D1.1 DNA replication

The question asks about the composition of daughter DNA molecules after one round of replication, which relates directly to the semi-conservative model of DNA replication.

In semi-conservative replication, the two parental strands separate, and each acts as a template. A new complementary strand is built alongside each original strand.

The result is two daughter molecules. Each molecule contains one original (parental) strand and one newly synthesised strand — this is the definition of semi-conservative replication.

The option stating that each daughter molecule contains one original parental strand and one newly synthesised strand perfectly describes the semi-conservative model.

The question asks about the strand composition of daughter DNA molecules — this tests knowledge of semi-conservative replication.

This would describe a conservative model where the original molecule is fully preserved separately — but that model was disproved by the Meselson-Stahl experiment. Eliminate this option.

This describes the conservative replication model, where one daughter molecule keeps both old strands and one gets two new strands. This is incorrect and was ruled out experimentally.

This describes the dispersive model, in which parental and new DNA are interspersed randomly throughout both strands of each daughter molecule. This model was also disproved.

The remaining option — each daughter molecule contains one original parental strand and one newly synthesised strand — correctly describes semi-conservative replication.

The enzyme responsible for opening the double helix at the start of replication is helicase. It moves along the DNA molecule and separates the two strands.

The two strands of DNA are held together by hydrogen bonds between complementary base pairs (A–T and C–G). Helicase breaks these hydrogen bonds to separate the strands.

Helicase does not break the covalent (phosphodiester) bonds of the sugar-phosphate backbone — the individual strands remain chemically intact after unwinding.

The correct answer is helicase; breaks hydrogen bonds between complementary base pairs.

The question asks for both the correct enzyme and the correct bond type involved in strand separation.

DNA polymerase does not separate strands — it synthesises new strands. It also forms covalent bonds rather than breaking them. Eliminate this option.

DNA ligase joins (seals) fragments by forming covalent bonds — it does not break bonds or unwind DNA. Eliminate this option.

While helicase is correctly identified, it does not break covalent bonds in the backbone. Breaking covalent bonds in the backbone would destroy the strand. The bonds helicase breaks are hydrogen bonds between base pairs.

Helicase; breaks hydrogen bonds between complementary base pairs is the only option that correctly names both the enzyme and the bond type.