Q.1 Which enzyme is responsible for unwinding the DNA helix during replication?
DNA ligase
DNA helicase
DNA polymerase I
Topoisomerase
Explanation - DNA helicase unwinds the double-stranded DNA at the replication fork, allowing polymerases to access single strands for synthesis.
Correct answer is: DNA helicase
Q.2 What is the role of DNA ligase in DNA replication?
Proofreading nucleotides
Joining Okazaki fragments
Unwinding DNA
Synthesizing RNA primers
Explanation - DNA ligase seals the nicks between Okazaki fragments on the lagging strand by forming phosphodiester bonds.
Correct answer is: Joining Okazaki fragments
Q.3 In prokaryotes, which RNA polymerase subunit recognizes the promoter region?
Alpha
Beta
Beta’
Sigma
Explanation - The sigma subunit of RNA polymerase binds to specific promoter sequences, guiding transcription initiation.
Correct answer is: Sigma
Q.4 Which of the following is a stop codon?
AUG
UAA
GUG
CAG
Explanation - UAA, UAG, and UGA are stop codons that terminate translation by not encoding any amino acid.
Correct answer is: UAA
Q.5 Which molecule acts as the adapter in protein synthesis?
mRNA
tRNA
rRNA
snRNA
Explanation - tRNA carries specific amino acids and pairs with codons on mRNA, acting as the adapter between nucleic acid and protein.
Correct answer is: tRNA
Q.6 The lac operon is primarily regulated by which mechanism?
Repression and induction
Methylation
Transposition
Attenuation
Explanation - The lac operon is repressed by the lac repressor in absence of lactose and induced when lactose is present.
Correct answer is: Repression and induction
Q.7 Which enzyme synthesizes RNA primers during replication?
Helicase
Primase
Ligase
Topoisomerase
Explanation - Primase lays down short RNA primers to provide free 3’-OH groups for DNA polymerase to extend.
Correct answer is: Primase
Q.8 Which process increases genetic variability in bacteria?
Binary fission
Transcription
Translation
Horizontal gene transfer
Explanation - Horizontal gene transfer methods like transformation, transduction, and conjugation introduce new genetic material into bacteria.
Correct answer is: Horizontal gene transfer
Q.9 Which bacterial structure is essential for conjugation?
Flagella
Fimbriae
Sex pilus
Capsule
Explanation - The sex pilus is required for transfer of plasmid DNA during bacterial conjugation.
Correct answer is: Sex pilus
Q.10 Which type of mutation changes a codon to a stop codon?
Missense
Silent
Nonsense
Frameshift
Explanation - A nonsense mutation converts a codon into a stop codon, leading to premature termination of translation.
Correct answer is: Nonsense
Q.11 Which of the following best describes transposons?
Plasmids
Mobile genetic elements
Restriction enzymes
Promoter regions
Explanation - Transposons, or jumping genes, can move from one site to another within the genome, altering gene expression.
Correct answer is: Mobile genetic elements
Q.12 The Shine-Dalgarno sequence is found in:
DNA promoter
mRNA
tRNA anticodon loop
rRNA
Explanation - The Shine-Dalgarno sequence in bacterial mRNA helps ribosome binding for translation initiation.
Correct answer is: mRNA
Q.13 Which enzyme removes RNA primers during DNA replication in bacteria?
DNA polymerase I
DNA polymerase III
Ligase
Helicase
Explanation - DNA polymerase I has exonuclease activity that removes RNA primers and fills in the gaps with DNA nucleotides.
Correct answer is: DNA polymerase I
Q.14 In bacterial transcription, termination by rho protein is an example of:
Intrinsic termination
Rho-dependent termination
Alternative splicing
Post-transcriptional modification
Explanation - Rho protein binds to RNA and facilitates dissociation of RNA polymerase from DNA during transcription termination.
Correct answer is: Rho-dependent termination
Q.15 What is an operon?
A DNA repair enzyme
A cluster of genes under one promoter
A plasmid
A transposon
Explanation - An operon is a functional unit of DNA containing multiple genes regulated together by a single promoter and operator.
Correct answer is: A cluster of genes under one promoter
Q.16 Which repair mechanism corrects thymine dimers?
Mismatch repair
Photoreactivation
Base excision repair
Nucleotide excision repair
Explanation - Photolyase enzymes directly reverse UV-induced thymine dimers in photoreactivation repair.
Correct answer is: Photoreactivation
Q.17 Which type of RNA has catalytic activity?
tRNA
mRNA
rRNA
snRNA
Explanation - The rRNA component of ribosomes acts as a ribozyme, catalyzing peptide bond formation during translation.
Correct answer is: rRNA
Q.18 Which bacterial genetic transfer method requires a bacteriophage?
Transformation
Conjugation
Transduction
Recombination
Explanation - In transduction, bacteriophages carry DNA from one bacterial cell to another, enabling horizontal gene transfer.
Correct answer is: Transduction
Q.19 Which part of a plasmid ensures autonomous replication?
Origin of replication
Antibiotic resistance gene
Promoter
Operator
Explanation - The origin of replication is the DNA sequence where plasmid replication is initiated independently of chromosomal DNA.
Correct answer is: Origin of replication
Q.20 Which amino acid does the start codon AUG code for?
Methionine
Valine
Glycine
Proline
Explanation - AUG is the universal start codon that codes for methionine, initiating translation.
Correct answer is: Methionine
Q.21 Which DNA replication model is followed in prokaryotes?
Conservative
Dispersive
Semiconservative
Rolling circle
Explanation - In semiconservative replication, each new DNA molecule consists of one parental and one newly synthesized strand.
Correct answer is: Semiconservative
Q.22 Which enzyme relieves supercoiling ahead of the replication fork?
Helicase
Ligase
Topoisomerase
Primase
Explanation - Topoisomerases relieve torsional strain by cutting and rejoining DNA strands during replication.
Correct answer is: Topoisomerase
Q.23 Which regulatory protein binds to the operator region of the lac operon?
CAP protein
Lac repressor
Sigma factor
RNA polymerase
Explanation - The lac repressor binds to the operator region, blocking transcription in the absence of lactose.
Correct answer is: Lac repressor
Q.24 Which base pairing is correct in DNA?
A–C and G–T
A–T and G–C
A–G and C–T
A–U and G–C
Explanation - Adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds.
Correct answer is: A–T and G–C
Q.25 Which of the following describes an inducible operon?
Always active
Turned on in presence of substrate
Turned off by product
Controlled only by repressors
Explanation - Inducible operons, such as the lac operon, are activated when the substrate is available to metabolize.
Correct answer is: Turned on in presence of substrate