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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 will add the free DNA nucleotides using complementary base pairing (A-T and C-G) to the 3’ end of the primer this will allow the new DNA strand to form. Adenine pairs with thymine, thymine with adenine, cytosine with guanine and guanine with cytosine. A primer is needed to start replication.

1. Leading strand is synthesised continuously. DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand 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.