401. Which codon usually marks the beginning of a translational unit?
ⓐ. UGA
ⓑ. UAA
ⓒ. AUG
ⓓ. UAG
Correct Answer: AUG
Explanation: AUG generally serves as the initiator codon in translation. It establishes the starting point from which codons are read in the correct frame. Because the code is comma-less, choosing the right starting point is essential for the proper amino-acid sequence. AUG therefore has a special role beyond simply coding for methionine.
402. Which feature correctly describes untranslated regions or UTRs in mRNA?
ⓐ. They are removed from all mRNAs before translation begins.
ⓑ. They always code for stop codons only.
ⓒ. They are translated into small peptides before the main protein is made.
ⓓ. They are regions at the ends of mRNA that are not translated but help efficient translation.
Correct Answer: They are regions at the ends of mRNA that are not translated but help efficient translation.
Explanation: UTRs are present at the ends of many mRNA molecules but do not themselves code for amino acids. Even though they are untranslated, they contribute to proper handling and efficient use of the mRNA during translation. Their presence shows that not every part of a mature mRNA is part of the protein-coding sequence. They are therefore functionally important even without being translated.
403. Which statement correctly compares the start and stop points of a translational unit?
ⓐ. It begins at a promoter and ends at a terminator.
ⓑ. It begins at AUG and ends at one of the stop codons.
ⓒ. It begins at UAA and ends at AUG.
ⓓ. It begins at the anticodon and ends at the amino-acid acceptor site.
Correct Answer: It begins at AUG and ends at one of the stop codons.
Explanation: Translation starts where the ribosome encounters the initiator codon, usually AUG. It continues through successive codons until a stop codon such as UAA, UAG, or UGA is reached. That codon range defines the translational unit. The promoter and terminator belong to transcription, not translation.
404. Which sequence best represents the major phases of translation?
ⓐ. initiation → elongation → termination
ⓑ. elongation → initiation → termination
ⓒ. initiation → termination → elongation
ⓓ. termination → elongation → initiation
Correct Answer: initiation → elongation → termination
Explanation: Translation begins with initiation, when the ribosome assembles at the start codon with the initiator tRNA. It then proceeds through elongation, during which amino acids are added one by one to the growing chain. Finally, it ends at termination when a stop codon is reached. This sequence reflects the normal order of protein synthesis.
405. Which event is most characteristic of the elongation phase of translation?
ⓐ. recognition of the stop codon by release factors
ⓑ. addition of amino acids to the growing polypeptide chain
ⓒ. export of hnRNA from the nucleus
ⓓ. binding of RNA polymerase to the promoter
Correct Answer: addition of amino acids to the growing polypeptide chain
Explanation: Elongation is the stage in which the polypeptide actually grows. During this phase, successive charged tRNAs enter according to codon sequence, peptide bonds form, and the ribosome moves along the mRNA. This repeated cycle lengthens the protein chain step by step. It is therefore the main synthetic phase of translation.
406. Assertion: Ribosomes are not made only of proteins.
Reason: Ribosomal RNA contributes both to ribosome structure and to catalytic activity during translation.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion.
ⓑ. Assertion is true, but Reason is false.
ⓒ. Assertion is false, but Reason is true.
ⓓ. Both Assertion and Reason are true, and Reason correctly explains Assertion.
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Explanation: Ribosomes are ribonucleoprotein complexes, so they include both RNA and proteins. The RNA component is not merely present as support; it also contributes functionally, including catalytic activity in peptide-bond formation. This directly explains why ribosomes cannot be described as protein-only structures. The reason therefore fully supports the assertion.
407. During initiation of translation, the first tRNA to bind at the start codon is usually the
ⓐ. release-factor-bound tRNA
ⓑ. initiator tRNA carrying methionine
ⓒ. tRNA carrying a stop signal
ⓓ. ribosomal RNA molecule
Correct Answer: initiator tRNA carrying methionine
Explanation: Translation begins when the start codon is recognized by a specialized initiator tRNA. This tRNA usually carries methionine and helps establish the correct reading frame for the entire protein. Its binding marks the true beginning of the translational unit. Without this step, the ribosome would not know where to start reading codons correctly.
408. Which event is most directly associated with termination of translation?
ⓐ. binding of RNA polymerase to the promoter
ⓑ. addition of a methyl guanosine cap
ⓒ. charging of tRNA with amino acid
ⓓ. recognition of a stop codon and release of the completed polypeptide
Correct Answer: recognition of a stop codon and release of the completed polypeptide
Explanation: Translation ends when the ribosome encounters a stop codon on the mRNA. Since stop codons do not specify amino acids, the chain is not extended further. Instead, termination factors help release the completed polypeptide from the ribosome. This brings the synthesis of that protein to an end.
409. Which statement best describes peptide-bond formation during translation?
ⓐ. It links adjacent amino acids into a growing polypeptide chain.
ⓑ. It joins exons together after transcription.
ⓒ. It connects codons directly to DNA bases.
ⓓ. It seals Okazaki fragments during replication.
Correct Answer: It links adjacent amino acids into a growing polypeptide chain.
Explanation: The main chemical event of translation is the formation of peptide bonds between successive amino acids. As each new amino acid is brought by tRNA, it is joined to the existing chain. Repetition of this step builds the polypeptide in a defined sequence. Protein synthesis is therefore a polymerization process based on peptide-bond formation.
410. Which molecule helps end translation by acting at a stop codon instead of a tRNA?
ⓐ. histone
ⓑ. DNA ligase
ⓒ. release factor
ⓓ. sigma factor
Correct Answer: release factor
Explanation: Stop codons are not matched by amino-acid-carrying tRNAs. Instead, they are recognized by release factors, which help terminate translation. These factors promote release of the completed polypeptide from the ribosome. Their action explains how protein synthesis ends even though no tRNA corresponds to a stop codon.
411. Which event occurs repeatedly during elongation of translation?
ⓐ. synthesis of DNA from an RNA template
ⓑ. binding of RNA polymerase to a terminator
ⓒ. removal of introns from hnRNA
ⓓ. codon recognition followed by addition of the corresponding amino acid
Correct Answer: codon recognition followed by addition of the corresponding amino acid
Explanation: Elongation is the repetitive phase of translation in which the ribosome reads codons one after another. For each codon, the matching charged tRNA enters, and its amino acid is added to the growing chain. This cycle continues until a stop codon is reached. The stepwise nature of elongation gives proteins their specific amino-acid order.
412. Movement of the ribosome along mRNA during translation is important because it allows
ⓐ. DNA replication to begin at the origin
ⓑ. introns to be removed from the transcript
ⓒ. histones to bind newly synthesized protein
ⓓ. the next codon to be brought into position for decoding
Correct Answer: the next codon to be brought into position for decoding
Explanation: Translation proceeds codon by codon along the mRNA. After one amino acid is added, the ribosome shifts so that the next codon can be read. This movement is necessary for continued elongation of the polypeptide chain. Without it, translation would stall at a single codon.
413. Which statement best explains why the start codon is important in translation?
ⓐ. It establishes the correct reading frame for the protein sequence.
ⓑ. It signals the release of the completed polypeptide.
ⓒ. It removes untranslated regions from mRNA.
ⓓ. It converts tRNA into rRNA.
Correct Answer: It establishes the correct reading frame for the protein sequence.
Explanation: Because the genetic code is read continuously in triplets, the point where translation begins determines how all later codons will be grouped. The start codon therefore does more than begin protein synthesis; it sets the frame for correct decoding. A wrong starting point would change the downstream amino-acid sequence. This is why AUG has such special importance.
414. Which statement correctly compares initiation and elongation in translation?
ⓐ. Initiation removes introns, whereas elongation adds the 5' cap.
ⓑ. Initiation ends at a stop codon, whereas elongation begins at the promoter.
ⓒ. Initiation establishes the ribosome at the start codon, whereas elongation adds amino acids sequentially.
ⓓ. Initiation forms DNA, whereas elongation forms RNA.
Correct Answer: Initiation establishes the ribosome at the start codon, whereas elongation adds amino acids sequentially.
Explanation: Initiation is the stage where the ribosome assembles with the mRNA and initiator tRNA at the start codon. Once this arrangement is in place, elongation begins and amino acids are added one by one according to the codon sequence. The two phases are therefore consecutive but functionally distinct. One starts the process, and the other builds the chain.
415. A mutation removes the start codon from an mRNA. Which effect is most directly expected?
ⓐ. peptide bonds will form more rapidly than normal
ⓑ. translation may fail to initiate properly
ⓒ. stop codons will be converted into sense codons
ⓓ. the mRNA will become DNA immediately
Correct Answer: translation may fail to initiate properly
Explanation: The start codon is the signal that tells the ribosome where to begin reading the message. If it is absent, proper initiation becomes difficult or impossible. As a result, the ribosome may not establish the correct reading frame for protein synthesis. This can severely reduce or prevent production of the intended polypeptide.
416. Assertion: Translation ends only after a specific signal is reached on mRNA.
Reason: A stop codon does not code for an amino acid and instead triggers termination.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion.
ⓑ. Assertion is true, but Reason is false.
ⓒ. Assertion is false, but Reason is true.
ⓓ. Both Assertion and Reason are true, and Reason correctly explains Assertion.
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Explanation: Translation does not end at a random point. It stops when a stop codon enters the ribosome and is recognized as a termination signal. Since stop codons do not specify amino acids, chain elongation cannot continue beyond them in the usual way. The reason therefore directly explains why termination requires a specific signal.
417. Which description best matches untranslated regions or UTRs in an mRNA molecule?
ⓐ. They are introns removed before the mature mRNA leaves the nucleus.
ⓑ. They are promoter sequences present only in DNA.
ⓒ. They are codons that always specify stop signals.
ⓓ. They are non-translated sequences flanking the translated region and help efficient translation.
Correct Answer: They are non-translated sequences flanking the translated region and help efficient translation.
Explanation: UTRs are parts of mature mRNA that are present but not translated into amino acids. They occur near the ends of the transcript, outside the start-to-stop codon region. Even though they do not code for protein, they help the mRNA function efficiently during translation. This makes them important but non-coding parts of the transcript.
418. If a mature mRNA contains a 5' UTR, a coding region, and a 3' UTR, which part defines the translational unit?
ⓐ. only the 5' UTR
ⓑ. only the coding region from start codon to stop codon
ⓒ. the entire mRNA from cap to tail
ⓓ. only the 3' UTR
Correct Answer: only the coding region from start codon to stop codon
Explanation: The translational unit is the portion of mRNA that is actually read to produce a polypeptide. It begins at the initiator codon and ends at a stop codon. The UTRs are present in the mature mRNA and help efficient translation, but they are not themselves translated into amino acids. This is a subtle distinction between mRNA structure and translated sequence.
419. Which conclusion is supported simultaneously by the RNA world concept and by the catalytic role of 23S rRNA in the ribosome?
ⓐ. RNA can function not only as genetic material in some systems but also as a catalyst.
ⓑ. RNA is more stable than DNA for long-term storage in all organisms.
ⓒ. Ribosomes are composed only of RNA and no proteins.
ⓓ. Every catalytic RNA molecule must also be a genetic material.
Correct Answer: RNA can function not only as genetic material in some systems but also as a catalyst.
Explanation: The RNA world concept proposes that RNA could once act as the first genetic material, while the ribozyme activity of 23S rRNA shows that RNA can also be catalytic. Together, these ideas support the broader view that RNA is a multifunctional molecule. This makes RNA a plausible early biomolecule in evolution. The evidence does not mean that all RNA is genetic material or that ribosomes lack proteins.
420. A DNA template strand has the sequence 3'-TACCCAATCATT-5'. What polypeptide will be formed from the corresponding mRNA, assuming standard translation begins at the first AUG?
ⓐ. methionine-glycine only
ⓑ. methionine-proline-isoleucine-histidine
ⓒ. methionine-proline only
ⓓ. methionine-glycine-isoleucine
Correct Answer: methionine-glycine only
Explanation: The mRNA transcribed from the template is 5'-AUGGGUUAGUAA-3'. Translation starts at AUG, giving methionine, then GGU gives glycine. The next codon is UAG, which is a stop codon, so translation ends there. This question combines template reading, transcription, codon recognition, and termination in one step.
