section 24.1
DNA Replication
547
F I G U R E 2 4 -1
Semiconservative replication of DNA. The two replicas consist of one
parental strand plus one daughter strand. Each base in a daughter strand is
selected by the requirement that it form a base pair with the parental base.
integrates into the bacterial chromosome, it can replicate
its own DNA and bacterial DNA as well during
conju-
gation,
a process in which bacteria physically join and
transfer DNA from the donor bacterium to the recipient
bacterium. In this situation, bacteria replicate DNA si-
multaneously in a bidirectional manner from their own
origin of replication (oric) and unidirectionally from the
origin of the integrated F factor (oriF). Such replication
clearly raises topological problems during DNA replica-
tion (Figure 24-3). Studies of bacterial and plasmid DNA
replication led to the concept of a
replicon,
which is:
1. A site on DNA consisting of a sequence of
nucleotides that defines an origin of replication, and
2. Structural genes coding for proteins that recognize
and bind to the origin to initiate DNA replication.
F I G U R E 2 4 - 3
Replication of bacterial DNA with an integrated F factor. There are two
origins of DNA replication, oric and oriF. Replication is bidirectional from
oric and unidirectional from oriF. Somehow the cell solves this topological
dilemma when cell division occurs.
These genes and the segment of DNA that is replicated
from the origin are collectively defined as a replicon. Thus,
a bacterial chromosome or a plasmid is a single replicon,
whereas a single human chromosome may contain thou-
sands of replicons.
Discontinuous DNA Replication
In the diagram of replication shown in Figure 24-1,
both daughter molecules are shown as having continuous
strands. In truth, DNA cannot replicate by copying both
strands continuously because DNA polymerases, the en-
zymes that synthesize new DNA, can
only
add nucleotides
to a 3'-OH group. Because of the antiparallel nature of the
F I G U R E 2 4 - 2
Rolling-circle or
a
replication. Newly synthesized DNA is shown in color.
