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containing one ‘old’ and one ‘new’ strand; the process is therefore known assemi-

conservative replication. The ideas behind DNA synthesis, replication and the

enzymes involved in them have been adopted in many molecular biology techniques

and form the basis of many manipulations in genetic engineering.

Leading

strand

(a) (b) (c) (d)

RNA primers

Lagging

5  strand

5 

Replication

fork

5 

5 

5 

5 ^5 

3 

3 

5 

5  3 

3  5 

Direction of DNA replication

Newly synthesised DNA strand

Origin of

replication

Fig. 5.13DNA replication. (a) Double-stranded DNA separates at the origin of replication. RNA polymerase

synthesises short DNA primer strands complementary to both DNA strands. (b) DNA polymerase III

synthesises new DNA strands in a 5’ to 3’ direction, complementary to the exposed, old DNA strands, and

continuing from the 3’end of each RNA primer. Consequently DNA synthesis is in the same direction as

DNA replication for one strand (the leading strand) and in the opposite direction for the other (the lagging

strand). RNA primer synthesis occurs repeatedly to allow the synthesis of fragments of the lagging strand.

(c) As the replication fork moves away from the origin of replication, DNA polymerase III continues the

synthesis of the leading strand, and synthesises DNA between RNA primers of the lagging strand. (d) DNA

polymerase I removes RNA primers from the lagging strand and fills the resulting gaps with DNA.

DNA ligase then joins the resulting fragments, producing a continuous DNA strand.

Supercoiled

double-stranded

DNA DNA

helicase

DNA

helicase

Single-stranded DNA

binding proteins

5 

Replication fork

3 Single-stranded DNA

Single-stranded

binding proteins

Single-stranded DNA

Fig. 5.12Initial events at the replication fork involving DNA unwinding.

153 5.5 Functions of nucleic acids