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Stage one

The DNA is unwound and unzipped.

• the helix structure is unwound
• special molecules break the weak hydrogen bonds between bases, which are holding the two strands together
• this process occurs at several locations on a DNA molecule

Stage two

DNA polymerase adds the free DNA nucleotides to the 3’ end of the primer.

This uses complementary base pairing (A-T and C-G):

• adenine pairs with thymine
• thymine pairs with adenine
• cytosine pairs with guanine
• guanine pairs with cytosine

This allows the new DNA strand to form. A primer is needed to start replication.

1. Leading strand is synthesised continuously:
   • DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand
   • this happens in a 5’ to 3’ direction
2. Lagging strand is synthesised in fragments:
   • nucleotides cannot be added to the phosphate (5’) end because DNA polymerase can only add DNA nucleotides in a 5’ to 3’ direction
   • the lagging strand is therefore synthesised in fragments
   • the fragments are then sealed together by an enzyme called ligase

Stage three

The two new strands twist to form a double helix. Each is identical to the original strand.