201. Consider the following statements about tRNA.
I. Its anticodon can base-pair with a complementary codon on mRNA.
II. An amino acid is attached at the amino-acid acceptor end.
III. The anticodon and amino acid occupy the same functional site.
IV. It acts as an adapter between nucleotide information and amino-acid placement.
ⓐ. I, II and III only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I and III only
Correct Answer: I, II and IV only
Explanation: tRNA performs the adapter function required to convert nucleotide information into an amino-acid sequence. One region of the molecule contains the anticodon, which can pair with the complementary codon exposed on mRNA. A different end serves as the amino-acid acceptor site, where the appropriate amino acid becomes attached before translation. Keeping these two functional regions separate is essential: the anticodon identifies the codon being read, while the acceptor end carries the amino acid that will enter the growing polypeptide. By combining codon recognition with amino-acid carriage in one molecule, tRNA provides the physical link between the genetic code and ordered protein synthesis. The two functional ends of tRNA must remain coordinated, since codon recognition without delivery of the corresponding amino acid would compromise translation fidelity.
202. A tRNA is represented by a cloverleaf-like arrangement. Region P is a three-base sequence in one loop that pairs with mRNA, while Region Q lies at the \(3'\) end and carries an activated amino acid. What are the functions of P and Q?
ⓐ. P binds the ribosome permanently; Q contains the mRNA codon
ⓑ. P terminates transcription; Q binds the DNA template
ⓒ. P carries the amino acid; Q recognises the mRNA codon
ⓓ. P is the anticodon; Q is the amino-acid acceptor end
Correct Answer: P is the anticodon; Q is the amino-acid acceptor end
Explanation: The three-base sequence that pairs directly with an mRNA codon is the anticodon, so Region P performs codon recognition. Region Q is located at the \(3'\) end and is described as carrying an activated amino acid, identifying it as the amino-acid acceptor end. These spatially separated functions explain the adapter role of tRNA. The anticodon reads nucleotide information through complementary pairing, while the opposite end delivers the amino acid corresponding to that codon. Protein synthesis depends on this combination: recognition alone would not supply an amino acid, and amino-acid carriage without codon recognition would not place residues in the sequence specified by the mRNA.
203. An mRNA codon is written \(5'\text{-AUG-}3'\). If a tRNA anticodon pairs directly and antiparallel to this codon, the anticodon written opposite it is:
ⓐ. \(3'\text{-UAC-}5'\)
ⓑ. \(5'\text{-UAC-}3'\)
ⓒ. \(3'\text{-AUG-}5'\)
ⓓ. \(5'\text{-GUA-}3'\)
Correct Answer: \(3'\text{-UAC-}5'\)
Explanation: Codon-anticodon recognition depends on both complementarity and antiparallel orientation. The mRNA codon runs \(5'\rightarrow3'\) as AUG. The bases directly paired with A, U and G are U, A and C respectively, giving the complementary sequence UAC. Since the two interacting nucleotide sequences are antiparallel, the anticodon placed directly opposite \(5'\text{-AUG-}3'\) must run \(3'\text{-UAC-}5'\). Writing the same bases as \(5'\text{-UAC-}3'\) would preserve complementarity as a sequence list but not the antiparallel orientation required for direct pairing. Codon recognition by tRNA is governed by the same basic polarity principle that operates in other nucleic-acid interactions.
204. A mutation changes the anticodon of a particular tRNA, but the same amino acid can still be attached normally to its acceptor end. What is the most direct consequence?
ⓐ. The tRNA must become a DNA molecule before entering a ribosome
ⓑ. The carried amino acid is automatically converted into another amino acid
ⓒ. The tRNA may fail to recognise its original mRNA codon correctly
ⓓ. Every codon on the mRNA changes its nucleotide sequence
Correct Answer: The tRNA may fail to recognise its original mRNA codon correctly
Explanation: The anticodon provides the sequence-recognition part of the tRNA adapter function. If that three-base region is altered, the tRNA can lose normal complementarity with the codon it previously recognised, even when its amino-acid acceptor end remains structurally able to carry the same amino acid. The immediate defect is in codon recognition rather than in the chemistry of the amino acid itself. Since the mRNA sequence has not changed, the mutation does not alter every codon in the message. This changed condition separates the two major functional regions of tRNA: the acceptor end handles amino-acid carriage, whereas the anticodon determines where that charged tRNA can correctly participate during translation.
205. Assertion: A tRNA molecule has a characteristic folded structure that places different functional regions in distinct positions.
Reason: Its anticodon participates in recognition of a complementary mRNA codon.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, but Reason does not correctly explain Assertion
Explanation: tRNA has a characteristic folded organisation with functionally distinct regions, including an anticodon-containing region and an amino-acid acceptor end. The reason is also valid: the anticodon base-pairs with the corresponding mRNA codon during translation. However, stating one function of the anticodon does not by itself explain why the whole tRNA molecule adopts its characteristic folded structure or why all of its functional regions are spatially organised as they are. The two statements are biologically related but not linked as a complete cause-and-explanation pair. A correct assertion-reason analysis requires more than both statements being true; the reason must specifically account for the assertion rather than merely describe one feature contained within it.
206. The following events lead to formation of a charged tRNA.
P. The appropriate amino acid is available.
Q. ATP is used during amino-acid activation.
R. The activated amino acid becomes linked to its cognate tRNA.
S. The charged tRNA becomes available for protein synthesis.
The most appropriate order is:
ⓐ. P, Q, R, S
ⓑ. Q, R, P, S
ⓒ. R, P, Q, S
ⓓ. P, R, S, Q
Correct Answer: P, Q, R, S
Explanation: Before an amino acid can participate in ribosomal protein synthesis, it must be linked to the appropriate tRNA. The amino acid is first available as a substrate and undergoes an ATP-dependent activation step. The activated amino acid is then attached to its cognate tRNA, producing a charged or aminoacylated tRNA. That charged tRNA is the form used during translation to deliver the amino acid to the growing polypeptide. The sequence reflects functional dependency: attachment cannot produce a charged tRNA before the relevant amino acid has been activated, and the ribosome cannot use the amino acid through the tRNA adapter until charging has occurred. ATP use is positioned before formation of the translation-ready charged tRNA.
207. A cell contains normal mRNAs, ribosomes, tRNAs and amino acids, but ATP required for amino-acid activation becomes unavailable. What is the most direct effect on translation?
ⓐ. Ribosomes begin reading mRNA in the \(3'\rightarrow5'\) direction
ⓑ. Stop codons are converted into amino-acid codons
ⓒ. tRNA anticodons are replaced by DNA sequences
ⓓ. Charged-tRNA formation falls, limiting amino-acid delivery
Correct Answer: Charged-tRNA formation falls, limiting amino-acid delivery
Explanation: Amino acids are activated in an ATP-dependent step before they become linked to their appropriate tRNAs. If ATP required for this activation is unavailable, normal charging of tRNAs is impaired even though the tRNA molecules, mRNAs and ribosomes themselves remain present. Translation requires charged tRNAs as substrates that can both recognise codons and deliver amino acids. An uncharged tRNA lacks the amino acid needed for addition to the growing polypeptide. Loss of ATP at this step does not reverse the direction in which mRNA is read or change codon identities. The effect arises earlier, at preparation of the amino-acid-bearing adapters needed for continued translation.
208. Four mixtures are tested for formation of charged tRNA.
| Mixture | Amino acid | Appropriate tRNA | ATP | Charged tRNA formed |
|---|
| P | Present | Present | Present | Yes |
| Q | Present | Present | Absent | No |
| R | Present | Absent | Present | No |
| S | Absent | Present | Present | No |
What conclusion is best supported by the complete pattern?
ⓐ. ATP alone can generate a charged tRNA without an amino acid
ⓑ. An amino acid alone determines codon recognition during translation
ⓒ. Charging requires an amino acid, its matching tRNA and ATP
ⓓ. tRNA charging occurs only when ATP is absent
Correct Answer: Charging requires an amino acid, its matching tRNA and ATP
Explanation: Mixture P is the only condition in which all three required components are available, and it is also the only mixture in which charged tRNA is formed. Removing ATP in Q prevents the activation step. Removing the appropriate tRNA in R eliminates the molecule to which the amino acid must be attached, while removing the amino acid in S removes the cargo itself. The pattern is stronger than a single comparison because each necessary component is independently omitted once. Taken together, the results support a requirement for the amino acid, the corresponding tRNA and ATP during formation of the charged tRNA that will subsequently participate in translation.
209. Consider the following statements about charged and uncharged tRNA.
I. A charged tRNA carries an amino acid at its acceptor end.
II. An uncharged tRNA lacks the amino acid that should be delivered during translation.
III. Charging changes the tRNA anticodon into the corresponding mRNA codon.
IV. Charged tRNAs provide amino-acid-bearing substrates for ribosomal protein synthesis.
ⓐ. I, II and III only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I and III only
Correct Answer: I, II and IV only
Explanation: Charging links an appropriate amino acid to the acceptor end of its tRNA, creating the amino-acid-bearing form used during translation. An uncharged tRNA retains its nucleotide structure and anticodon but lacks the amino acid required for incorporation into a growing polypeptide. Charging does not rewrite the anticodon into a codon; the anticodon remains the recognition sequence that pairs with a complementary triplet on mRNA. The charged tRNA can perform both parts of the adapter task at once: its anticodon identifies the correct codon position, while its attached amino acid supplies the residue that translation needs. The distinction between charged and uncharged tRNA is a difference in amino-acid attachment, not a replacement of the anticodon sequence.
210. Match each component with its principal role during translation. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Small ribosomal subunit | 1. Provides the codon sequence to be read |
| Q. Large ribosomal subunit | 2. Participates in formation of the complete translating ribosome after initiation begins |
| R. Catalytic ribosomal RNA | 3. Helps position the mRNA during initiation |
| S. mRNA | 4. Contributes directly to peptide-bond formation |
ⓐ. P-4, Q-3, R-2, S-1
ⓑ. P-2, Q-1, R-4, S-3
ⓒ. P-3, Q-4, R-1, S-2
ⓓ. P-3, Q-2, R-4, S-1
Correct Answer: P-3, Q-2, R-4, S-1
Explanation: Translation begins with the small ribosomal subunit associating with the mRNA near the start region, so P matches the initiation-positioning role. The large subunit subsequently joins to form the complete translating ribosome, giving Q its stated relation. Ribosomal RNA is not merely structural: catalytic rRNA contributes directly to peptide-bond formation during protein synthesis. The mRNA supplies the sequence of codons that determines the order in which amino acids are selected. These roles cooperate within one molecular machine, but they are not interchangeable. The mRNA supplies information, tRNAs interpret that information, and the ribosome provides the organised structural and catalytic environment in which the polypeptide is assembled. Correct assembly of these roles allows initiation, codon reading and peptide-bond formation to occur as coordinated parts of one translation process.
211. Ribosomal subunits assemble normally on an mRNA, and charged tRNAs still recognise their codons. However, the catalytic rRNA responsible for peptide-bond formation is selectively inactivated. What is the most direct outcome?
ⓐ. Peptide-bond formation is impaired despite amino-acid positioning
ⓑ. The mRNA sequence is converted back into its template DNA sequence
ⓒ. tRNAs lose their anticodons before entering the ribosome
ⓓ. The start codon automatically becomes a stop codon
Correct Answer: Peptide-bond formation is impaired despite amino-acid positioning
Explanation: The altered condition leaves several earlier translation functions intact. Ribosomal assembly occurs, charged tRNAs are available, and codon-anticodon recognition still positions amino-acid-bearing tRNAs according to the mRNA sequence. The disrupted function is the catalytic contribution of ribosomal RNA to formation of peptide bonds between adjacent amino acids. Amino acids may reach the correct positions, yet the polypeptide cannot be extended efficiently if those bonds are not formed. This observation demonstrates that the ribosome is more than a passive platform holding mRNA and tRNAs. Its RNA component contributes directly to catalysis, linking ribosomal structure with the chemical reaction required to build the polypeptide chain.
212. Assertion: Ribosomal RNA participates directly in the catalytic activity required for peptide-bond formation.
Reason: DNA polymerase catalyses peptide-bond formation between amino acids during translation.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is true, but Reason is false
Explanation: The assertion reflects an important feature of the ribosome: rRNA contributes directly to the catalytic centre involved in peptide-bond formation. This is why the ribosome can be considered a ribonucleoprotein machine with an RNA component that performs more than a structural role. The reason is false. DNA polymerase functions in DNA synthesis, where it adds deoxyribonucleotides to a growing DNA strand; it is not the catalyst responsible for joining amino acids during translation. The two processes use different substrates and molecular machinery. Recognising the catalytic role of rRNA also supports the broader biological significance of RNA, which can act not only as an information carrier or adapter but also as a catalytic molecule.
213. Four events involved in the start of translation are listed.
P. The small ribosomal subunit associates with the mRNA near the start codon.
Q. The initiator tRNA recognises the start codon.
R. The large ribosomal subunit joins the complex.
S. Elongation of the polypeptide begins.
The appropriate order is:
ⓐ. Q, P, R, S
ⓑ. P, Q, R, S
ⓒ. P, R, Q, S
ⓓ. R, P, Q, S
Correct Answer: P, Q, R, S
Explanation: Translation initiation begins when the small ribosomal subunit associates with the mRNA in the region of the start codon. The initiator tRNA then recognises the AUG start codon through codon-anticodon pairing, establishing the beginning of the translated reading frame. The large ribosomal subunit subsequently joins to form the complete functional ribosome. Only after this initiation complex has been assembled can the repeated elongation cycle proceed efficiently. The sequence follows the dependence of one event on the previous one: the start site must be located, the initiator tRNA must establish the first codon-amino-acid relation, and the complete ribosome must be formed before sustained polypeptide extension begins.
214. An mRNA is arranged from \(5'\) to \(3'\) as Region P-start codon-coding region-stop codon-Region Q. P and Q do not contribute amino acids to the polypeptide but support efficient translation. P and Q are:
ⓐ. promoter and terminator
ⓑ. intron and exon
ⓒ. anticodon and acceptor end
ⓓ. \(5'\) UTR and \(3'\) UTR
Correct Answer: \(5'\) UTR and \(3'\) UTR
Explanation: Untranslated regions flank the translated portion of a mature mRNA. The \(5'\) UTR lies before the start codon, while the \(3'\) UTR lies after the stop codon. These regions are part of the mRNA but do not supply amino acids to the polypeptide sequence. Their designation as untranslated does not mean they are biologically meaningless; they contribute to efficient translation and mRNA function. Promoters and terminators are DNA features associated with transcription, whereas introns are normally removed from mature eukaryotic mRNA. The supplied positions identify the regions directly: P lies before translation begins, making it the \(5'\) UTR, and Q lies beyond the termination signal, making it the \(3'\) UTR.
215. The regions of an mRNA are described below.
| Region | Description |
|---|
| P | \(5'\) UTR before the start codon |
| Q | Start codon followed by amino-acid-coding codons up to the last sense codon |
| R | Stop codon |
| S | \(3'\) UTR after the stop codon |
Which region directly specifies the amino-acid sequence of the polypeptide?
ⓐ. P only
ⓑ. R only
ⓒ. Q only
ⓓ. S only
Correct Answer: Q only
Explanation: Region Q begins with the start codon and includes the sense codons that specify the amino acids of the polypeptide. The stop codon in R marks termination but does not contribute an amino acid, so it is a translational signal rather than part of the amino-acid sequence. Regions P and S are untranslated regions positioned on either side of the coding portion. They can be important for efficient translation and other aspects of mRNA function, yet their nucleotides are not translated into the polypeptide under the stated arrangement. The table separates three categories that are often confused: untranslated regulatory regions, codons that specify amino acids, and the stop signal that ends translation.
216. Assertion: The \(5'\) and \(3'\) untranslated regions of mRNA can be important for efficient translation even though they do not encode amino acids in the polypeptide.
Reason: These regions lie outside the amino-acid-coding sequence but contribute to proper use of the mRNA during translation.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: The designation untranslated refers to whether the nucleotide sequence is converted into amino-acid sequence, not to whether the region has biological significance. The \(5'\) UTR lies before the start codon and the \(3'\) UTR lies after the stop codon, so neither region contributes amino acids to the translated product. Even so, these regions contribute to efficient use of a mature mRNA during translation. The reason captures the distinction between coding and functional importance: a sequence can lie outside the polypeptide-coding region while still supporting effective use of the mRNA by the translational machinery. The assertion and reason are linked by this coding-versus-function distinction. The Reason directly accounts for the Assertion by separating absence of amino-acid coding from absence of function; untranslated regions can still influence translation efficiency.
217. A mutation deletes most of the \(5'\) UTR of an mRNA but leaves the start codon and entire amino-acid-coding sequence unchanged. Translation efficiency falls sharply, although the rare polypeptides that are produced have the normal sequence. Which interpretation fits the observation?
ⓐ. The deletion shifted the coding frame by one nucleotide
ⓑ. The mutation replaced the normal stop codon with AUG
ⓒ. Every amino-acid codon was altered by removal of the UTR
ⓓ. UTR loss reduces translation but not protein sequence
Correct Answer: UTR loss reduces translation but not protein sequence
Explanation: The start codon and the complete amino-acid-coding sequence remain unchanged, so the information specifying the polypeptide has not been altered. The reduced output instead follows removal of a \(5'\) untranslated region that contributes to efficient translation. When translation does occur, the same codons are read in the same frame, which explains why the resulting polypeptide sequence remains normal. The case demonstrates that untranslated does not mean functionless. An mRNA region can influence how effectively the translational machinery uses the message without itself supplying amino acids. This separates effects on translation efficiency from mutations that directly alter codons and thereby change protein sequence.
218. An mRNA has a normal AUG start codon, but the initiator tRNA has an altered anticodon that can no longer pair with AUG. Other charged tRNAs and both ribosomal subunits are normal. What is the most direct consequence?
ⓐ. Loss of AUG recognition by initiator tRNA impairs translation initiation
ⓑ. The large ribosomal subunit converts AUG into a stop codon
ⓒ. Elongating tRNAs restore the initiator anticodon before translation begins
ⓓ. The mRNA is replicated into DNA to replace the defective tRNA
Correct Answer: Loss of AUG recognition by initiator tRNA impairs translation initiation
Explanation: Translation initiation depends on correct pairing between the start codon and the specialised initiator tRNA. The mRNA still contains AUG, and the ribosomal subunits remain structurally available, but the altered anticodon removes the required codon-anticodon match at the start site. This prevents the initiator tRNA from establishing the first codon-amino-acid relationship needed for normal assembly of the translating complex. Ordinary elongating tRNAs act after initiation and do not repair or rewrite an initiator anticodon. The large ribosomal subunit also does not change the nucleotide sequence of AUG. The defect is narrowly located at molecular recognition during initiation, not in DNA replication or mRNA sequence. This changed condition shows why a correct start codon alone is insufficient: the translation machinery must also contain an adapter capable of recognising that codon in the proper initiation context.
219. During elongation, charged tRNAs can still recognise their codons and peptide bonds can form, but the ribosome is experimentally prevented from moving along the mRNA after a bond is made. What is the most direct effect?
ⓐ. The existing mRNA is converted into a DNA template
ⓑ. Every tRNA becomes permanently attached to the stop codon
ⓒ. Elongation stalls because the ribosome cannot advance to the next codon
ⓓ. The ribosome begins synthesising the polypeptide from its carboxyl end toward its amino end
Correct Answer: Elongation stalls because the ribosome cannot advance to the next codon
Explanation: Protein synthesis requires repeated cycles rather than a single codon-recognition event. After an amino acid is added and a peptide bond is formed, the ribosome must move along the mRNA so that the next codon can be positioned for recognition by another charged tRNA. If this movement is blocked, the machinery may complete the local reaction already under way but cannot advance efficiently to the next triplet. The polypeptide stops extending even though charged tRNAs and catalytic activity remain available. The defect is mechanical progression along the message, not a change in mRNA chemistry or polypeptide direction. Codon-by-codon movement is what converts a linear mRNA sequence into an ordered amino-acid sequence.
220. A translating ribosome reaches UAA in the established reading frame. Which event most directly distinguishes termination from ordinary elongation?
ⓐ. A tRNA carrying a special stop amino acid pairs with UAA and extends the chain
ⓑ. A release factor recognises UAA and releases the completed polypeptide
ⓒ. DNA polymerase binds UAA and removes the final amino acid
ⓓ. The ribosome reverses direction and rereads all preceding codons
Correct Answer: A release factor recognises UAA and releases the completed polypeptide
Explanation: UAA is a termination codon rather than an amino-acid codon. During elongation, sense codons are recognised through charged tRNAs that deliver amino acids to the growing chain. At a stop codon, this pattern changes: no ordinary aminoacyl-tRNA contributes a terminating amino acid. Instead, a release factor recognises the termination signal and promotes release of the completed polypeptide from the translation machinery. The ribosomal complex can then dissociate rather than continuing codon-by-codon elongation. This difference distinguishes termination mechanistically from ordinary sense-codon reading. Stop codons specify an action in the translation process, ending polypeptide synthesis, rather than specifying an additional residue in the final protein.
