanation: In RNA, the pyrimidine Uracil (U) is used instead of Thymine (T). Uracil pairs with Adenine during transcription and in RNA-RNA interactions.
35. Which reaction in DNA replication is catalysed by DNA ligase?
A) Addition of new nucleotides to the lagging strand. B) Addition of new nucleotides to the leading strand. C) Base pairing of the template and the newly formed DNA strand. D) Formation of a phosphodiester bond between the 3′-OH of one Okazaki fragment and the 5′-phosphate of the next on the lagging strand.
Correct Answer: D) Formation of a phosphodiester bond between the 3′-OH of one Okazaki fragment and the 5′-phosphate of the next on the lagging strand.
Explanation: DNA ligase acts as “molecular glue.” It seals the nicks in the sugar-phosphate backbone by forming a phosphodiester bond, which is essential for joining Okazaki fragments on the lagging strand.
36. During which phase of the cell cycle is DNA replicated?
A) G1 phase. B) S phase. C) G2 phase. D) M phase.
Correct Answer: B) S phase.
Explanation: The S phase (Synthesis phase) of the cell cycle is specifically dedicated to DNA replication, ensuring that each daughter cell receives a complete set of genetic instructions.
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37. What would be the effect on the primary structure of a protein if a single base was deleted from an mRNA transcript?
A) No effect. B) A single amino acid residue is changed. C) A complete change in amino acid sequence from the point of the deletion. D) A premature termination of the chain at the point of mutation.
Correct Answer: C) A complete change in amino acid sequence from the point of the deletion.
Explanation: This is known as a frameshift mutation. Since mRNA is read in triplets (codons), deleting one base shifts the “reading frame” for every subsequent codon, typically resulting in a completely different and non-functional protein.
38. Base substitutions:
A) May result in nonsense mutations B) Can affect splicing C) Are always pathogenic D) Can affect gene expression
Correct Answer: A) May result in nonsense mutations
Explanation: A base substitution (point mutation) can change a codon to a “Stop” codon, which is called a nonsense mutation. This leads to the premature termination of protein synthesis.
39. A nucleotide contains:
A) Sugar, nitrogen base B) Sugar, a nitrogen-containing base and a phosphate molecule C) Monomer fat and polysaccharide D) Sugar, glycerol and phosphate
Correct Answer: B) Sugar, a nitrogen-containing base and a phosphate molecule
Explanation: A nucleotide is the basic building block of nucleic acids (DNA and RNA). It is composed of three components: a pentose sugar, a nitrogenous base, and at least one phosphate group.
40. The sugar found in RNA is:
A) Galactose B) Fructose C) Ribose D) Deoxyribose
Correct Answer: C) Ribose
Explanation: RNA (Ribonucleic Acid) contains the pentose sugar Ribose, which has a hydroxyl (-OH) group on the 2′ carbon.
41. The sugar found in DNA is:
A) Galactose B) Fructose C) Ribose D) Deoxyribose
Correct Answer: D) Deoxyribose
Explanation: DNA (Deoxyribonucleic Acid) contains Deoxyribose, which lacks one oxygen atom at the 2′ carbon compared to ribose.
42. Deoxyribose is a:
A) pentose sugar B) hexose sugar C) Pyrimidine of DNA D) Nitrogenous base
Correct Answer: A) pentose sugar
Explanation: Both ribose and deoxyribose are pentose sugars, meaning they contain a five-carbon ring structure.
43. Contain an anti-codon:
A) tRNA B) mRNA C) rRNA D) cDNA
Correct Answer: A) tRNA
Explanation: tRNA (transfer RNA) molecules have an anti-codon loop that base-pairs with the complementary codon on the mRNA, ensuring the correct amino acid is added to the growing polypeptide chain.
44. Which of the following is not a requirement for protein synthesis?
A) Ribosome B) Peptidyl transferase C) Spliceosome D) Amino acyl-tRNA synthetase
Correct Answer: C) Spliceosome
Explanation: The spliceosome is involved in RNA processing (removing introns) in the nucleus. Protein synthesis (translation) occurs in the cytoplasm and requires ribosomes, enzymes, and tRNA, but not the spliceosome.
45. The enzyme required for transcription is:
A) Restriction enzymes B) DNA polymerase C) RNA polymerase D) RNAse
Correct Answer: C) RNA polymerase
Explanation: RNA polymerase is the primary enzyme that reads the DNA template and assembles the complementary RNA strand during transcription.
46. A restriction enzyme capable of making an internal cut in DNA is a:
A) Restriction exonuclease B) Restriction endonuclease C) RNA polymerase D) RNAse
Correct Answer: B) Restriction endonuclease
Explanation: Endonucleases cut DNA at specific sequences within the molecule, whereas exonucleases remove nucleotides one by one from the ends of the DNA strand.
47. Single-stranded unpaired extensions formed by a restriction enzyme upon cleavage are called:
A) blunt end B) sticky end C) Flush end D) none of these
Correct Answer: B) sticky end
Explanation: Sticky ends are overhangs of single-stranded DNA that can easily hydrogen-bond with complementary sequences, making them very useful for creating recombinant DNA.
48. All of the following are thermostable polymerases except:
A) Taq polymerase B) Vent polymerase C) DNA polymerase III D) Pfu polymerase
Correct Answer: C) DNA polymerase III
Explanation: DNA polymerase III is the primary replication enzyme in E. coli and is not heat-stable. The other three are derived from extremophiles and are used in PCR because they can survive high temperatures.
49. The first step in PCR is:
A) Denaturation B) annealing C) primer extension D) none of these
Correct Answer: A) Denaturation
Explanation: Denaturation involves heating the DNA (typically to 94-98°C) to separate the double-stranded template into two single strands by breaking the hydrogen bonds between bases.
50. The process of binding of a primer to a denatured DNA strand is called:
A) Denaturation B) annealing C) renaturation D) none of these
Correct Answer: B) annealing
Explanation: Annealing occurs when the temperature is lowered (usually to 50-65°C), allowing synthetic DNA primers to bind to their complementary sequences on the single-stranded DNA template.