Biology MCQs with Answers and Explanations

Phenylketonuria (PKU) is inherited as an autosomal recessive trait. If two carriers have a child, what is the probability the child will be unaffected (no mutant alleles)?

A. 100%
B. 75%
C. 50%
D. 25%

Explanation:
For two carriers (Aa × Aa), the offspring genotype ratio is 1 AA: 2 Aa: 1 aa. 75% (AA + Aa) are unaffected (though 50% are carriers).

An allele is:

A. A mutated chromosome
B. A variant form of a gene at a particular locus
C. A gene on the Y chromosome
D. A plasmid containing genetic information

Explanation:
An allele is one of the alternative forms of the same gene found at the same chromosomal locus.

What is the fundamental unit of heredity that occupies a specific locus on a chromosome?

A. Base pair
B. Gene
C. Chromatin
D. Nucleotide

Explanation:
A gene is the hereditary unit at a chromosome locus. It consists of DNA sequences encoding a functional product (RNA or protein).

Alternative splicing occurs during gene expression at the level of:

A. DNA replication
B. Transcription initiation
C. Pre-mRNA processing (RNA splicing)
D. Protein folding

Explanation:
Alternative splicing of pre-mRNA allows different exons to be joined or skipped, producing multiple mRNA variants from one gene during RNA processing.

The 5′ cap added to eukaryotic mRNAs is:

A. A string of adenine residues at the 5′ end
B. A modified guanosine (7-methylguanosine) linked 5′→5′
C. A poly-uridine sequence at the 3′ end
D. A phosphate group at the 5′ end

Explanation:
The 5′ cap is a 7-methylguanosine joined via a 5′-to-5′ triphosphate linkage to the first nucleotide of the mRNA, protecting it from degradation and aiding translation.

Nucleosomes are composed of DNA wrapped around an octamer of histones. How many base pairs of DNA are wrapped around one nucleosome core?

A. ~50 bp
B. ~146 bp
C. ~300 bp
D. ~10 bp

Explanation:
About 146 base pairs of DNA wrap ~1.7 turns around the histone octamer (2 copies each of H2A, H2B, H3, H4) in a nucleosome core.

Which structural feature is shared by B-DNA, A-DNA, and Z-DNA?

A. Right-handed double helix
B. 10 bases per turn
C. Uniform major groove
D. Two strands paired by base complementarity

Explanation:
All DNA forms (B, A, Z) are double-stranded helices with complementary base pairing. However, B and A are right-handed (10 and ~11 bp/turn), and Z is left-handed.

In a DNA molecule, 5′ to 3′ synthesis means:

A. New nucleotides are added at the 5′ end of the new strand
B. New nucleotides are added at the 3′ end of the new strand
C. DNA is always read 5′ to 3′ on the template
D. The sugar backbone runs from 5′ to 3′

Explanation:
DNA polymerases add nucleotides to the 3′-OH end of a growing strand, so synthesis proceeds in the 5′→3′ direction (extending at the 3′ end).

Why is the G0 phase considered distinct from G1 in the cell cycle?

A. G0 cells are rapidly dividing
B. G0 is a quiescent/nondividing state outside the normal cycle
C. G0 occurs immediately after the M phase in all cells
D. Cells in G0 cannot re-enter the cell cycle under any condition

Explanation:
G0 is a resting or terminal differentiation state where cells are metabolically active but not dividing. Cells can re-enter G1 from G0 given appropriate signals.

In gene therapy using a viral vector, what is a major concern related to using retroviruses?

A. They cannot infect human cells
B. They only integrate into mitochondrial DNA
C. Insertional mutagenesis by random integration into the host genome
D. They cannot carry large genes

Explanation:
Retroviral vectors integrate randomly into the host genome, risking insertional mutagenesis (disrupting host genes), which can cause cancer or other issues.