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Genetic Code and Translation: Deciphering the Blueprint of Life
1. The following are all characteristics of the genetic code except:
a) Universal
b) Degenerate
c) Non-overlapping
d) Ambiguous
Correct Answer:
d) Ambiguous
Explanation:
The genetic code has several key characteristics:
Universal → The same codons specify the same amino acids across nearly all organisms, with few exceptions (e.g., mitochondrial genomes).
Degenerate → Multiple codons can code for the same amino acid (e.g., leucine has six different codons).
Non-overlapping → Each nucleotide is read only once as part of a single codon (triplet).
2. Which of the following statements is NOT true concerning the role of aminoacyl-tRNA synthetases?
a) Charging an aminoacyl-tRNA synthetase (loading) with an amino acid is an energetically favorable reaction.
b) For every amino acid, there are several aminoacyl-tRNA synthetases.
c) An amide bond is formed between an amino acid and tRNA.
d) They have the ability to identify and remove the incorrectly loaded amino acid.
Correct Answer:
b) For every amino acid, there are several aminoacyl-tRNA synthetases.
Explanation:
Aminoacyl-tRNA synthetases are enzymes responsible for attaching the correct amino acid to its corresponding tRNA, a process known as tRNA charging.
(a) True – Charging a tRNA with an amino acid is an energetically unfavorable reaction, but it is made favorable by ATP hydrolysis.
(b) False – There is one specific aminoacyl-tRNA synthetase for each amino acid (i.e., 20 aminoacyl-tRNA synthetases in most organisms), ensuring high specificity. Several synthetases do not exist for a single amino acid.
(c) False wording but conceptually relevant – The bond formed between the amino acid and tRNA is an ester bond, NOT an amide bond. However, this option might be tricky depending on the context.
(d) True – Many aminoacyl-tRNA synthetases have proofreading (editing) activity to ensure that only the correct amino acid is attached to the tRNA, preventing translation errors.
3. Which of the following changes in mRNA (resulting from a point mutation) would result in the synthesis of a protein identical to the normal protein?
a) UCA → UAA
b) UCA → CCA
c) UCA → UCU
d) UCA → ACA
Correct Answer:
c) UCA → UCU
Explanation:
- UCA codes for serine (Ser) in the standard genetic code.
- A silent mutation occurs when a point mutation changes a codon but does not alter the encoded amino acid, meaning the protein remains unchanged.
- (c) UCA → UCU is a silent mutation because both UCA and UCU code for serine, ensuring the protein remains identical.
Why the Other Options Are Incorrect:
(a) UCA → UAA → This creates a stop codon (UAA), resulting in a premature termination and a truncated protein.
(b) UCA → CCA → This changes the amino acid from serine (Ser) to proline (Pro), leading to a missense mutation that alters protein structure.
(d) UCA → ACA → This changes the amino acid from serine (Ser) to threonine (Thr), which could affect protein function.
Thus, option (c) UCA → UCU is the only mutation that results in an identical protein due to redundancy in the genetic code.
4. The initiation of translation in eukaryotes requires which of the following?
a) Initiation factor (IF-2)
b) Elongation factor (EF-2)
c) A 40S ribosomal subunit
d) Formyl Methionyl-tRNA (fMet)
Correct Answer:
c) A 40S ribosomal subunit
Explanation:
Eukaryotic translation initiation involves the 40S ribosomal subunit, which binds to the mRNA and scans for the AUG start codon before assembling with the 60S subunit to form the complete 80S ribosome.
Why the Other Options Are Incorrect:
(a) IF-2 → IF-2 is a prokaryotic initiation factor not used in eukaryotic translation. Instead, eIF2 (eukaryotic initiation factor 2) plays a key role in bringing the initiator tRNA (Met-tRNAᵐᵉᵗ) to the ribosome.
(b) EF-2 → EF-2 is an elongation factor not involved in initiation. It helps in ribosomal translocation during elongation.
(d) Formyl Methionyl-tRNA (fMet) → Eukaryotes use methionine (Met-tRNAᵐᵉᵗ) instead of N-formylmethionine (fMet), which is specific to prokaryotic translation.
5. Which of the following statements about prokaryotic translation is incorrect?
a) Transcription and translation are simultaneous
b) Initiation takes place with the help of Methionyl-tRNA (fMet)
c) 20S and 60S subunits together form a ribosome of 80S
d) Chloramphenicol inhibits prokaryotic translation
Correct Answer (Incorrect Statement):
c) 20S and 60S subunits together form a ribosome of 80S
Explanation:
(a) True → In prokaryotes, transcription and translation occur simultaneously because there is no nuclear membrane, allowing ribosomes to begin translating mRNA while it is still being transcribed.
(b) True → Prokaryotic translation initiates with N-formylmethionine (fMet-tRNAᶠᴹᵉᵗ), which helps distinguish start codons from internal methionine codons.
(c) False → Prokaryotic ribosomes consist of a 30S small subunit and a 50S large subunit, which together form a 70S ribosome (not 80S). 80S ribosomes are found in eukaryotes (40S + 60S).
(d) True → Chloramphenicol is an antibiotic that inhibits bacterial translation by binding to the 23S rRNA of the 50S ribosomal subunit, preventing peptide bond formation.
6. Which of the following components is required in prokaryotic protein synthesis?
a) Aminoacyl-tRNA synthetase
b) Helicase
c) Topoisomerase
d) Primase
Correct Answer:
a) Aminoacyl-tRNA synthetase
Explanation:
(a) Aminoacyl-tRNA synthetase (Correct) → This enzyme is essential for translation because it attaches the correct amino acid to its corresponding tRNA, allowing the tRNA to participate in protein synthesis.
(b) Helicase (Incorrect) → Helicase is involved in DNA replication, where it unwinds the DNA double helix but is not needed for protein synthesis.
(c) Topoisomerase (Incorrect) → This enzyme relieves supercoiling during DNA replication and transcription, but not translation.
(d) Primase (Incorrect) → Primase synthesizes RNA primers during DNA replication, but it has no role in translation.
7. With respect to translation, which of the following statements about the role of methionine is incorrect?
a) It is always the first amino acid to be incorporated into the growing polypeptide chain.
b) It is only found at the N-terminal end of the proteins.
c) It requires a codon AUG for its incorporation in the peptide chain.
d) Non-formylated form of methionine is used for the initiation of eukaryotic protein synthesis.
Correct Answer (Incorrect Statement):
b) It is only found at the N-terminal end of the proteins.
Explanation:
(a) False (but appears correct at first glance) → Methionine is usually the first amino acid incorporated during translation. However, in some cases, it can be removed post-translationally by enzymes.
(b) Incorrect (Correct Answer) → Although methionine is the first amino acid added, it is not always retained at the N-terminal end because many proteins undergo post-translational modifications, where methionine may be removed.
(c) True → Methionine incorporation into the polypeptide chain requires the AUG start codon.
(d) True → Eukaryotic cells use a non-formylated methionine (Met-tRNAᵐᵉᵗ) for translation initiation, whereas prokaryotes use N-formylmethionine (fMet-tRNAᶠᴹᵉᵗ).
8. A 58-year-old male has recently been diagnosed with hemochromatosis. He presents with bronze discoloration of the skin, elevated plasma glucose, and high ferritin levels. Genetic analysis reveals a tyrosine (Tyr) substitution for cysteine (Cys) in the HFE gene.
This disease results from which of the following mutations?
a) Silent
b) Nonsense
c) Missense
d) Frameshift
Correct Answer:
c) Missense
Explanation:
- A missense mutation occurs when a single nucleotide change leads to the substitution of one amino acid for another in the protein sequence.
- In this case, the HFE gene has a missense mutation, where tyrosine (Tyr) replaces cysteine (Cys), leading to altered protein function.
- Hemochromatosis is often caused by the C282Y (Cys → Tyr at position 282) mutation in the HFE gene, disrupting iron regulation and leading to iron overload.
Why the Other Options Are Incorrect:
(a) Silent → A silent mutation changes a codon but does not alter the amino acid. Since Tyr and Cys are different amino acids, this is not a silent mutation.
(b) Nonsense → A nonsense mutation converts a codon into a stop codon (UAA, UGA, or UAG), leading to premature termination of translation. This case does not involve a stop codon.
(d) Frameshift → A frameshift mutation results from insertions or deletions that shift the reading frame, affecting all subsequent codons. This case involves a single amino acid substitution, not a frameshift.
9. A 34-year-old female develops a nonproductive cough and a low-grade fever. The attending physician suspects Mycoplasma pneumoniae infection and starts empirical treatment with Erythromycin.
Which of the following describes the mechanism of action of Erythromycin?
a) Inhibits the 50S ribosomal subunit.
b) Inhibits the initiation factor-1 (IF-1).
c) Binds to the Shine-Dalgarno sequence.
d) Inhibits the incoming aminoacyl tRNA.
Correct Answer:
a) Inhibits the 50S ribosomal subunit.
Explanation:
- Erythromycin is a macrolide antibiotic that targets bacterial ribosomes to inhibit protein synthesis.
- It specifically binds to the 23S rRNA of the 50S ribosomal subunit, preventing translocation of the ribosome along the mRNA, which halts protein elongation.
- This makes it bacteriostatic, meaning it prevents bacterial growth rather than killing bacteria directly.
Why the Other Options Are Incorrect:
(b) Inhibits the initiation factor-1 (IF-1) → IF-1 is an initiation factor in prokaryotic translation, but erythromycin acts during elongation, not initiation.
(c) Binds to the Shine-Dalgarno sequence → The Shine-Dalgarno sequence is a ribosome-binding site on bacterial mRNA, but erythromycin does not bind here. Instead, antibiotics like aminoglycosides affect codon recognition at this stage.
(d) Inhibits the incoming aminoacyl-tRNA → Tetracyclines (not erythromycin) work by preventing aminoacyl-tRNA from binding to the A-site of the ribosome.
10. A 4-year-old girl is brought to the Pediatric OPD with fever and a “whooping” cough. The pediatrician learns that she has not been properly immunized and has yet to receive the Pertussis vaccine.
The mechanism by which pertussis toxin causes cell damage is through the inhibition of:
a) Translocase
b) Peptidyl transferase
c) Elongation factor (EF-2)
d) Aminoacyl-tRNA synthetase
Correct Answer:
a) Translocase
Explanation:
- Pertussis toxin, produced by Bordetella pertussis, does not directly inhibit translation like diphtheria toxin does.
- Instead, pertussis toxin ADP-ribosylates the Gαi protein, leading to increased cyclic AMP (cAMP) levels, which disrupts cellular signaling.
- However, Bordetella pertussis also produces adenylate cyclase toxin, which can impair host immune responses.
- The characteristic whooping cough results from the accumulation of thick mucus due to toxin-mediated effects on the respiratory tract.
Why the Other Options Are Incorrect:
(b) Peptidyl transferase → This enzyme forms peptide bonds during translation, but it is not targeted by pertussis toxin.
(c) Elongation factor (EF-2) → Diphtheria toxin (not pertussis toxin) inhibits EF-2, halting protein synthesis.
(d) Aminoacyl-tRNA synthetase → This enzyme charges tRNA with amino acids, but it is not affected by pertussis toxin.
11. An 18-year-old girl has been advised to start Tetracycline for the treatment of acne.
Which of the following statements best describes the mechanism of action of Tetracycline?
Tetracycline:
a) Binds reversibly to the 30S subunit and distorts it.
b) Inhibits prokaryotic Peptidyl Transferase.
c) Inactivates the prokaryotic elongation factors.
d) Structurally binds to the aminoacyl-tRNA.
Correct Answer:
a) Binds reversibly to the 30S subunit and distorts it.
Explanation:
- Tetracyclines are bacteriostatic antibiotics that act by binding reversibly to the 30S ribosomal subunit of bacteria.
- This prevents aminoacyl-tRNA from binding to the A-site of the ribosome, thereby halting protein synthesis.
- By distorting the ribosome, tetracyclines disrupt translation and inhibit bacterial growth.
Why the Other Options Are Incorrect:
(b) Inhibits prokaryotic Peptidyl Transferase → Peptidyl transferase catalyzes peptide bond formation and is found in the 50S subunit. It is inhibited by Chloramphenicol, not Tetracycline.
(c) Inactivates the prokaryotic elongation factors → Elongation factors like EF-Tu and EF-G help move ribosomes along mRNA. These are inhibited by Diphtheria toxin, not Tetracycline.
(d) Structurally binds to the aminoacyl-tRNA → Tetracycline does not bind directly to aminoacyl-tRNA but rather prevents its binding to the ribosome.
12. Which of the following is NOT an example of post-translational modifications?
a) Covalent modification
b) Allosteric modification
c) Gamma carboxylation
d) Trimming
Correct Answer:
b) Allosteric modification
Explanation:
Post-translational modifications (PTMs) are chemical modifications that occur after a protein is synthesized (translated). These modifications affect protein function, stability, localization, and interaction with other molecules.
Why the Other Options Are Correct Post-Translational Modifications:
(a) Covalent modification→ Includes phosphorylation, acetylation, methylation, ubiquitination, and other irreversible or reversible modifications that regulate protein activity.
(c) Gamma carboxylation → A specific PTM where glutamate residues are carboxylated, mainly in clotting factors (e.g., prothrombin) to enhance calcium binding.
(d) Trimming → Involves removal of peptides from inactive precursor proteins (e.g., activation of insulin or zymogens like trypsinogen).
Why (b) Allosteric Modification is NOT a Post-Translational Modification:
Allosteric modification refers to the non-covalent binding of an effector molecule that induces a conformational change, altering protein function.
This is not a chemical modification of the protein itself, but rather a regulatory interaction (e.g., hemoglobin’s oxygen binding).
13. A 34-year-old woman presents to the Medical OPD with high-grade fever, dry cough, headache, loss of appetite, and abdominal pain for the past seven days. She is diagnosed with typhoid fever and started on Chloramphenicol.
The drug acts by inhibiting prokaryotic:
a) Translocase
b) Peptidyl transferase
c) Initiation factor (IF-2)
d) Aminoacyl-tRNA synthetase
Correct Answer:
b) Peptidyl transferase
Explanation:
- Chloramphenicol is a broad-spectrum antibiotic that works by inhibiting prokaryotic peptidyl transferase, an enzyme in the 50S ribosomal subunit responsible for catalyzing peptide bond formation during translation.
- By blocking peptidyl transferase activity, elongation of the polypeptide chain is halted, preventing bacterial protein synthesis.
Why the Other Options Are Incorrect:
(a) Translocase → Translocase (also known as elongation factor EF-G) facilitates ribosome movement along the mRNA. Chloramphenicol does not affect translocation—Macrolides (e.g., Erythromycin) do.
(c) Initiation factor (IF-2) → IF-2 helps in binding the initiator tRNA (fMet-tRNA) to the ribosome during translation initiation. Chloramphenicol does not inhibit this process.
(d) Aminoacyl-tRNA synthetase → This enzyme charges tRNA with the correct amino acid before translation. Chloramphenicol does not affect this step—Mupirocin does.
