129 DNA synthesis is continuous on one template strand of DNA and discontinuous on the other

(a) Theta model

Unwinding and replication

Leading strand

Unwinding and replication

Leading strand

DNA unwinds at the origin.

3"

5 'Origin mm VV

DNA unwinds at the origin.

Lagging strand

3"

5 'Origin mm VV

strand / Origin

I Unwinding

Leading W and replication strand

^ DNA synthesis of the lagging strand proceeds discontin-uously in the direction opposite that of unwinding.

At each fork, DNA synthesis of the leading strand proceeds continuously in the same direction as that of unwinding.

(b) Rolling-circle model

^ As the DNA molecule unwinds, the 5' end is progressively displaced.

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