MCAT (Biological and Biochemical) 5 — Questions and Answers
Question 1: Which process allows a single gene to produce multiple protein isoforms from the same pre-mRNA?
- DNA methylation
- Alternative splicing (Correct answer)
- Histone acetylation
- RNA editing
Correct answer: Alternative splicing
Alternative splicing allows different combinations of exons to be included or excluded, producing multiple distinct mRNA and protein isoforms from a single gene.
Question 2: A patient lacks C3 complement protein. Which immune function is most directly impaired?
- Antibody class switching
- Opsonization and phagocytosis (Correct answer)
- T cell activation
- NK cell killing
Correct answer: Opsonization and phagocytosis
C3b, derived from C3 cleavage, is a major opsonin that coats pathogens and enhances phagocytosis; C3 deficiency severely impairs this innate defense.
Question 3: Which neurotransmitter is primarily responsible for the 'fight-or-flight' response at the adrenal medulla?
- Acetylcholine (Correct answer)
- Norepinephrine
- Dopamine
- Serotonin
Correct answer: Acetylcholine
The adrenal medulla is innervated by preganglionic sympathetic fibers that release acetylcholine, which stimulates chromaffin cells to secrete epinephrine and norepinephrine.
Question 4: Which event occurs during the S phase of the cell cycle?
- Chromosome condensation
- Spindle formation
- DNA replication (Correct answer)
- Cytokinesis
Correct answer: DNA replication
During the S (synthesis) phase of interphase, the entire genome is replicated so that each chromosome consists of two identical sister chromatids.
Question 5: Which amino acid is most commonly found at the active site of serine proteases?
- Cysteine
- Histidine
- Serine (Correct answer)
- Aspartate
Correct answer: Serine
Serine proteases (e.g., trypsin, chymotrypsin) use a catalytic triad of Ser-His-Asp, but serine is the nucleophile that attacks the peptide bond.
Question 6: What is the net ATP yield from complete oxidation of one molecule of glucose via aerobic respiration (standard estimate)?
- 2 ATP
- 4 ATP
- 30-32 ATP (Correct answer)
- 36-38 ATP
Correct answer: 30-32 ATP
The modern accepted estimate is 30-32 ATP per glucose under physiological conditions, accounting for the P/O ratios of ~2.5 for NADH and ~1.5 for FADH2.
Question 7: Which DNA repair mechanism corrects thymine dimers caused by UV radiation?
- Base excision repair
- Mismatch repair
- Nucleotide excision repair (Correct answer)
- Homologous recombination
Correct answer: Nucleotide excision repair
Nucleotide excision repair (NER) removes bulky lesions like UV-induced thymine dimers by excising a ~12-24 nucleotide oligonucleotide containing the lesion.
MCAT (Biological and Biochemical) 5
Which process allows a single gene to produce multiple protein isoforms from the same pre-mRNA?