Bacterial Reproduction: Bacterial Conjugation
o Bacteria (prokaryotes) CAN’T undergo meiosis or mitosis, don’t have centrioles, and they can ONLY undergo Asexual Reproduction.
o The type of cell division they are capable of undergoing is called binary fission, which exhibits exponential growth.
o In binary fission, the circular DNA is replicated in a process similar to replication in eukaryotes.
o Two DNA polymerases begin at the same point on the circle (ORI) and move in opposite directions making complementary single strands that combine with their template strands to form two complete DNA double stranded circles.
o The cell then divides, leaving one circular chromosome in each daughter cell.
o The daughter cells are genetically identical.
There are three types of genetic recombination that can occur in bacterium:
I. Bacterial Conjugation
o Bacterial Conjugation requires that one of the bacterium have a plasmid with the gene that codes for the sex pilus.
o If the plasmid can integrate into the chromosome of the host it’s called an episome.
o In order for a bacterium to initiate conjugation, it must contain a conjugative plasmid.
o Conjugative plasmids possess the gene for the sex pilus.
o The sex pilus a hollow protein tube that connects the two bacteria to allow the passage of DNA.
o The plasmid replicates differently than the circular chromosome.
o One strand is nicked, and one end of this strand begins to separate from its complement as its replacement is replicated.
o The loose strand is then replicated and fed through the pilus.
o Two plasmids of interest are: the F plasmid and the R plasmid.
o The F plasmid is called the fertility factor or F factor.
o It allows the production of the sex pilus.
o A bacterium with an F factor is designated as F+, and one without it is designated F-.
o The F plasmid can be in the form of an episome and if the sex pilus is made while the F factor is integrated into the chromosome, then some or the entire chromosome may be replicated and transferred.
Bacterial Conjugation Diagram
o R-plasmids often contain resistance genes coding for multiple antibiotic resistance.
o As well as resistance transfer genes, they also code for the production of a conjugation (sex) pilus.
o The conjugation pilus enables the donor bacterium to transfer a copy of the R-plasmid to a recipient bacterium, making it also multiple antibiotic resistant and able to produce a conjugation pilus.
o Conjugation is conservative because the donor retains a complete original copy of the plasmid after the transfer is complete.
II. Bacterial Transformation
Transformation is the process by which bacteria may incorporate DNA from the external environment into their genome.
III. Bacterial Transduction
o Sometimes the capsid of a bacteriophage will mistakenly encapsulate a DNA fragment of the host cell.
o When these new virions infect a new bacterium, they inject harmless bacterial DNA fragments instead of virulent viral DNA fragments.
o This type of genetic recombination is called transduction.
o The virus mediates that mediates transduction is called the vector.
o This can be done artificially in a lab.
o Mnemonic for transduction: the bacteriophage induces a change.
Bacterial Conjugation Citations