Bacterial Genetics
Transformation in prokaryotes refers to the uptake of naked DNA. It is one of the three
natural mechanisms of genetic exchange. Through this process cells acquire foreign
fragments of DNA, which they are then able to integrate into their own genomes either
through homologous recombination, or through the transposition of specific segments
of the incoming DNA referred to as cassettes. Either way the transformed cell may
benefit from the rescue of a defective or deleted allele, or may acquire novel functions
that become inserted into the host chromosome resulting in a change of both genotype
and phenotype.
Competency is a term used to describe a cell’s ability to take up the naked DNA. Some
strains produce proteins that facilitate this process. Others like E. coli are treated with
certain divalent cations like Ca++ which can make the cells competent.
Transduction is a process whereby DNA is introduced into a host cell with the help of a
phage particle or a bacterial virus. These bacteriophage are infectious particles and are
referred to as transducing phage. Phage infection ordinarily involves attachment to
specific receptors at the surface of host cell. The phage genome is then injected like a
hypodermic needle into the host. As the infection progresses new particles are
synthesized in the host. At the end of the infection the host is lysed and the phage
particles are released. Phage that follow this course are called lytic phage.
Often at some low frequency a small fragment of the host genome is inserted in to a new
phage particle in place of the phage genome. These particles are referred to as defective
phage and when they infect a new host they transduce (to lead across) the fragment of
DNA from the previous host into the new host. This fragment is then capable of
recombining with the genome of the new host, and as with transformation, the transduced
cell can be selected for and identified as a result of a change in phenotype. Often these
phage are able to rescue metablolic mutants
Biologists recognize two types of transduction: generalized transduction and specialized
transduction.
Generalized transduction results from infection with a transducing phage that has
randomly picked up a fragment of the host genome usually during a lytic infection.
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Specialized transduction follows in the specific case when the phage genome after it
enters the host, is silenced by repressors and the integrates into the host genome. A phage
that can integrate into a host chromosome is said to be a temperate phage and in its
integrated state is referred to as a prophage. A prophage can be induced under certain
conditions to excise itself from the host genome, and then go on to replicate and make
phage particles just as in the case of the lytic phage. Sometimes, at a low frequency,
when the prophage excises itself it takes a small fragment of the host chromosome to
which it was attached. This particle is capable of introducing the adjacent host DNA to a
new host upon subsequent infection.
Strains of bacteria that contain a prophage are called lysogens.
Phage infection can proceed either by the lysogenic or the lytic cycle.
The lytic cycle involves infection, replication and production of phage particles, and lyses
of the host bring about the release of phage.
The lysogenic cycle involves infection, and integration resulting in the silencing of the
phage genome. Prophage are said to piggyback on the host chromosome.
At a certain frequency prophage are induced out of their repressed slumber and then can
go on to enter the lytic cycle.
Conjugation is a third mechanism by which genetic material can be exchanged between
bacterial cells. It is often referred to as “bacterial sex”. Bacterial conjugation'
is the transfer of genetic material between bacteria
through direct cell-to-cell contact. Discovered in 1946 by
Joshua Lederberg and Edward Tatum.
Conjugation is mediated by a plasmid, a small extrachromosomal, circular DNA
molecule which is autonomous, that is capable of its own replication. These small
molecules are important for a number of different reasons.
Often they contain determinants or genes that confer resistance to specific antibiotics.
Such a plasmid is called an R factor. Plasmids capable of conjugation code for many
different genes that are required for the transfer. These genes are called tra genes.
The process of conjugation requires a sex pilus or conjugation bridge. This conduit is
composed often of a single protein. Cells that contain the plasmids are referred to as
“male” or “+” strains. Plus strains are only capable of conjugating with “minus” or
female cells that do not have a copy of the plasmid. In this way plasmid genes that confer
resistance to antibiotics, or which encode some other useful metabolic function can be
transferred to sensitive strains making them resistant.
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During conjugation the plasmid replicates in such a way that only one of the two strands
is transferred via the pilus. The other strand remains behind and is replicated. In this way
the male cell remains male, and the female cell becomes male.
Sometimes the plasmid integrates into the host genome, and when this plasmid moves to
a minus stain it is capable of bringing the host chromosome along with it. These strains
that have plasmids integrated into them are called hfrs for high frequency of
recombination. As with transformation and transduction the mobilized host chromosomal
material is capable of recombination with its new host.
Phage and plasmids that are capable of existing free or autonomous, as well as, in the
integrated state are called episomes. A plasmid that is capable if integrating into the host
chromosome therefore is an episome.
Horizontal Gene Transfer
Review food for thought
What are the three mechanisms responsible for genetic exchange in bacteria?
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By what mechanisms does the incoming DNA exchange with the host chromosome?
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What kind of cells are capable of taking up naked DNA?
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What is the definition of bacterial transformation?
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What is transduction?
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What is required for transduction?
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Distinguish between general and specialized transduction.
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Teachers
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Describe the lytic cycle of bacteriophage infection.
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What is a lysogen.
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How is a temperate phage different from a lytic phage?
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What is a prophage?
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What is the sex pilus?
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What is an episome?
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Why is an episomal plasmid that is capable of conjugating with a minus cell called an
Hfr?
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Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program
Summer 2008 Workshop in Biology and Multimedia for High School
Teachers
Harvard University Life Sciences – HHMI Outreach Program