MCAT (Biological and Biochemical) 4 — Questions and Answers
Question 1: Which base pairing rule describes complementary DNA strands?
- A pairs with C, G pairs with T
- A pairs with T, G pairs with C (Correct answer)
- A pairs with G, T pairs with C
- A pairs with U, G pairs with C
Correct answer: A pairs with T, G pairs with C
In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds (Chargaff's rules).
Question 2: Which muscle type has intercalated discs and is under involuntary control?
- Skeletal muscle
- Smooth muscle
- Cardiac muscle (Correct answer)
- Both cardiac and smooth muscle
Correct answer: Cardiac muscle
Cardiac muscle has intercalated discs containing gap junctions that allow coordinated electrical conduction, and it functions involuntarily.
Question 3: The lac operon in E. coli is an example of which type of gene regulation?
- Positive regulation only
- Negative regulation only
- Both positive and negative regulation (Correct answer)
- Post-translational regulation
Correct answer: Both positive and negative regulation
The lac operon is regulated negatively by the lac repressor (which blocks transcription without lactose) and positively by the CAP-cAMP complex (which enhances transcription when glucose is low).
Question 4: Which immunoglobulin class is the first to be produced during a primary immune response?
- IgG
- IgA
- IgM (Correct answer)
- IgE
Correct answer: IgM
IgM is the first antibody produced during a primary immune response due to its pentameric structure and early B cell expression before class switching occurs.
Question 5: What is the role of helicase during DNA replication?
- Synthesizes new DNA strands
- Unwinds the double helix by breaking hydrogen bonds (Correct answer)
- Joins Okazaki fragments
- Adds RNA primers
Correct answer: Unwinds the double helix by breaking hydrogen bonds
Helicase unwinds the DNA double helix by breaking the hydrogen bonds between complementary base pairs at the replication fork.
Question 6: Which type of hypersensitivity reaction involves IgE and mast cell degranulation?
- Type I (immediate hypersensitivity) (Correct answer)
- Type II (cytotoxic)
- Type III (immune complex)
- Type IV (delayed-type)
Correct answer: Type I (immediate hypersensitivity)
Type I hypersensitivity involves IgE antibodies bound to mast cells; upon re-exposure to antigen, cross-linking triggers mast cell degranulation and histamine release.
Question 7: During beta-oxidation of a 16-carbon fatty acid, how many cycles of beta-oxidation occur?
- 6
- 7 (Correct answer)
- 8
- 9
Correct answer: 7
A 16-carbon fatty acid (e.g., palmitate) undergoes 7 cycles of beta-oxidation to produce 8 acetyl-CoA, because each cycle removes 2 carbons and n/2 - 1 = 7.
MCAT (Biological and Biochemical) 4
Which base pairing rule describes complementary DNA strands?