to involve the preservation of one of the two parent strands in each of the two newly synthesised duplexes.
This is called semi-conservative replication, because half of the strands generated are old and half are new.
6.6.4.1.4 Origins and Direction of DNA Replication. In bacteria, fungi and viruses, DNA replica-
tion starts at distinct origins, but in higher eukaryotes such sites are far less easy to identify.^48 Replication of
all chromosomes proceeds in both directions, creating ‘bubbles’ that can be seen in electron micrographs
(Figure 6.27).
6.6.4.2 Priming of DNA Replication. The enzymes responsible for making a complementary copy
of a DNA are called DNA polymerases. But they can only elongate an existing duplex. Therefore, another
enzyme is needed to initiate the synthesis of a new strand on the DNA template. This enzyme is a specialised
RNA polymerase called a primase,^49 which occurs in both prokaryotes and eukaryotes. A short RNA
oligonucleotide is synthesised first on the DNA template strand and then a DNA strand is synthesised from
the 3-end of this short RNA molecule. The unwanted RNA primer is removed by a specialised ribonuclease
called RNase H, which digests only the RNA strand within an RNA–DNA duplex.
Genes and Genomes 233
Figure 6.26 DNA replication is semi-conservative. Only the top mode of DNA replication (semi-conservative
replication) is observed for replication of prokaryote and eukaryote chromosomes
Figure 6.27 DNA replication for eukaryotic and prokaryotic chromosomes is bi-directional. Newly synthesised DNA
is shown in red