Click on thumbnails for a larger view and a
description of the bacteria.
Bacterium DNA:
1. singular and circular
2. found in the nucleoid region of
the bacterial cell
3. contain a small double strand
ring of DNA called PLASMIDS.
Most bacteria reproduce by binary
fission. There is only one
origin of replication.
2 replication forks move in
opposite directions. Bacteria can completely divide
in 20 minutes under optimal conditions.
Transfer and recombination of
Bacterial genes:
There are three natural processes
of gene transfer in bacterium:
1. Transformation: Bacteria
picks up DNA from its environment. Avery's experiment.
2. Transduction: Gene transfer
from one bacterium to another using a bacteriophage.
a). General transduction:
random pieces of host DNA are packaged in within a
phage capsid during the lytic cycle.
b). Restricted transduction:
when a temperate phage takes some of the host's DNA
with it when it forms a new virus.
3. Conjugation:
transfer of genetic material between 2 cells that
are temporarily joined.
a). The cell donating the DNA extends
an external appendage called a sex pili.
b). This pili attaches to the cell
receiving the DNA.
c). A cytoplasmic bridge forms which
the DNA is transferred.
The ability for the sex pili to form
is conferred by genes in a plasmid called the F
factor. (fertility factor). Cells
with the F factor are designated F+ and cells without
it are designated F-. During conjugation between an
F+ and an F- bacterium both cells finish up as F+
forms. Sometime the the F factor inserts into the
circular chromosome. Integrated F factor is still
expressed. It is designated Hfr. (high frequency
of recombination).
Transfer of genetic material by
conjugation can help scientists map the chromosomes
of bacteria.
1. Specific strains of Hfr bacteria
always transfer genes in the same sequence.
2. The duration of conjugation determines
the number of genes transferred.
Here are some common morphological
types of bacteria. The Eubacteria
and the Archaebacteria,
which are collectively known as the "procaryotes:"
