Enzymes of replication: helicase & DNA polymerase

The big idea: Before a cell divides it must copy all of its DNA — this is DNA replication.

Two enzymes do the key work. Helicase unzips the double helix, and DNA polymerase builds a new strand against each separated half.

The result is two identical DNA molecules, each keeping one old (template) strand and one new strand — this is what 'semi-conservative' means.

DNA replication

The process of copying a DNA molecule to make two identical molecules, each with one original strand and one new strand.

Helicase

The enzyme that unwinds and unzips the double helix by breaking the hydrogen bonds between the paired bases.

DNA polymerase

The enzyme that builds each new strand by adding complementary nucleotides to a template strand and joining them with covalent bonds.

Template strand

One of the two original (parental) strands; its base sequence is read to decide which nucleotides are added to the new strand.

Complementary base pairing

The rule that A pairs with T and G pairs with C, so each template strand specifies exactly one new strand.

Two enzymes, two jobs: Keep the division of labour clear:

Helicase = unzips (separates the strands). DNA polymerase = builds (adds the new nucleotides).

Helicase goes first to open the helix; DNA polymerase follows to fill in the new strands.

The two strands of DNA are held together by hydrogen bonds between the paired bases (A–T, G–C). Along each strand, the nucleotides are joined into a backbone by much stronger covalent bonds.

The exam expects you to know which bond each enzyme acts on — and they act on different ones.

Helicase BREAKS hydrogen bonds: Helicase moves along the double helix and breaks the hydrogen bonds between the paired bases.

This unzips the two strands apart, separating the parental helix into two single template strands and creating the replication fork where copying happens.

DNA polymerase FORMS covalent bonds: DNA polymerase then reads each template strand and adds the complementary nucleotides (A opposite T, G opposite C).

As it joins each new nucleotide to the growing strand, it forms covalent bonds along the sugar–phosphate backbone.

It can only add nucleotides in one direction — the 5'->3' direction — building the new strand one nucleotide at a time.

A memory hook for the bonds: Helicase handles the Hydrogen bonds — and it breaks them.

Polymerase Puts nucleotides together — forming the covalent bonds of the new backbone.

Different enzyme, different bond, opposite action: one breaks, one forms.

How this is tested: On Paper 1 a 1-mark question often asks you to state the role of DNA polymerase — the answer is that it adds complementary nucleotides to the template strand to build the new strand.

A favourite Paper 1A matching item asks which bonds helicase and DNA polymerase respectively break or form: helicase breaks hydrogen bonds, DNA polymerase forms covalent bonds.

On Paper 2 the same idea appears inside a longer 'describe replication' answer — name the enzyme, then say exactly what it does.

✓ Why this scores the mark: The mark is for the action — adding nucleotides / building the new strand.

Just naming the enzyme is not enough; 'state the role' wants what it actually does.

Now contrast the two enzymes side by side: