As Figure 2.24 shows, this process does not occur in a haphazard manner. An enzyme called helicase unzips one region of the DNA molecule and nucleotides are added in a step-by-step process that links them to one another and to their complementary bases in an area known as the replication fork. 1 The first step in the process is the ‘unzipping’ of the two strands. DNA helicase moves along the double helix, unwinding the two strands, which separate from one another as the relatively weak hydrogen bonds between the bases are broken. 2 The unpaired nucleotides are exposed and each single strand now acts as a template for the formation of a new complementary strand. Free nucleotides move into place: C pairs with G and A pairs with T. 3 The free nucleotide bases form complementary pairs with the bases on the single DNA strands. DNA polymerase is the enzyme involved in linking the new nucleotides into place. Finally, the two new DNA molecules are rewound, each one forming a new double helix. The two new DNA strands that are produced are absolutely identical to the original strands. Complementary base pairing between the template strand and the new strand ensures that an accurate copy of the original DNA is made every time replication occurs. DNA replication is said to be semi-conservative because no DNA molecule is ever completely new. Every double helix contains one ‘original’ and one ‘new’ strand.
Taq DNA polymerase is used in the polymerase chain reaction (PCR) to produce multiple copies of DNA for forensic examination of small DNA samples (Subtopic 3.5). Taq polymerase is used because it is stable at high temperatures. It is named after the thermophilic (heat tolerant) bacterium Thermus aquaticus from which it was originally isolated.
3 base pairing between the bases on opposite strands, and condensation reactions between pentose and phosphate in the new strand make new polynucleotide strands - one strand acting as a template for the other
2 free nucleotides diffuse into position
C
1 hydrogen bonds between base pairs are broken - DNA ‘unzips’
A T
C G
A T
C C
G C
T
C
T
G
A
A
G
A
G
T
C
G two new strands
A C
G
A G
G
C
T
T
G
A
A
G C
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T
A
T
T
Figure 2.24 DNA replication.
C
A
G
T
C