Nucleic Acids in Chemistry and Biology

base pairing determines the identity of the nucleotide to be added at each step during the replication
process. In addition to DNA, many other components are required to ensure faithful copying of the DNA.

6.6.4.1.1 DNA Topology. DNA cannot be copied unless the complementary DNA strands are first

unwound. One problem is that because one strand is wound round the other, unwinding one region by
pulling the two strands apart leads to an increase in the number of superhelical turns (supercoils) in
another adjacent region (Section 2.3.5). The DNA replication machinery therefore needs to relieve these
extra turns as replication proceeds. This problem is solved by the use of a DNA topoisomerase^25 to relax
these extra supercoils as they are generated during replication.

6.6.4.1.2 Strand Polarity. The two DNA strands in a double helix have opposite polarities (Section

2.2). Every enzyme in nature that copies DNA or RNA into DNA or RNA does so by adding single nucleotides
to the 3-end of the elongating strand (i.e., replication proceeds in a 5to 3direction). If DNA replication
is to proceed in a given direction along a duplex, a ‘replication fork’ must migrate in that direction.^46 Only
one DNA strand can be copied in that direction, the ‘leading strand’ and the new DNA elongated continu-
ously. The other strand (the ‘lagging strand’) must be copied in the reverse direction and is elongated dis-
continuously^47 (Figure 6.25).

6.6.4.1.3 Semi-Conservative DNA Replication. In principle there are at least three different

ways that DNA could be copied (Figure 6.26). The correct mechanism was shown by Meselson and Stahl

232 Chapter 6

Figure 6.25 The replication fork and the polarity of DNA replication. Black lines indicate old DNA and red lines
indicate newly synthesised DNA. The direction of movement of the replication fork and the direction of
synthesis of DNA are both shown by red arrows

TFIIIA TFIIIC

Figure 6.24 DNA control elements and transcription factors involved in transcriptional initiation by RNA
polymerase III. TF: Transcription factor

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