Transcription/translation US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Transcription/translation. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Transcription/translation US Medical PG Question 1: A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?
- A. CUA (Correct Answer)
- B. CCC
- C. UAA
- D. CCA
- E. AUG
Transcription/translation Explanation: ***CUA***
- The **wobble hypothesis** allows for non-standard base pairing at the **third position** of the codon.
- Since CUU codes for leucine, a change in the third base to **A (CUA)** can often still code for the same amino acid due to degeneracy of the genetic code.
*CCC*
- This codon codes for **proline**, not leucine.
- A change in the **second letter** of the codon almost always results in a different amino acid.
*UAA*
- This is one of the **stop codons** (UAA, UAG, UGA), which signals the termination of translation.
- It does not code for any amino acid.
*CCA*
- This codon codes for **proline**, not leucine.
- Changing the first or second nucleotide typically results in a different amino acid.
*AUG*
- This codon codes for **methionine** and also serves as the **start codon** for protein synthesis.
- It does not code for leucine.
Transcription/translation US Medical PG Question 2: An investigator is comparing DNA replication in prokaryotes and eukaryotes. He finds that the entire genome of E. coli (4 × 106 base pairs) is replicated in approximately 30 minutes. A mammalian genome (3 × 109 base pairs) is usually replicated within 3 hours. Which of the following characteristics of eukaryotic DNA replication is the most accurate explanation for this finding?
- A. Replication inhibition at checkpoint
- B. Absence of telomerase enzyme activity
- C. DNA compaction in chromatin
- D. Simultaneous replication at multiple origins (Correct Answer)
- E. More efficient DNA polymerase activity
Transcription/translation Explanation: ***Simultaneous replication at multiple origins***
- Eukaryotic DNA replication initializes at **multiple origins of replication** along each chromosome, allowing synthesis to occur concurrently in many places.
- This strategy compensates for the much larger eukaryotic genome size, enabling its complete replication within a reasonable timeframe despite slower polymerase speed compared to prokaryotes.
*Replication inhibition at checkpoint*
- **Cell cycle checkpoints**, such as those in G1, S, and G2 phases, ensure the integrity of DNA replication and repair.
- While these checkpoints can *pause* or *inhibit* replication if errors occur, they do not fundamentally explain the *speed* or **efficiency** of replication across the entire genome.
*Absence of telomerase enzyme activity*
- **Telomerase** is an enzyme that maintains the ends of eukaryotic chromosomes (telomeres) by adding repetitive DNA sequences.
- Its presence or absence is related to telomere length regulation and cellular aging, not the overall speed of genome replication.
*DNA compaction in chromatin*
- Eukaryotic DNA is compact and organized into **chromatin** within the nucleus, which presents a challenge to replication by limiting access to the DNA.
- While enzymes must overcome this compaction, it is a *hindrance* rather than an enabler of replication speed. If anything, it would slow down replication.
*More efficient DNA polymerase activity*
- In actuality, **prokaryotic DNA polymerases** (e.g., DNA Pol III in *E. coli*) are generally more processive and faster than eukaryotic DNA polymerases.
- Therefore, more efficient polymerase activity is not a characteristic that would explain the relatively fast replication of a larger eukaryotic genome.
Transcription/translation US Medical PG Question 3: An investigator studying protein synthesis in human stem cells isolates tRNA molecules bound to mRNA molecules. The isolated tRNA molecules have inosine in the 5' position of the anticodon; of these, some are bound to adenine, some to cytosine, and some to uracil at the 3' position of the mRNA codon. Which of the following properties of the genetic code is best illustrated by this finding?
- A. Unambiguity
- B. Non-overlapping
- C. Degeneracy (Correct Answer)
- D. Specificity of the start codon
- E. Specificity of stop codons
Transcription/translation Explanation: ***Degeneracy***
- The finding that a single tRNA anticodon (with **inosine** at the 5' position) can bind to multiple different mRNA codons (ending in **adenine, cytosine, or uracil**) illustrates the concept of **degeneracy** in the genetic code.
- This **wobble hypothesis** allows fewer tRNAs to recognize more than one codon for a given amino acid, meaning multiple codons can code for the same amino acid.
*Unambiguity*
- The genetic code is unambiguous, meaning that each codon specifies **only one specific amino acid** (or a stop signal) and never two different amino acids.
- This finding, however, shows one tRNA recognizing multiple codons, not one codon coding for multiple amino acids.
*Non-overlapping*
- The **non-overlapping** nature of the genetic code means that each nucleotide in an mRNA sequence is read only once as part of a single codon, without sharing nucleotides between adjacent codons.
- This concept describes how codons are read sequentially, not the flexibility of codon-anticodon pairing.
*Specificity of the start codon*
- The **start codon (AUG)** specifically initiates translation, coding for methionine, and signals the beginning of a polypeptide chain.
- This finding relates to the wobble pairing at the 3' end of the codon, not the initiation of translation.
*Specificity of stop codons*
- **Stop codons (UAA, UAG, UGA)** specifically signal the termination of translation without coding for any amino acid.
- This finding describes the flexibility of codon-anticodon pairing, not the distinct function of termination codons.
Transcription/translation US Medical PG Question 4: An 18-year-old African-American woman comes to the physician for the evaluation of worsening fatigue that started 1 year ago. Physical examination shows mild jaundice and splenomegaly. Laboratory studies show:
Hemoglobin 10.4 g/dL
Mean corpuscular hemoglobin concentration 43% Hb/cell
Platelet count 220,000/mm3
Reticulocyte count 7%
A peripheral blood smear shows target cells and erythrocytes with hemoglobin crystals. Which of the following is the most likely underlying cause of this patient's findings?
- A. Acquired mutation of membrane-bound glycosylphosphatidylinositol anchor
- B. Decreased conversion of oxidized glutathione into its reduced form
- C. Replacement of glutamate by valine in beta-globin chain
- D. Replacement of glutamate by lysine in beta-globin chain (Correct Answer)
- E. Reduced production of beta-globin due to a mutation in the HbB gene
Transcription/translation Explanation: ***Replacement of glutamate by lysine in beta-globin chain***
- The combination of **mild jaundice**, **splenomegaly**, **anemia**, **elevated reticulocyte count**, and the presence of **target cells** and **hemoglobin C crystals** on peripheral smear is characteristic of **Hemoglobin C disease**.
- **Hemoglobin C** results from the **replacement of glutamate by lysine at position 6 of the beta-globin chain**, leading to abnormal hemoglobin and erythrocyte morphology.
*Acquired mutation of membrane-bound glycosylphosphatidylinositol anchor*
- This describes **Paroxysmal Nocturnal Hemoglobinuria (PNH)**, which is characterized by **hemolytic anemia**, **thrombosis**, and **bone marrow failure**.
- While it causes hemolytic anemia, the peripheral smear findings of **target cells** and **hemoglobin crystals** are not typical for PNH as seen in this patient.
*Decreased conversion of oxidized glutathione into its reduced form*
- This mechanism is characteristic of **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**, a cause of **hemolytic anemia**.
- G6PD deficiency typically presents with episodic hemolysis triggered by **oxidative stress**, and the characteristic peripheral smear findings are **Heinz bodies** and **bite cells**, not hemoglobin crystals.
*Replacement of glutamate by valine in beta-globin chain*
- This mutation describes **Sickle Cell Anemia**, which also causes hemolytic anemia and splenomegaly.
- While it leads to **sickle cells** on peripheral smear, **hemoglobin C crystals** and **target cells** are not the primary features; sickle cells and Howell-Jolly bodies would be expected.
*Reduced production of beta-globin due to a mutation in the HbB gene*
- This describes **Beta-thalassemia**, a group of disorders characterized by **reduced or absent beta-globin chain synthesis**.
- Beta-thalassemia typically presents with **microcytic hypochromic anemia** and **target cells**, but the presence of **hemoglobin C crystals** is not a feature.
Transcription/translation US Medical PG Question 5: In translation, the wobble phenomenon is best illustrated by the fact that:
- A. Charged tRNA contains energy needed for peptide bonds to form
- B. The last nucleotide provides specificity for the given amino acid
- C. A tRNA with the UUU anticodon can bind to either AAA or AAG codons (Correct Answer)
- D. There are more amino acids than possible codons
- E. The genetic code is preserved without mutations
Transcription/translation Explanation: ***A tRNA with the UUU anticodon can bind to either AAA or AAG codons***
- The **wobble phenomenon** allows for non-standard base pairing between the **first nucleotide (5' position) of the tRNA anticodon** and the **third nucleotide (3' position) of the mRNA codon**.
- In this example, a tRNA with anticodon **3'-UUU-5'** can bind to either **5'-AAA-3'** or **5'-AAG-3'** codons (both encoding lysine) due to the relaxed base-pairing rules at the wobble position.
- This flexibility means fewer tRNAs are needed to recognize all 61 sense codons, illustrating the **degeneracy of the genetic code**.
- According to Crick's wobble hypothesis, **U at the 5' position of the anticodon** can pair with either **A or G at the 3' position of the codon**.
*Charged tRNA contains energy needed for peptide bonds to form*
- While **charged tRNA** (aminoacyl-tRNA) does carry an amino acid activated for peptide bond formation, this statement describes the energy source for translation, not the wobble phenomenon.
- The energy for peptide bond formation comes from the **high-energy ester bond** linking the amino acid to the tRNA, not from the base pairing itself.
*The last nucleotide provides specificity for the given amino acid*
- The **last nucleotide** (3' position) of the mRNA codon is where **wobble pairing** occurs, meaning it does *not* always provide strict specificity for the amino acid due to the relaxed base-pairing rules.
- It is often the *first two nucleotides* of the codon that are most critical in determining the specific amino acid incorporated.
*There are more amino acids than possible codons*
- This statement is incorrect; there are **20 standard amino acids** and **61 sense codons** (three are stop codons), meaning there are more codons than amino acids, leading to **code degeneracy**.
- The concept of wobble base pairing helps explain how this degeneracy is managed efficiently, but the premise of this option is false.
*The genetic code is preserved without mutations*
- This statement refers to the **fidelity of DNA replication and repair** or the evolutionary conservation of the genetic code, not the mechanism of translation or wobble base pairing.
- The genetic code being largely universal and degenerate does not mean that mutations never occur, but rather that it is robust.
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