113 BIOLOGY - MICROBIOLOGY
Lecture 15: Microbial Genetics - Genetic transfer and recombination in bacteria
Genetic recombination refers to processes by which new genetic information is provided to a cell
- Genetic recombination may involve exchange of genes between molecules, which generates new
combinations of genes on a chromosome
= Homologous sequences in two DNA molecules may undergo a process of breaking and
rejoining - crossing over - and thus exchange sequences (Tortora et al., Figure 8.23)
= Crossing over occurs during meiosis in eukaryotes and during the processes of
transformation, Hfr-mediated conjugation and transduction (see below) in prokaryotes
- Genetic recombination may also involve acquisition of DNA for which there is no homologous
DNA in the "recipient", as in most bacterial plasmid transfers
- In a sense, a zygote is a recombinant cell since it contains a combination of genes not found cells
of either parent
Genetic material can be transferred between bacteria in several ways; all of these ways are
fundamentally different from the process of mating by which genetic recombinants are generated in
eucaryotic organisms
- In eukaryotes, we consider two "parents" making an equal contribution to the recombinant
genome of their "progeny"
- In bacteria, transfer of genetic material is one-way, from donor to recipient
= The donor provides (not necessarily of its own volition!) a fragment of its DNA
= The recipient has its genotype changed through recombination of its genome with the DNA
from the recipient
In 1928, Frederick Griffith demonstrated that live cells of an avirulent strain of Streptococcus
pneumoniae could be transformed by something (later shown to be DNA) from dead cells of a
virulent strain of the same species (Tortora et al., Figure 8.24)
- These experiments employed two strains of S. pneumoniae
= The virulent strain possessed a capsule that allowed to cause a lethal infection in mice
= The avirulent strain lacked the genetic information necessary to synthesize the capsule, and
thus could not infect mice
= Not surprisingly, heat-killed cells of the virulent strain were harmless to mice
- When avirulent S. pneumoniae cells were injected into mice along with killed virulent cells, the
mice died
= Cells of encapsulated S. pneumoniae could be recovered from the dead mice; these were fully
virulent in further experiments
= Griffith also showed that, when avirulent, capsuleless cells were incubated in vitro with heatkilled encapsulated cells, and spread onto plates of agar medium, many of the resultant
colonies were made up of cells producint capsules
- Griffith's conclusion, confirmed by other scientists, was that the avirulent cells had acquired the
genetic information needed to produce capsules from the dead virulent cells
- Later experiments, in the laboratory of Oswald Avery, demonstrated that the ability to produce
capsules could be transferred by purified DNA
- The phenomenon of transformation occurs in several, though by no means in all, species of
bacteria
= Today, transformation is understood to involve uptake of free DNA (released by dead donor
cells) by recipient cells
= The ability to take up DNA is a genetically-encoded characteristic of the competent recipients,
and may involve the presence of DNA receptors on the cell surface
- A fragment of donor DNA, after it is taken up by a competent recipient, may recombine with a
homologous sequence in the recipient's chromosome (Tortora et al., Figure 8.25)
Conjugation is a type of genetic recombination in bacteria mediated by plasmids
- Plasmids are circular DNA molecules that are replicated independently of the cell's chromosome
- Some plasmids encode gene that products allow them to move from one cell into another
= Among these products are sex pili which cause the cell carrying the plasmid - the donor - to
stick to other cells (Tortora et al., Figure 8.26)
= Following attachment, the plasmid is is replicated and a single strand is transferred (maybe
through the sex pilus, maybe not) to the recipient cell, where the complementary strand is
synthesized (Tortora et al., Figure 8.27(a))
- In Escherichia coli, the F factor was the first plasmid observed to transfer between cells during
conjugation (Tortora et al., Figure 8.26)
= F+ donors transfer the F factor to F- recipients, which become F+ cells as a result (Tortora et
al., Figure 8.27(a))
= In some cells, the F factor recombines with the chromosome, forming an Hfr (High
frequency of recombination) donor (Tortora et al., Figure 8.27(b))
= When the integrated F factor begins to replicate and move into an F- recipient, it carries in a
portion of the donor's chromosome (Tortora et al., Figure 8.27(c)); once in the recipient, the
fragment of donor chromosome can recombine with the recipient's chromosome
= Usually, only a portion of the donor chromosome is transferred; consequently, the recipient
remains F- (Why?)
- Bacteria "acquire" resistance to antibiotics by serving as recipients of plasmids that include
genes encoding proteins that determine antibiotic resistance
Generalized transduction in a process by which donor DNA is carried into a recipient cell inside of
a bacterial virus, or bacteriophage (Tortora et al., Figure 8.28)
- Bacteriophages (or simply, phages) reproduce by injecting their DNA into cells, where it is
expressed to synthesize phage proteins and enzymes used in construction of the progeny phage
particles
- During the infection, copies of the phage DNA are synthesized by replication and packaged into
newly-synthesized protein coats
- Sometimes, fragments of the infected cell's chromosome (which is usually broken up during the
infection) are "accidentally" packaged into phage particles
- These transducing particles - donor DNA in a phage protein coat - can inject the donor DNA
into recipient cells, where it can recombine with the recipient's chromosome
Acquisition and transfer of extrachromosomal genetic elements by horizontal gene transfer
contribute to the evolution of bacteria
- Plasmids are very common in many species of bacteria, and may include genes that enable their
hosts to survive in new environments
= Plasmids may be conjugative, allowing the plasmid to be transferred from one cell to another
= Plasmids may carry genes for virulence factors, proteins that enhance the pathogenicity of a
bacterium; it is a plasmid-encoded toxin that makes Escherichia coli O157 a dangerous
pathogen while most E. coli strains are harmless commensal bacteria
= R factors are plasmids that carry genes for proteins that provide resistance to antibiotics
A single plasmid may carry genes for resistance to multiple antibiotics (Tortora et al.,
Figure 8.29); if the plasmid is conjugative, a bacterium can simultaneously acquire
resistance to multiple antibiotics
The spread of R factors has compromised our ability to treat bacterial infections
- Transposons are segments of DNA that can move from one DNA molecule to another
= The phenomenon of transposition was discovered by Barbara McClintock working with corn
= The simplest transposons, insertion sequences, contain only a gene for the transposase
enzyme that catalyzes transposition and recognition sites in the form of inverted repeats
(Tortora et al., Figure 8.30(a))
= Complex transposons (Tortora et al., Figure 8.30(b)) may consist of multiple insertion
sequences along with additional genes