ABSTRACT
Transfer and Evolution” section. The chromosome and plas-
mids may also carry “integrons.” These are DNA elements
that can sponsor the integration into them of multiple “gene
cassettes” by a type of site-specific recombination that is
similar to that used by lysogenic bacteriophage when they
integrate into the host chromosome. The “gene cassettes”
can carry genes that provide resistance to antibacterial
DNA Replication, Mutation, and Repair
Bacterial chromosomal DNA is replicated bidirectionally
from a single origin in a semiconservative manner (that is,
each new double-stranded DNA molecule has one single
strand of DNA conserved from the parent double helix and
one newly synthesized DNA single strand). At each of the
two resulting replication forks, one newly synthesized
single strand of DNA is polymerized as a long, continuous
piece and the other in small pieces (Okazaki fragments)
that are then ligated together (in both cases the parental
single strands act as templates). Thus, replication is also
said to be semidiscontinuous. Both new single strands at
each replication fork appear to be synthesized by a single
replication apparatus, which contains two catalytic centers,
one for each strand. Plasmids are also replicated by semi-
conservative mechanisms from single origins, although
with details that may differ from those of chromosomal
replication. Mechanisms for replication of bacteriophage
genomes vary depending on their type (RNA or DNA) and
form (single or double stranded), but in all cases, including
replication of double-stranded chromosomes and plasmids,
the standard base pairing rules allow the template strand to
accurately determine the base sequence of the newly
synthesized strand.
