Q.1 What is the primary function of the ribosome during translation?
Replicate DNA
Synthesize RNA
Assemble amino acids into proteins
Transport mRNA out of the nucleus
Explanation - Ribosomes read mRNA codons and catalyze the formation of peptide bonds, linking amino acids into a polypeptide chain.
Correct answer is: Assemble amino acids into proteins
Q.2 Which of the following RNA molecules carries the genetic code from DNA to the ribosome?
tRNA
rRNA
mRNA
snRNA
Explanation - Messenger RNA (mRNA) is transcribed from DNA and transports the codon sequence to the ribosome for translation.
Correct answer is: mRNA
Q.3 How many nucleotides make up a codon?
1
2
3
4
Explanation - Each codon consists of three nucleotides, which together encode a single amino acid or a stop signal.
Correct answer is: 3
Q.4 Which amino acid is coded by the codon AUG?
Methionine
Tryptophan
Phenylalanine
Leucine
Explanation - AUG is the start codon and codes for methionine, initiating protein synthesis.
Correct answer is: Methionine
Q.5 What is the role of transfer RNA (tRNA) in translation?
Catalyzes peptide bond formation
Carries amino acids to the ribosome
Synthesizes mRNA
Helps splice introns
Explanation - tRNA molecules have an anticodon that pairs with mRNA codons and a specific attached amino acid.
Correct answer is: Carries amino acids to the ribosome
Q.6 Which of the following is NOT a stop codon?
UAA
UAG
UGA
UAC
Explanation - UAA, UAG, and UGA are stop codons; UAC codes for the amino acid tyrosine.
Correct answer is: UAC
Q.7 During translation, what is the order of the three stages?
Elongation, Initiation, Termination
Initiation, Elongation, Termination
Termination, Initiation, Elongation
Elongation, Termination, Initiation
Explanation - Translation proceeds through initiation (ribosome assembly), elongation (amino acid addition), and termination (release of the polypeptide).
Correct answer is: Initiation, Elongation, Termination
Q.8 In prokaryotes, where does translation typically begin relative to transcription?
After transcription is fully completed
Before transcription starts
Simultaneously with transcription
Only after the mRNA is exported from the nucleus
Explanation - Prokaryotes lack a nucleus, so ribosomes can translate mRNA while it is still being transcribed.
Correct answer is: Simultaneously with transcription
Q.9 What is the significance of the wobble hypothesis?
It explains the flexibility in base pairing at the third codon position.
It describes how ribosomes unwind DNA.
It details the mechanism of DNA replication.
It defines the start codon selection.
Explanation - The wobble hypothesis (Crick) states that the third base of a codon can form non-standard base pairs, allowing fewer tRNAs to recognize multiple codons.
Correct answer is: It explains the flexibility in base pairing at the third codon position.
Q.10 Which of the following statements about the genetic code is FALSE?
It is nearly universal across organisms.
It is ambiguous; one codon can code for multiple amino acids.
It is degenerate; multiple codons can code for the same amino acid.
It is non-overlapping.
Explanation - The genetic code is unambiguous: each codon specifies only one amino acid (or a stop signal).
Correct answer is: It is ambiguous; one codon can code for multiple amino acids.
Q.11 Which ribosomal subunit binds to the mRNA first during bacterial initiation?
50S large subunit
30S small subunit
Both subunits bind simultaneously
None; initiation uses a different ribonucleoprotein
Explanation - In bacteria, the 30S subunit, together with initiation factors and fMet-tRNA, first binds the mRNA at the Shine‑Dalgarno sequence.
Correct answer is: 30S small subunit
Q.12 What is the function of the Shine‑Dalgarno sequence in prokaryotic mRNA?
Signal termination of translation
Serve as the start codon
Align the ribosome with the start codon
Recruit RNA polymerase
Explanation - The Shine‑Dalgarno sequence upstream of the start codon pairs with a complementary region in the 16S rRNA, positioning the ribosome correctly.
Correct answer is: Align the ribosome with the start codon
Q.13 During elongation, which enzyme catalyzes peptide bond formation?
RNA polymerase
DNA ligase
Peptidyl transferase
Helicase
Explanation - Peptidyl transferase, a ribozyme activity of rRNA in the large subunit, forms peptide bonds between amino acids.
Correct answer is: Peptidyl transferase
Q.14 What is the typical length (in nucleotides) of a bacterial operon mRNA that encodes a single protein?
30–60 nucleotides
100–300 nucleotides
500–1500 nucleotides
2000–3000 nucleotides
Explanation - Most bacterial mRNAs encoding a single protein are a few hundred nucleotides long, sufficient for an average protein coding region.
Correct answer is: 500–1500 nucleotides
Q.15 Which of the following best describes a frameshift mutation?
Insertion or deletion of nucleotides in multiples of three
Substitution of a single nucleotide
Insertion or deletion of nucleotides not in multiples of three
Conversion of a codon into a stop codon
Explanation - Frameshift mutations shift the reading frame, altering downstream codon grouping and usually resulting in nonfunctional proteins.
Correct answer is: Insertion or deletion of nucleotides not in multiples of three
Q.16 Which of the following amino acids is encoded by the most codons?
Leucine
Methionine
Tryptophan
Histidine
Explanation - Leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG).
Correct answer is: Leucine
Q.17 What is the role of the eukaryotic initiation factor eIF2?
It binds the ribosomal large subunit.
It transports the initiator tRNA to the ribosome.
It unwinds mRNA secondary structures.
It signals termination.
Explanation - eIF2 forms a ternary complex with GTP and Met‑tRNAi^Met, delivering the initiator tRNA to the 40S ribosomal subunit.
Correct answer is: It transports the initiator tRNA to the ribosome.
Q.18 In eukaryotes, the 5' cap of mRNA primarily serves to:
Mark the mRNA for degradation
Facilitate ribosome binding and protect mRNA from exonucleases
Encode the start codon
Signal polyadenylation
Explanation - The 7‑methylguanosine cap is recognized by initiation factors, promoting translation and protecting the transcript.
Correct answer is: Facilitate ribosome binding and protect mRNA from exonucleases
Q.19 Which of the following best explains why the genetic code is considered 'redundant'?
Each amino acid is coded by a single codon.
Multiple codons can encode the same amino acid.
Some codons encode more than one amino acid.
All codons are used equally in genomes.
Explanation - Redundancy (degeneracy) means that several codons can specify the same amino acid, providing a buffer against mutations.
Correct answer is: Multiple codons can encode the same amino acid.
Q.20 What is the significance of the Kozak consensus sequence in eukaryotic mRNA?
It signals polyadenylation.
It is the binding site for RNA polymerase.
It enhances translation initiation at the start codon.
It marks intron-exon boundaries.
Explanation - The Kozak sequence (gccRccAUGG) surrounds the start codon and improves ribosomal recognition in eukaryotes.
Correct answer is: It enhances translation initiation at the start codon.
Q.21 During translation, what is the function of release factors (RF1, RF2) in prokaryotes?
Recruit the small ribosomal subunit
Catalyze peptide bond formation
Recognize stop codons and promote polypeptide release
Attach amino acids to tRNA
Explanation - Release factors bind stop codons in the A site and trigger hydrolysis of the bond linking the polypeptide to the tRNA.
Correct answer is: Recognize stop codons and promote polypeptide release
Q.22 Which of the following statements about mitochondrial translation is TRUE?
Mitochondria use the same genetic code as the nuclear genome.
Mitochondrial ribosomes are identical to bacterial ribosomes.
Mitochondrial mRNAs lack a 5' cap but have poly(A) tails.
Mitochondria translate proteins using the cytoplasmic ribosomes.
Explanation - Mitochondrial transcripts are typically uncapped but polyadenylated, and mitochondria have their own ribosomes distinct from both cytoplasmic and bacterial ribosomes.
Correct answer is: Mitochondrial mRNAs lack a 5' cap but have poly(A) tails.
Q.23 What is the effect of a premature stop codon introduced by a nonsense mutation?
It leads to a longer protein.
It results in a truncated, usually nonfunctional protein.
It has no effect on the protein product.
It changes a single amino acid without altering length.
Explanation - A premature stop codon halts translation early, producing a shortened protein that often loses function.
Correct answer is: It results in a truncated, usually nonfunctional protein.
Q.24 In eukaryotes, what is the primary role of the poly(A) tail?
Facilitate ribosome entry
Signal transcription termination
Enhance mRNA stability and translation efficiency
Encode the start codon
Explanation - The poly(A) tail protects mRNA from degradation and interacts with translation initiation factors to improve efficiency.
Correct answer is: Enhance mRNA stability and translation efficiency
Q.25 Which of the following best describes 'codon bias'?
Preference for certain codons over others to encode the same amino acid within a genome.
Use of different codons to encode different amino acids.
Random distribution of codons across the genome.
Mutations that convert codons into stop signals.
Explanation - Codon bias reflects the preferential use of specific synonymous codons, often correlated with tRNA abundance.
Correct answer is: Preference for certain codons over others to encode the same amino acid within a genome.
Q.26 What is the main difference between the genetic codes of most bacteria and vertebrate mitochondria?
Mitochondria use a double-stranded RNA code.
Bacterial genomes lack stop codons.
Some codons are reassigned to different amino acids in mitochondria.
Bacterial codons are read in groups of four nucleotides.
Explanation - Mitochondrial genomes have a slightly altered code; for example, AUA codes for methionine instead of isoleucine.
Correct answer is: Some codons are reassigned to different amino acids in mitochondria.
Q.27 During translation, what is the significance of the A, P, and E sites on the ribosome?
They are binding sites for DNA polymerase.
They hold tRNAs during amino acid addition: Aminoacyl (A), Peptidyl (P), Exit (E).
They are regions where mRNA is spliced.
They are the three subunits of the ribosome.
Explanation - The A site accepts incoming tRNA, the P site holds the growing peptide chain, and the E site is where deacylated tRNA exits.
Correct answer is: They hold tRNAs during amino acid addition: Aminoacyl (A), Peptidyl (P), Exit (E).
Q.28 Which of the following amino acids is coded by only one codon?
Leucine
Methionine
Serine
Valine
Explanation - Methionine is uniquely encoded by the single codon AUG, which also serves as the start codon.
Correct answer is: Methionine
Q.29 What is the effect of a point mutation that changes a codon from AAA to AAG?
It changes lysine to glutamic acid.
It changes lysine to arginine.
It does not change the amino acid (silent mutation).
It creates a premature stop codon.
Explanation - Both AAA and AAG code for lysine, so the protein sequence remains unchanged.
Correct answer is: It does not change the amino acid (silent mutation).
Q.30 Which of the following best explains why the genetic code is said to be 'non-overlapping'?
Each nucleotide is read three times.
Each nucleotide belongs to only one codon.
Codons share nucleotides with adjacent codons.
A single codon can encode multiple amino acids.
Explanation - In a non-overlapping code, nucleotides are read sequentially in groups of three without overlap.
Correct answer is: Each nucleotide belongs to only one codon.
Q.31 Which of the following is a common method for experimentally determining the translation start site of a protein?
Northern blotting
Ribosome profiling
Southern blotting
Chromatin immunoprecipitation
Explanation - Ribosome profiling maps ribosome-protected mRNA fragments, revealing the positions of active translation initiation.
Correct answer is: Ribosome profiling
Q.32 During translation, the hydrolysis of which molecule provides the energy required for peptide bond formation?
ATP
GTP
CTP
UTP
Explanation - GTP hydrolysis by elongation factors (EF‑Tu/EF‑G) fuels translocation and peptide bond formation.
Correct answer is: GTP
Q.33 What is the functional consequence of the 'leaky scanning' mechanism in eukaryotes?
Ribosomes skip the first AUG and initiate translation at downstream AUGs.
RNA polymerase skips promoter regions.
tRNA molecules bypass the A site.
Ribosomes translate introns.
Explanation - Leaky scanning allows alternative start codons downstream of the first AUG to be used, producing protein isoforms.
Correct answer is: Ribosomes skip the first AUG and initiate translation at downstream AUGs.
Q.34 Which of the following correctly pairs a codon with its corresponding amino acid in the standard genetic code?
UUU – Phenylalanine
UAA – Lysine
AUG – Tryptophan
CGG – Serine
Explanation - UUU codes for phenylalanine; UAA is a stop codon, AUG codes for methionine, CGG codes for arginine.
Correct answer is: UUU – Phenylalanine
Q.35 Which process ensures that only correctly charged tRNAs enter the ribosome during translation?
Proofreading by aminoacyl‑tRNA synthetases
Splicing of introns
RNA editing
DNA methylation
Explanation - Aminoacyl‑tRNA synthetases attach the correct amino acid to their cognate tRNA and have proofreading activity to prevent mischarging.
Correct answer is: Proofreading by aminoacyl‑tRNA synthetases
Q.36 Which of the following is true about the mitochondrial genetic code in humans?
UGA codes for tryptophan instead of a stop signal.
AUG codes for valine.
All codons are identical to the universal code.
UGG is a stop codon.
Explanation - In human mitochondria, the UGA codon encodes tryptophan rather than serving as a stop codon.
Correct answer is: UGA codes for tryptophan instead of a stop signal.
Q.37 What is the role of the elongation factor EF‑Tu in bacterial translation?
Catalyzes peptide bond formation
Delivers aminoacyl‑tRNA to the A site in a GTP‑dependent manner
Facilitates ribosome subunit dissociation
Recognizes stop codons
Explanation - EF‑Tu·GTP forms a complex with aminoacyl‑tRNA and transports it to the ribosomal A site; GTP hydrolysis releases the tRNA.
Correct answer is: Delivers aminoacyl‑tRNA to the A site in a GTP‑dependent manner
Q.38 Which of the following best explains why the genetic code is said to be 'nearly universal'?
All organisms use exactly the same codon assignments.
Only a few exceptions exist, such as in mitochondria and some protozoa.
Each species has a completely unique code.
The code changes drastically between prokaryotes and eukaryotes.
Explanation - The standard code is shared by most organisms, with limited variations in organelles and certain lineages.
Correct answer is: Only a few exceptions exist, such as in mitochondria and some protozoa.
Q.39 During translation termination in eukaryotes, which factor recognizes the stop codon?
eRF1
eIF4E
EF‑G
eIF2
Explanation - Eukaryotic release factor 1 (eRF1) binds all three stop codons and promotes hydrolysis of the peptide‑tRNA bond.
Correct answer is: eRF1
Q.40 Which type of mutation is most likely to be silent?
A point mutation in the third base of a codon
A deletion of two nucleotides
A substitution that creates a premature stop codon
A frameshift mutation
Explanation - Due to degeneracy, changes in the third position often do not alter the encoded amino acid.
Correct answer is: A point mutation in the third base of a codon
Q.41 Which of the following statements about the ribosomal RNA (rRNA) is correct?
rRNA is encoded only in the nuclear genome.
rRNA has catalytic activity and forms the peptidyl transferase center.
rRNA functions solely as a scaffold without enzymatic roles.
rRNA is translated into proteins.
Explanation - rRNA is a ribozyme; the large subunit rRNA provides the peptidyl transferase activity essential for peptide bond formation.
Correct answer is: rRNA has catalytic activity and forms the peptidyl transferase center.
Q.42 What is the most common cause of a missense mutation?
Insertion of a stop codon
Deletion of a codon
Substitution of one nucleotide that changes the encoded amino acid
Duplication of an entire gene
Explanation - A missense mutation results from a single‑base change that leads to a different amino acid in the protein.
Correct answer is: Substitution of one nucleotide that changes the encoded amino acid
Q.43 In which cellular compartment does the majority of protein synthesis occur in eukaryotic cells?
Nucleus
Mitochondria
Cytoplasm (ribosomes)
Endoplasmic reticulum lumen
Explanation - Free ribosomes in the cytoplasm translate most proteins; ribosomes attached to the ER translate secretory/membrane proteins.
Correct answer is: Cytoplasm (ribosomes)
Q.44 Which of the following best describes the term 'polysome'?
A single ribosome bound to an mRNA
A cluster of ribosomes simultaneously translating a single mRNA
A type of RNA polymerase
A protein complex that degrades mRNA
Explanation - Polysomes (polyribosomes) increase translational efficiency by allowing multiple ribosomes to translate the same mRNA concurrently.
Correct answer is: A cluster of ribosomes simultaneously translating a single mRNA
Q.45 Which of the following antibiotics specifically targets the bacterial ribosomal 30S subunit?
Chloramphenicol
Erythromycin
Tetracycline
Streptomycin
Explanation - Streptomycin binds to the 30S subunit, causing misreading of mRNA and inhibiting protein synthesis in bacteria.
Correct answer is: Streptomycin
Q.46 What is the consequence of a mutation that changes the anticodon of a tRNA, but not its attached amino acid?
The tRNA will no longer be charged with its amino acid.
The ribosome will incorporate the wrong amino acid at the corresponding codon.
Translation will stop prematurely.
The mutation has no effect on translation.
Explanation - If the anticodon no longer matches the codon, the tRNA may be mispaired, leading to misincorporation of its attached amino acid.
Correct answer is: The ribosome will incorporate the wrong amino acid at the corresponding codon.
Q.47 Which of the following mechanisms can rescue a ribosome stalled on a damaged mRNA lacking a stop codon?
Nonsense‑mediated decay
Trans‑translation mediated by tmRNA
RNA interference
Spliceosome activity
Explanation - In bacteria, tmRNA (ssrA) adds a tag peptide and a stop codon to stalled ribosomes, allowing termination and degradation of the incomplete protein.
Correct answer is: Trans‑translation mediated by tmRNA
Q.48 Which experimental technique can be used to measure the rate of protein synthesis in live cells?
Western blotting
Pulse‑chase labeling with radioactive amino acids
Chromatin immunoprecipitation (ChIP)
DNA sequencing
Explanation - Pulse‑chase experiments track incorporation of labeled amino acids into proteins over time, revealing synthesis rates.
Correct answer is: Pulse‑chase labeling with radioactive amino acids
Q.49 What is the main functional difference between the initiator tRNA (Met‑tRNAi) and elongator Met‑tRNA in eukaryotes?
Only the initiator tRNA can bind GTP.
The initiator tRNA carries formyl‑methionine.
The initiator tRNA is recognized by initiation factors and can bind the P site directly.
There is no functional difference.
Explanation - Met‑tRNAi is specially recognized by eIF2 and can directly occupy the P site at the start of translation.
Correct answer is: The initiator tRNA is recognized by initiation factors and can bind the P site directly.
Q.50 Which of the following best explains why the genetic code is said to be 'redundant but not ambiguous'?
Each amino acid has multiple codons, but each codon codes for only one amino acid.
Each codon can encode multiple amino acids.
Some codons encode no amino acids.
All codons are used equally in all organisms.
Explanation - Redundancy (degeneracy) means several codons map to the same amino acid; unambiguity means no codon maps to more than one amino acid.
Correct answer is: Each amino acid has multiple codons, but each codon codes for only one amino acid.
Q.51 During translation, which step is directly responsible for moving the ribosome three nucleotides forward on the mRNA?
Peptide bond formation
Translocation mediated by EF‑G (bacterial) or eEF2 (eukaryotic)
Initiation complex assembly
Termination by release factors
Explanation - EF‑G/eEF2 uses GTP hydrolysis to shift the ribosome, moving tRNAs from A to P and P to E sites.
Correct answer is: Translocation mediated by EF‑G (bacterial) or eEF2 (eukaryotic)
Q.52 Which of the following statements about codon usage bias is TRUE?
It is only observed in viral genomes.
It correlates with the abundance of corresponding tRNAs in the cell.
It has no impact on protein expression levels.
All organisms have identical codon usage patterns.
Explanation - Highly expressed genes often use codons that match abundant tRNAs, optimizing translation efficiency.
Correct answer is: It correlates with the abundance of corresponding tRNAs in the cell.
Q.53 In eukaryotic cells, where does the majority of mitochondrial protein synthesis occur?
In the cytosolic ribosomes
Within the mitochondrial matrix by mitochondrial ribosomes
On the rough endoplasmic reticulum
In the nucleus
Explanation - Mitochondria possess their own ribosomes that translate the small set of mitochondrially encoded proteins inside the matrix.
Correct answer is: Within the mitochondrial matrix by mitochondrial ribosomes
Q.54 Which of the following best describes the function of the 'A' site on the ribosome during translation?
It holds the growing polypeptide chain.
It receives the incoming aminoacyl‑tRNA.
It releases the deacylated tRNA.
It binds the mRNA cap structure.
Explanation - The A (aminoacyl) site is where the next aminoacyl‑tRNA enters the ribosome.
Correct answer is: It receives the incoming aminoacyl‑tRNA.
Q.55 Which of the following is a characteristic of a 'recoding' event during translation?
Standard codon–anticodon pairing is altered to incorporate non‑standard amino acids.
Ribosomes skip all stop codons.
All tRNAs are replaced by synthetic analogues.
The mRNA is transcribed in reverse.
Explanation - Recoding includes events like selenocysteine insertion, where a stop codon is reinterpreted to add a specialized amino acid.
Correct answer is: Standard codon–anticodon pairing is altered to incorporate non‑standard amino acids.
Q.56 What is the primary reason that the start codon AUG is conserved across nearly all organisms?
It is the only codon that can bind tRNA.
It encodes methionine, which is chemically essential for all proteins.
It provides a universal signal for translation initiation and codes for methionine.
It is the most abundant codon in genomes.
Explanation - AUG serves as the initiation codon and introduces methionine, establishing a common start point for protein synthesis.
Correct answer is: It provides a universal signal for translation initiation and codes for methionine.
Q.57 Which of the following describes the mechanism of 'ribosomal frameshifting' used by some viruses?
The ribosome pauses and shifts reading frame by one nucleotide, often facilitated by slippery sequences.
The ribosome skips the start codon.
tRNA molecules are replaced by DNA.
The ribosome dissociates and reassembles on the same mRNA.
Explanation - Programmed ribosomal frameshifting allows viruses to produce multiple proteins from overlapping reading frames.
Correct answer is: The ribosome pauses and shifts reading frame by one nucleotide, often facilitated by slippery sequences.
Q.58 Which of the following is NOT a typical feature of a prokaryotic promoter that influences translation efficiency?
Shine‑Dalgarno sequence
TATA box
Transcription start site (+1)
Spacer region between Shine‑Dalgarno and start codon
Explanation - The TATA box is a eukaryotic promoter element; prokaryotes use the Shine‑Dalgarno sequence for translation initiation.
Correct answer is: TATA box
Q.59 What is the major effect of a mutation that changes a codon from GAA (glutamic acid) to GAG (glutamic acid)?
It changes the amino acid to lysine.
It creates a stop codon.
It is a silent mutation.
It causes a frameshift.
Explanation - Both GAA and GAG encode glutamic acid, so the protein sequence remains unchanged.
Correct answer is: It is a silent mutation.
Q.60 In the context of translation, what does the term 'ribosomal stalk' refer to?
A flexible region of the large subunit that interacts with translation factors
The site where tRNA enters the ribosome
The part of the ribosome that binds mRNA
A protein that degrades mRNA
Explanation - The ribosomal stalk (L7/L12 proteins in bacteria) assists in binding GTP‑binding translation factors during elongation.
Correct answer is: A flexible region of the large subunit that interacts with translation factors
Q.61 Which of the following best explains the term 'translational fidelity'?
The speed at which transcription occurs.
The accuracy with which the ribosome incorporates the correct amino acid in response to each codon.
The number of ribosomes present in a cell.
The ability of DNA polymerase to proofread.
Explanation - Translational fidelity measures how precisely the genetic code is interpreted during protein synthesis.
Correct answer is: The accuracy with which the ribosome incorporates the correct amino acid in response to each codon.
Q.62 Which of the following statements about the 'peptidyl transferase center' is correct?
It is composed of proteins only.
It is formed by rRNA and catalyzes peptide bond formation.
It binds ATP for energy.
It is located on the small ribosomal subunit.
Explanation - The peptidyl transferase activity resides in the 23S rRNA of the large ribosomal subunit, making it a ribozyme.
Correct answer is: It is formed by rRNA and catalyzes peptide bond formation.
Q.63 A mutation that changes a codon from UGG (tryptophan) to UGA has what effect?
It replaces tryptophan with a stop signal, truncating the protein.
It replaces tryptophan with serine.
It has no effect (silent mutation).
It changes tryptophan to arginine.
Explanation - UGA is a stop codon; the mutation creates a nonsense mutation leading to premature termination.
Correct answer is: It replaces tryptophan with a stop signal, truncating the protein.
Q.64 Which of the following is a common feature of eukaryotic mRNAs that is absent in most bacterial mRNAs?
Shine‑Dalgarno sequence
5' methyl‑guanosine cap
Operon structure
Poly‑cistronic transcription
Explanation - Eukaryotic mRNAs possess a 5' cap that protects the transcript and aids ribosome recruitment, whereas bacterial mRNAs generally do not.
Correct answer is: 5' methyl‑guanosine cap
Q.65 Which of the following best describes the role of 'elongation factor G' (EF‑G) in bacteria?
It brings aminoacyl‑tRNA to the A site.
It catalyzes peptide bond formation.
It mediates translocation of the ribosome along mRNA.
It recognizes stop codons.
Explanation - EF‑G, powered by GTP hydrolysis, shifts the ribosome by one codon after peptide bond formation.
Correct answer is: It mediates translocation of the ribosome along mRNA.
Q.66 What is the primary determinant for the selection of the correct start codon in eukaryotic translation?
The presence of a Shine‑Dalgarno sequence
The Kozak consensus sequence surrounding the AUG
The length of the poly(A) tail
The presence of a 5' cap
Explanation - A strong Kozak context (gccRccAUGG) enhances initiation at the correct AUG.
Correct answer is: The Kozak consensus sequence surrounding the AUG
Q.67 Which of the following best explains why formyl‑methionine (fMet) is used as the initiator amino acid in prokaryotes?
Formyl‑methionine is the only amino acid that can bind tRNA.
Formylation helps distinguish the initiator tRNA from elongator Met‑tRNA.
It allows translation to start without a ribosome.
It is required for peptide bond formation.
Explanation - fMet‑tRNA^fMet is specifically recognized by initiation factors and the P site, ensuring proper initiation.
Correct answer is: Formylation helps distinguish the initiator tRNA from elongator Met‑tRNA.
Q.68 What is the functional consequence of a ribosome encountering a strong secondary structure in the 5' UTR of an mRNA?
Increased translation efficiency
Ribosome stalling and reduced translation initiation
Immediate termination of translation
Conversion of mRNA into DNA
Explanation - Stable hairpins can impede ribosome scanning or binding, decreasing translation rates.
Correct answer is: Ribosome stalling and reduced translation initiation
Q.69 Which of the following best describes the phenomenon of 'translational readthrough'?
Ribosomes skip every other codon.
Ribosomes continue translation beyond a stop codon, incorporating an amino acid.
tRNA molecules are degraded before use.
mRNA is spliced during translation.
Explanation - Readthrough can occur when a near‑cognate tRNA pairs with a stop codon, leading to an extended protein.
Correct answer is: Ribosomes continue translation beyond a stop codon, incorporating an amino acid.
Q.70 Which of the following is the correct order of the three nucleotide bases in a codon that codes for serine (one of its six codons)?
UCU
UAA
AUG
GGC
Explanation - Serine is encoded by six codons: UCU, UCC, UCA, UCG, AGU, and AGC.
Correct answer is: UCU
Q.71 In the context of translation, what does the term 'ribosome profiling' refer to?
Sequencing of ribosomal RNA genes
Mapping the positions of ribosomes on mRNAs genome‑wide
Measuring ribosome size using electron microscopy
Analyzing ribosome DNA replication
Explanation - Ribosome profiling (Ribo‑seq) isolates ribosome‑protected mRNA fragments and sequences them to infer translation activity.
Correct answer is: Mapping the positions of ribosomes on mRNAs genome‑wide
Q.72 Which of the following best describes the effect of a 'suppressor tRNA'?
It binds to the Shine‑Dalgarno sequence.
It inserts an amino acid at a stop codon, allowing translation to continue.
It degrades mRNA after translation.
It blocks the A site of the ribosome.
Explanation - Suppressor tRNAs have anticodons that recognize stop codons and are charged with an amino acid, rescuing translation.
Correct answer is: It inserts an amino acid at a stop codon, allowing translation to continue.
Q.73 Which of the following enzymes catalyzes the attachment of an amino acid to its corresponding tRNA?
RNA polymerase
Aminoacyl‑tRNA synthetase
DNA ligase
Peptidyl‑transferase
Explanation - Aminoacyl‑tRNA synthetases recognize specific tRNAs and catalyze the formation of aminoacyl‑tRNA (charging).
Correct answer is: Aminoacyl‑tRNA synthetase
Q.74 What is the main reason that a ribosome cannot translate a non‑capped eukaryotic mRNA efficiently?
The ribosome requires a 5' cap to bind initiation factors.
The cap is necessary for splicing.
Capless mRNA lacks a stop codon.
The cap provides the start codon.
Explanation - The 5' cap is recognized by eIF4E, which recruits the 43S pre‑initiation complex; without it, translation initiation is severely reduced.
Correct answer is: The ribosome requires a 5' cap to bind initiation factors.
Q.75 Which of the following best characterizes the term 'ribosomal scanning' in eukaryotes?
Ribosome moves along DNA to find promoters.
Ribosome slides along the mRNA 5' UTR to locate the start codon.
Ribosome degrades the poly(A) tail.
Ribosome assembles the spliceosome.
Explanation - After binding the cap, the small subunit scans downstream until it encounters an AUG in a favorable Kozak context.
Correct answer is: Ribosome slides along the mRNA 5' UTR to locate the start codon.
Q.76 In translation, what does the term 'polypeptide chain elongation' refer to?
Attachment of the ribosome to the mRNA
Addition of successive amino acids to the growing protein
Removal of the stop codon
Splicing of introns from the mRNA
Explanation - Elongation is the phase where peptide bonds are formed, extending the nascent polypeptide.
Correct answer is: Addition of successive amino acids to the growing protein
Q.77 Which of the following statements about the mitochondrial ribosome (mitoribosome) is TRUE?
It contains a higher proportion of protein than rRNA compared to bacterial ribosomes.
It uses the same antibiotics as bacterial ribosomes.
It translates nuclear-encoded mRNAs.
It is identical in size to the cytosolic ribosome.
Explanation - Mitoribosomes have reduced rRNA content and increased protein mass relative to bacterial ribosomes.
Correct answer is: It contains a higher proportion of protein than rRNA compared to bacterial ribosomes.
Q.78 What is the typical effect of a mutation that changes a codon from GGU (glycine) to GGU (glycine) in a different reading frame?
No effect; the protein remains unchanged.
A frameshift occurs, altering downstream amino acids.
Creates a premature stop codon.
Changes glycine to alanine.
Explanation - Even though the codon itself is unchanged, shifting the reading frame changes how subsequent nucleotides are grouped, leading to a frameshift.
Correct answer is: A frameshift occurs, altering downstream amino acids.
Q.79 Which of the following best explains the term 'ribosome rescue'?
Recycling of ribosomes after translation termination.
Removal of ribosomes stalled on damaged or truncated mRNA.
Assembly of new ribosomal subunits.
Binding of ribosomes to DNA.
Explanation - Rescue pathways (e.g., tmRNA, ArfA) release stalled ribosomes, allowing them to re-enter the translation cycle.
Correct answer is: Removal of ribosomes stalled on damaged or truncated mRNA.
Q.80 In eukaryotic translation, which factor is responsible for recognizing the 5' cap structure?
eIF2
eIF4E
eIF3
eIF5
Explanation - eIF4E binds the 7‑methylguanosine cap and recruits other initiation factors to form the eIF4F complex.
Correct answer is: eIF4E
Q.81 Which of the following best describes a 'cryptic' start codon?
A start codon located deep within an intron.
A non‑canonical start codon (e.g., CUG) that can be used under certain conditions.
A start codon that is hidden by secondary structure.
A start codon that is also a stop codon.
Explanation - Cryptic initiation sites can be used when the canonical AUG is inaccessible, leading to alternative protein isoforms.
Correct answer is: A non‑canonical start codon (e.g., CUG) that can be used under certain conditions.
Q.82 Which of the following best explains why the genetic code is considered 'degenerate'?
Some amino acids are encoded by multiple codons.
The code can change during evolution.
Codons overlap each other.
Each codon can code for multiple amino acids.
Explanation - Degeneracy arises because many amino acids have synonymous codons, providing redundancy in the code.
Correct answer is: Some amino acids are encoded by multiple codons.
Q.83 During translation, what is the functional significance of the 'P' site on the ribosome?
It binds the incoming aminoacyl‑tRNA.
It holds the growing polypeptide chain attached to tRNA.
It releases the completed protein.
It interacts with the 5' cap of mRNA.
Explanation - The peptidyl (P) site contains the tRNA linked to the nascent polypeptide during elongation.
Correct answer is: It holds the growing polypeptide chain attached to tRNA.
Q.84 Which of the following best describes the term 'codon reassignment' in mitochondrial genomes?
Changing the reading frame of all genes.
Using a different start codon for every protein.
Using a codon to encode a different amino acid than in the universal code.
Removing stop codons from the genome.
Explanation - Mitochondria often repurpose certain codons (e.g., AUA → Met) compared with the standard nuclear code.
Correct answer is: Using a codon to encode a different amino acid than in the universal code.
Q.85 Which of the following techniques can directly visualize ribosome positions on a single mRNA molecule in real time?
Cryo‑electron microscopy
Single‑molecule fluorescence resonance energy transfer (smFRET)
Northern blotting
PCR amplification
Explanation - smFRET allows observation of ribosome dynamics on individual mRNA strands at the single‑molecule level.
Correct answer is: Single‑molecule fluorescence resonance energy transfer (smFRET)
Q.86 Which of the following best explains why the codon UGG does not have synonymous alternatives?
It encodes the sole amino acid tryptophan.
It is a stop codon.
It is a rare codon not used in most organisms.
It codes for a post‑translational modification.
Explanation - Ugg is the only codon for tryptophan in the standard genetic code, making it non‑degenerate.
Correct answer is: It encodes the sole amino acid tryptophan.
Q.87 In which cellular compartment does the initial processing (capping, splicing, polyadenylation) of eukaryotic pre‑mRNA occur?
Cytoplasm
Mitochondria
Nucleus
Endoplasmic reticulum
Explanation - Pre‑mRNA undergoes capping, splicing, and poly(A) addition within the nucleus before export to the cytoplasm.
Correct answer is: Nucleus
Q.88 Which of the following is a known example of a 'recoding' event that incorporates a non‑standard amino acid into a protein?
Selenocysteine insertion at UGA codon
Leucine insertion at AUG codon
Methionine insertion at UAA codon
Phenylalanine insertion at UAG codon
Explanation - In the presence of a SECIS element, UGA is recoded to insert selenocysteine rather than acting as a stop signal.
Correct answer is: Selenocysteine insertion at UGA codon