Replication of DNA molecules is catalysed by an enzyme called

Step 1: Analyze the question The question asks which enzyme catalyses DNA replication, which is the copying of a DNA molecule before cell division.

Step 2: Explain the concept During DNA replication:

• The double helix is unwound and the two strands separate (helicase does this).
• Each strand acts as a template.
• Free nucleotides pair with the template by complementary base pairing (A with T, G with C).
• DNA polymerase joins these nucleotides into a new strand. It is the main enzyme that catalyses replication.

Step 3: Check each option

• A. Polymerase: Correct. DNA polymerase builds the new DNA strand.
• B. Ptyalin: This is salivary amylase. It digests starch in the mouth, so it is not involved in DNA replication.
• C. Pepsin: This is a stomach enzyme that digests proteins, so it is not involved either.
• D. Amylase: This digests starch to maltose, so it is also unrelated.

Step 4: Answer The correct option is A. polymerase.

Tip: Link the word to the job. Polymer means "many units," and polymerase joins many nucleotides into a long chain. The other options are all digestive enzymes (ptyalin, pepsin and amylase), and digestive enzymes do not copy DNA. In an exam, any name ending in "-ase" that has to do with nucleic acids, such as polymerase, is a strong clue.