DNA, Gene Structure, and DNA Replication - Insect Molecular Genetics - page 21

Figure 1.11 During DNA replication, only part of the DNA molecule upzips to allow synthesis of
new DNA strands. Helicases break the hydrogen bonds. In this example, replication begins at an
origin of replication. Eukaryotes have many origins of replication along their chromosomes so that
replication can occur rapidly. To keep the strands from reannealing at the replication forks where
synthesis is occurring, single-strand binding proteins (SSBs) attach.
1.17 Replication of DNA Requires an RNA Primer
Another complication of DNA synthesis is that synthesis an RNA primer
( Figure 1.13 ). Apparently, the first few (50-75) nucleotides attached to either
the leading or lagging strands are not deoxyribonucleotides but rather ribo-
nucleotides that are put in place by an RNA polymerase called primase . Once
these ribonucleotides have been polymerized on the DNA template, the primase
detaches, and DNA polymerase is able to synthesize DNA ( Figure 1.14 ).
1.18 Ligation of Replicated DNA Fragments
After the Okazaki fragments (sequences complementary to the lagging strand of
DNA) are produced, they must be joined together to produce a continuous strand
( Figure 1.12 ). On the lagging strand, DNA polymerase III of Escherichia coli stops