Biomedical Engineering Reference
In-Depth Information
(a)
5
Strands separate
3
(b)
5
3
3
RNA primer
5
(c)
5
3
3
DNA synthesized
5
(d)
5
3
5
RNA primer removed
by DNA polymerase
3
(e)
5
3
5
3
FIGURE 10.3 Initiation of DNA synthesis requires the formation of an RNA primer.
proteins bind to DNA at the origin of replication, break hydrogen bonds in the local region of
the origin, and force the two DNA strands apart. When DNA replication begins, the two
strands separate to form a Y-shaped structure called a replication fork.
Movement of the fork must be facilitated by the energy-dependent action of DNA gyrase
and unwinding enzymes.InE. coli, the chromosome is circular. In E. coli (but not all organisms),
Original
(a)
Fork
Growth
3 3
5
3
3
5
5
Growth
(b)
Growth
ligase
R Pol
Pol III
Ribonucleotides
Growth
1
Pol I
2
4
3
FIGURE 10.4 Schematic of the steps in the replication of the bacterial chromosome. (a) A replicating bacterial
chromosome at a stage shortly after replication begun at the origin. The newly polymerized strands of DNA (wavy
lines) are synthesized in the C5-to-C3 direction, using the preexisting DNA strands (solid lines) as a template. The
process creates two replication forks, which travel in opposite directions until they meet on the opposite side of
the circular chromosome, completing the replication process. (b) A detailed view of one replicating fork, showing
the process where short lengths of DNA are synthesized and eventually joined to produce a continuous new strand
of DNA. For illustration, four short segments of nucleic acid are illustrated at various stages: (1) primer RNA
(thickened area) is being synthesized by an RNA polymerase (R Pol); Successively, in (2) DNA is being polymerized
to it by DNA polymerase III (pol III); in (3) a preceding primer RNA is being hydrolyzed, while DNA is being
polymerized in its place by the exonuclease and polymerase activities of DNA polymerase I (pol I); finally, (4) the
completed short segment of DNA is joined to the continuous strand by the action of DNA ligase.
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