DNA replication practice questions.

During DNA replication, state the enzyme that catalyses the addition of new nucleotides to the growing strand in the 5′→3′ direction using the template strand.

• DNA helicase
• DNA polymerase
• DNA ligase
• DNA primase

Answer: DNA polymerase

DNA polymerase adds free deoxyribonucleotides to the 3′ end of the new strand, synthesising it in the 5′→3′ direction using complementary base pairing.

State the type of chemical bond broken between the two strands of DNA when helicase separates the double helix during replication.

• Covalent phosphodiester bonds
• Hydrogen bonds between bases
• Peptide bonds
• Glycosidic bonds

Answer: Hydrogen bonds between bases

Helicase breaks the hydrogen bonds holding complementary base pairs together; the sugar–phosphate backbone (phosphodiester bonds) remains intact.

One strand of DNA has the base sequence 5′-A T G C A G-3′. Determine the sequence of the complementary strand written 3′→5′.

• 3′-T A C G T C-5′
• 3′-A T G C A G-5′
• 3′-U A C G U C-5′
• 3′-T A G C T C-5′

Answer: 3′-T A C G T C-5′

Complementary base pairing gives A–T, T–A, G–C, C–G, so 5′-ATGCAG-3′ pairs with 3′-TACGTC-5′ (DNA uses thymine, not uracil).

Explain why DNA polymerase requires a primer before it can begin synthesising a new strand.

• Because it can only add nucleotides to an existing 3′ end
• Because it must first unwind the double helix
• Because it can only add nucleotides to an existing 5′ end
• Because it needs to break hydrogen bonds

Answer: Because it can only add nucleotides to an existing 3′ end

DNA polymerase cannot start a strand from scratch; it extends an existing 3′ end, so a short RNA primer laid down by primase provides the free 3′-OH group needed.

A sample of double-stranded DNA is found to contain 30 % adenine. Calculate the percentage of guanine present, applying Chargaff's rules.

• 30 %
• 20 %
• 70 %
• 40 %

Answer: 20 %

A = T = 30 %, so A + T = 60 %; the remaining 40 % is G + C, and since G = C, guanine = 20 %.

Which describes how the leading strand is synthesised relative to the movement of the replication fork?

• Continuously, toward the fork
• In fragments, away from the fork
• Continuously, away from the fork
• In fragments, towards the fork

Answer: Continuously, toward the fork

The leading strand is synthesised continuously in the 5′→3′ direction, following the replication fork as it opens.