Biology MCQs with Answers and Explanations

Which characteristic is common to Type II restriction endonucleases?

A. They require ATP to cut DNA
B. They recognize and cut at specific palindromic DNA sequences
C. They cut single-stranded RNA
D. They are not used in DNA cloning

Explanation:
Type II restriction enzymes recognize specific palindromic DNA sequences and cleave double-stranded DNA at those sites, without requiring ATP.

Sanger (dideoxy) sequencing terminates DNA chain elongation using:

A. Restriction enzymes
B. Dideoxynucleotides (ddNTPs)
C. RNA polymerase
D. DNA methyltransferases

Explanation:
In Sanger sequencing, modified nucleotides (ddNTPs) lacking a 3′-OH terminate extension when incorporated, allowing determination of the DNA sequence by fragment lengths.

Which of the following components is not required for a standard PCR reaction?

A. Template DNA
B. DNA polymerase
C. Primers
D. DNA ligase

Explanation:
PCR requires a thermostable DNA polymerase, primers, dNTPs, and template DNA. DNA ligase is not needed because PCR does not splice DNA fragments.

Why does the lagging strand of DNA require more DNA ligase activity during replication than the leading strand?

A. It contains more errors to fix
B. It is synthesized more continuously
C. It is synthesized discontinuously in Okazaki fragments
D. It is shorter than the leading strand

Explanation:
The lagging strand is made in short Okazaki fragments, each of which must be joined by DNA ligase. The leading strand is continuous and requires minimal ligation.

Histone acetyltransferases (HATs) play what role in transcription?

A. Deacetylate histones to compact chromatin
B. Add acetyl groups to histones, opening chromatin
C. Methylate DNA at promoters
D. Phosphorylate RNA polymerase II

Explanation:
HATs add acetyl groups to lysine residues on histones, reducing their positive charge and loosening DNA-histone interactions, which promotes transcription.

DNA methylation at CpG islands in gene promoters typically leads to:

A. Transcriptional activation
B. Transcriptional silencing
C. Increased mutation rate
D. Immediate DNA repair

Explanation:
Methylation of cytosines in promoters condenses chromatin and represses transcription, silencing the gene.

Which enzyme untangles DNA during replication by cutting and re-ligating both strands?

A. DNA ligase
B. Topoisomerase I
C. Topoisomerase II
D. DNA helicase

Explanation:
Topoisomerase II (a type II enzyme) makes double-strand breaks to relieve supercoils or decatenate DNA, then re-ligates the strands.

DNA polymerases proofread the newly synthesized DNA strand using which activity?

A. 5′→3′ exonuclease
B. 3′→5′ exonuclease
C. Primase activity
D. Helicase activity

Explanation:
Many DNA polymerases have 3′→5′ exonuclease activity that excises incorrectly paired nucleotides immediately after incorporation (proofreading).

Which mechanism allows a single eukaryotic gene to encode multiple protein isoforms?

A. Alternative splicing
B. Alternative sigma factors
C. Gene duplication
D. Horizontal gene transfer

Explanation:
Alternative splicing of a pre-mRNA can include or exclude exons, yielding different mRNAs and thus different protein isoforms from one gene.

In eukaryotic pre-mRNA splicing, which nucleotides are found at the 5′ and 3′ ends of most introns?

A. GU at 5′ and AG at 3′ ends
B. GA at both ends
C. AA at 5′ and TT at 3′ ends
D. CT at 5′ and TC at 3′ ends

Explanation:
The consensus splice donor site at the 5′ end of introns is “GU,” and the splice acceptor site at the 3′ end is “AG” in most eukaryotic genes.

Biology MCQs