Genetic Code and Translation Questions and Answers

Q: Which one of the following compounds is NOT a translation inhibitor?

The compound that is not a translation inhibitor is rifampicin.Rifampicin binds the β subunit of bacterial RNA polymerase and blocks transcription, the step before translation, which is why it is used against Mycobacterium tuberculosis.The other three act on the ribosome. Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase. Cycloheximide blocks translocation on eukaryotic 80S ribosomes. Puromycin mimics the 3′ end of an aminoacyl-tRNA, is joined to the growing chain and causes its premature release.

Q: Two RNAs shown below were used separately as templates in an in vitro translation system, which can generate proteins in all possible reading frames.RNA1: 5′ − (AG)n − 3′RNA2: 5′ − (AAG)n − 3′The RNA1 translated product contained Arg and Glu.The RNA2 translated product contained Arg, Glu, and Lys.Which one of the following codons directs the incorporation of Arg?

Arg is encoded by AGA.RNA1, (AG)n, reads AGA GAG AGA ... in every frame, so its only codons are AGA and GAG, giving an alternating Arg-Glu product. RNA2, (AAG)n, gives a different codon in each frame: AAG, AGA and GAA, one homopolymer each of Lys, Arg and Glu.Arg appears in both products, so its codon must be one both templates contain. The only codon common to the two sets is AGA, so AGA codes for Arg. It follows that GAG, the other codon of RNA1, is Glu; then GAA, the remaining RNA2 codon besides AGA, also gives Glu, leaving AAG for Lys.AAG and GAA are absent from RNA1, and GAG is absent from RNA2, so none of them could account for Arg in both products.

Q: Which one of the following statements about gene expression is INCORRECT?

The incorrect statement is Protein can be reverse-translated to mRNA.The central dogma allows information to pass from DNA to RNA (transcription), from RNA back to DNA (reverse transcription, by retroviral reverse transcriptase) and from mRNA to protein (translation). It does not allow transfer from protein back to nucleic acid: no enzyme reads an amino acid sequence to build mRNA, and the degeneracy of the code would leave the sequence undetermined anyway.The other three statements describe processes that do occur.

Q: Codon bias is correlated with the relative frequencies of which one of the following types of RNA?

Codon bias correlates with the relative abundance of tRNA.Synonymous codons for the same amino acid are not used equally. In highly expressed genes the preferred codons are those read by the most abundant tRNA species in the cell, which speeds and improves the accuracy of translation. Codon usage tables and tRNA gene copy numbers track each other across organisms.mRNA carries the codons but its abundance does not explain which synonym is preferred. rRNA forms the ribosome and is the same for every codon, and siRNA acts in gene silencing, not in decoding.

Q: In the genetic code, 61 codons code for 20 amino acids. How many types of aminoacyl tRNA synthetases are required for correct matching of tRNAs to their corresponding amino acids?

The answer is 20.Each aminoacyl-tRNA synthetase is specific for one amino acid and attaches it to every tRNA that carries that amino acid, including all the isoaccepting tRNAs that read its different synonymous codons. So 20 amino acids need 20 synthetases, not one per codon.61 is the number of sense codons and 64 the total number of codons including the three stop codons; neither counts synthetases. 40 is not tied to any feature of the code. Fewer than 61 distinct tRNAs are needed in any case, because wobble pairing lets one anticodon read more than one codon.

Q: In prokaryotes, the sequence in mRNA that is recognized by the small subunit of ribosome during its translation is known as :

The sequence is the Shine-Dalgarno sequence.In bacteria a purine-rich sequence (consensus AGGAGG) lies a few nucleotides upstream of the start codon on the mRNA. It pairs with a complementary sequence at the 3' end of the 16S rRNA in the small (30S) ribosomal subunit, which positions the ribosome so that the initiation codon sits in the P site.AUG is the start codon itself, read by the initiator tRNA rather than recognised by base-pairing with rRNA. UAG is a stop codon. The Pribnow box (TATAAT) is a promoter element in DNA recognised by RNA polymerase, not a sequence in mRNA.

Q: A messenger RNA is 222 nucleotides long including the initiator and termination codons. The number of amino acids in the protein translated from this mRNA is :

The protein has 73 amino acids.Each codon is three nucleotides, so the mRNA stretch described holds:\[ \begin{aligned} \frac{222}{3} &= 74 \text{ codons} \\ 74 - 1 &= 73 \text{ amino acids} \end{aligned} \]The initiator codon AUG codes for methionine and is counted, but the termination codon codes for no amino acid; it is read by a release factor that ends translation. So one is subtracted for the stop codon only.74 is the number of codons, the trap of forgetting that the stop codon adds nothing. 221 and 111 come from counting nucleotides or pairs instead of triplets.

Q: The enzyme involved in the following reaction is...........Amino acid + ATP + tRNA \(\rightleftharpoons\) aminoacyl tRNA + AMP + PPi

The enzyme is aminoacyl tRNA synthetase.Aminoacyl tRNA synthetases charge tRNAs in two steps. The amino acid first reacts with ATP to form aminoacyl-AMP, releasing pyrophosphate; the activated amino acid is then transferred to the 2' or 3' hydroxyl of the tRNA's terminal adenosine, releasing AMP. The overall reaction is the one in the stem. Each of the twenty amino acids has its own synthetase, which is why they are called the second genetic code.Peptidyl transferase forms peptide bonds on the ribosome, RNA polymerase makes RNA, and 'aminoacyl tRNA transferase' is not the name of this ligase.

Q: Protein synthesis in eukaryotees is inhibited by :

Protein synthesis in eukaryotes is inhibited by cycloheximide.Cycloheximide, from Streptomyces griseus, binds the E site of the eukaryotic 60S ribosomal subunit and blocks translocation during elongation. It does not affect bacterial 70S ribosomes, which is why it is used in the lab to stop cytosolic protein synthesis in eukaryotic cells, and to suppress fungi in bacterial culture media.Chloramphenicol (50S, peptidyl transferase) and streptomycin (30S) act on prokaryotic ribosomes. Cephalosporin C is a β-lactam that inhibits bacterial cell wall synthesis, not protein synthesis.

The pair of amino-acids which does NOT undergo post-translational modification is

A.Asn-His
B.Tyr-Ser
C.Asn-Ser
D.Ala-Gly

Exam Year(s)

GATE Life Sciences - 2012,

Answer: D

The pair that does not undergo post-translational modification is Ala-Gly.

Post-translational modifications act on reactive side chains, and alanine (a methyl group) and glycine (a hydrogen atom) have none to modify. (An N-terminal glycine can be myristoylated, but that acts on the chain's terminal amino group, not on a side chain.)

Every other pair includes a commonly modified residue. Asparagine is the site of N-linked glycosylation. Serine and tyrosine are phosphorylated by kinases, and serine also carries O-linked sugars. Histidine can be phosphorylated (as in bacterial two-component signalling) and methylated.

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Which one of the following compounds is NOT a translation inhibitor?

A.Chloramphenicol
B.Cycloheximide
C.Puromycin
D.Rifampicin

Exam Year(s)

GATE Life Sciences - 2022,

Answer: D

The compound that is not a translation inhibitor is rifampicin.

Rifampicin binds the β subunit of bacterial RNA polymerase and blocks transcription, the step before translation, which is why it is used against Mycobacterium tuberculosis.

The other three act on the ribosome. Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase. Cycloheximide blocks translocation on eukaryotic 80S ribosomes. Puromycin mimics the 3′ end of an aminoacyl-tRNA, is joined to the growing chain and causes its premature release.

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Two RNAs shown below were used separately as templates in an in vitro translation system, which can generate proteins in all possible reading frames.

RNA₁: 5′ − (AG)_n − 3′
RNA₂: 5′ − (AAG)_n − 3′

The RNA₁ translated product contained Arg and Glu.
The RNA₂ translated product contained Arg, Glu, and Lys.

Which one of the following codons directs the incorporation of Arg?

A.AAG
B.AGA
C.GAA
D.GAG

Exam Year(s)

GATE Life Sciences - 2022,

Answer: B

Arg is encoded by AGA.

RNA₁, (AG)_n, reads AGA GAG AGA ... in every frame, so its only codons are AGA and GAG, giving an alternating Arg-Glu product. RNA₂, (AAG)_n, gives a different codon in each frame: AAG, AGA and GAA, one homopolymer each of Lys, Arg and Glu.

Arg appears in both products, so its codon must be one both templates contain. The only codon common to the two sets is AGA, so AGA codes for Arg. It follows that GAG, the other codon of RNA₁, is Glu; then GAA, the remaining RNA₂ codon besides AGA, also gives Glu, leaving AAG for Lys.

AAG and GAA are absent from RNA₁, and GAG is absent from RNA₂, so none of them could account for Arg in both products.

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Which one of the following statements about gene expression is INCORRECT?

A.DNA is transcribed to mRNA.
B.mRNA can be reverse-transcribed to DNA.
C.mRNA can be translated to protein.
D.Protein can be reverse-translated to mRNA.

Exam Year(s)

GATE Life Sciences - 2024,

Answer: D

The incorrect statement is Protein can be reverse-translated to mRNA.

The central dogma allows information to pass from DNA to RNA (transcription), from RNA back to DNA (reverse transcription, by retroviral reverse transcriptase) and from mRNA to protein (translation). It does not allow transfer from protein back to nucleic acid: no enzyme reads an amino acid sequence to build mRNA, and the degeneracy of the code would leave the sequence undetermined anyway.

The other three statements describe processes that do occur.

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Codon bias is correlated with the relative frequencies of which one of the following types of RNA?

A.mRNA
B.rRNA
C.siRNA
D.tRNA

Exam Year(s)

GATE Life Sciences - 2024,

Answer: D

Codon bias correlates with the relative abundance of tRNA.

Synonymous codons for the same amino acid are not used equally. In highly expressed genes the preferred codons are those read by the most abundant tRNA species in the cell, which speeds and improves the accuracy of translation. Codon usage tables and tRNA gene copy numbers track each other across organisms.

mRNA carries the codons but its abundance does not explain which synonym is preferred. rRNA forms the ribosome and is the same for every codon, and siRNA acts in gene silencing, not in decoding.

Discuss in Forum

In the genetic code, 61 codons code for 20 amino acids. How many types of aminoacyl tRNA synthetases are required for correct matching of tRNAs to their corresponding amino acids?

A.61
B.64
C.40
D.20

Exam Year(s)

IIT JAM - 2026,

Answer: D

The answer is 20.

Each aminoacyl-tRNA synthetase is specific for one amino acid and attaches it to every tRNA that carries that amino acid, including all the isoaccepting tRNAs that read its different synonymous codons. So 20 amino acids need 20 synthetases, not one per codon.

61 is the number of sense codons and 64 the total number of codons including the three stop codons; neither counts synthetases. 40 is not tied to any feature of the code. Fewer than 61 distinct tRNAs are needed in any case, because wobble pairing lets one anticodon read more than one codon.

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In prokaryotes, the sequence in mRNA that is recognized by the small subunit of ribosome during its translation is known as :

A.UAG sequence
B.Shine-Dalgarno sequence
C.Pribnow box sequence
D.AUG sequence

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

The sequence is the Shine-Dalgarno sequence.

In bacteria a purine-rich sequence (consensus AGGAGG) lies a few nucleotides upstream of the start codon on the mRNA. It pairs with a complementary sequence at the 3' end of the 16S rRNA in the small (30S) ribosomal subunit, which positions the ribosome so that the initiation codon sits in the P site.

AUG is the start codon itself, read by the initiator tRNA rather than recognised by base-pairing with rRNA. UAG is a stop codon. The Pribnow box (TATAAT) is a promoter element in DNA recognised by RNA polymerase, not a sequence in mRNA.

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A messenger RNA is 222 nucleotides long including the initiator and termination codons. The number of amino acids in the protein translated from this mRNA is :

A.221
B.111
C.73
D.74

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The protein has 73 amino acids.

Each codon is three nucleotides, so the mRNA stretch described holds:

\[ \begin{aligned} \frac{222}{3} &= 74 \text{ codons} \\ 74 - 1 &= 73 \text{ amino acids} \end{aligned} \]

The initiator codon AUG codes for methionine and is counted, but the termination codon codes for no amino acid; it is read by a release factor that ends translation. So one is subtracted for the stop codon only.

74 is the number of codons, the trap of forgetting that the stop codon adds nothing. 221 and 111 come from counting nucleotides or pairs instead of triplets.

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The enzyme involved in the following reaction is...........

Amino acid + ATP + tRNA \(\rightleftharpoons\) aminoacyl tRNA + AMP + PPi

A.Aminoacyl tRNA transferase
B.Aminoacyl tRNA synthetase
C.Peptidyl transferase
D.RNA polymerase

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The enzyme is aminoacyl tRNA synthetase.

Aminoacyl tRNA synthetases charge tRNAs in two steps. The amino acid first reacts with ATP to form aminoacyl-AMP, releasing pyrophosphate; the activated amino acid is then transferred to the 2' or 3' hydroxyl of the tRNA's terminal adenosine, releasing AMP. The overall reaction is the one in the stem. Each of the twenty amino acids has its own synthetase, which is why they are called the second genetic code.

Peptidyl transferase forms peptide bonds on the ribosome, RNA polymerase makes RNA, and 'aminoacyl tRNA transferase' is not the name of this ligase.

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10.

Protein synthesis in eukaryotees is inhibited by :

A.Chloramphenicol
B.Streptomycin
C.Cephalosporin C
D.Cycloheximide

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

Protein synthesis in eukaryotes is inhibited by cycloheximide.

Cycloheximide, from Streptomyces griseus, binds the E site of the eukaryotic 60S ribosomal subunit and blocks translocation during elongation. It does not affect bacterial 70S ribosomes, which is why it is used in the lab to stop cytosolic protein synthesis in eukaryotic cells, and to suppress fungi in bacterial culture media.

Chloramphenicol (50S, peptidyl transferase) and streptomycin (30S) act on prokaryotic ribosomes. Cephalosporin C is a β-lactam that inhibits bacterial cell wall synthesis, not protein synthesis.

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