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Summary Genetics Chapter 7
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Chapter 7 genetic transfer and mapping in bacteria and bacteriophagesGenetic transfer mechanisms in Like sexual reproduction in eukaryotes, genetic transfer in bacteria enhances
bacteria genetic diversity. Transfer of genetic material from one bacterium to another can
occur in three ways:
The patterns of inheritance in bacteria 1. Conjugation: Involves direct physical contact
are studied using different 2. Transduction: Involves viruses
mechanisms than eukaryotes
3. Transformation: Involves uptake from the environment
Conjugation Bernard Davis later showed that the bacterial strains that seem to genetic
properties must make physical contact for transfer to occur. Using a U-tube.
Minimal medium = a growth medium
that contains the essential nutrients + = functional gene
for a wild-type (nonmutant) bacterial - = mutation caused
species to grow. gene to be inactive
Heterotrophes = bacterial strains that Conjugation = refers
had different nutritional growth to the transfer of
requirements. DNA from one
bacterium to
Auxotroph = a stain that cannot another, following
synthesize a particular nutrient and direct cell-to cell
needs that nutrient to be added to its contact.
growth medium.
Interrupted mating
Prototroph = does not need a = a process by
particular nutrient included in its which conjugation is stopped using a blender. May be used to determine the
growth medium. relative position of bacterial genes.
A histidine, methionine auxotroph Many, but not all, species of bacteria can conjugate. Moreover, only certain
cannot synthesize histidine and strains of a bacterium can act as donor cells. Those strains contain a small circular
methionine. piece of DNA termed the F factor (for Fertility factor, = plasmid). Strains
containing the F factor are designated F+ or F’, located on plasmids. Those lacking
The process of conjugation may it are F– . Plasmid is the general term used to describe extra-chromosomal DNA.
introduce new alleles into the Plasmids, which are
recipient strain, altering its genotype. transmitted via
conjugation, such as
An episome is a segment of bacterial F factors, are
DNA that can exist as a plasmid or be termed conjugative
integrated into the bacterial plasmids. These
chromosome. plasmids carry
genes required for
conjugation.
The protein complex that initiates the The first step in conjugation is the contact between donor and recipient cells. This
process of conjugation following is mediated by sex pili (or F pili) which are made only by F + strains. These pili act as
contact between two bacteria is attachment sites for the F– bacteria. Once contact is made, the pili shorten,
called the relaxosome. contract. Donor and recipient cell are drawn closer together. A conjugation bridge
is formed between the two cells.
A structure called sex pilus allows the The second step is the transfer of the F factor DNA. Genes within F factor encode
transfer of genetic information during protein complex = relaxosome, this complex recognizes DNA sequence in F factor
conjugation. => origin of transfer. Upon recognition, relaxosome cuts one DNA strand and
catalyzes separation -> only the cut DNA strand is called tDNA and is transferred
Donor strains can convert recipient to recipient cell. Protein in relaxosome called relaxase remains bound to cut DNA
strains into donor strains. strand = complex = single strand DNA + relaxase = nucleoprotein.
, The nucleoprotein is recognized by a
coupling factor that promotes entry
into the exporter (protein complex
spans both inner and outer
membranes of donor cell). Once
pumped out of the donor cell it
travels through conjugantion bridge
in recipient cell. Relaxase joins ends
of linear DNA strand, this is
replicated and becomes double-
stranded. The other strand
remaining in donor cell -> DNA
replication restores F factor to
double-strand condition.
Reciepient cell has acquired an F
factor: F- -> F+ cell.
Genetic composition of donor cell
did not change.
In some cases, the F factor may
carry genes that were once found
on the bacterial chromosome.
These types of F factors are called F’
factors (F ‘prime’ factors).
F’ factors can be transferred through conjugation. This may introduce new genes
into the recipient and thereby alter its genotype:
Hfr strains: F genes on chromosome These strains are derived from F+
High frequency of recombination. strains. Hfr cell: F factor
Strains that are very efficient at integrates in bacterial
transferring chromosomal genes. chromosome; formed by F factor
alignment with similar region in
bacterial chromosome ->
Episome = is a F factor that can recombination.
integrate into the chromosome.
William Hayes demonstrated that
When F factor is imprecisely excised - conjugation between an Hfr and an F– strain involves the transfer of a portion of the Hfr
> F’ factor is created and carries a bacterial chromosome
portion of bacterial chromosome. The origin of transfer of the integrated F factor determines the starting point and direction
of the transfer process. The cut, or nicked site is the starting point that will enter the F –
cell. One strand of Hfr chromsome begins to enter the F- cell in a linear manner. Because
it takes too long to enter the whole Hfr cell, usually only a portion of the Hfr is transmitted
into the F- cell.
Once inside
the F- cell,
chromosomal
material from
Hfr can swap
Mating that would have the highest
or recombine,
frequency of chromosomal gene with the
transfer: Hfr X F- homologous
region.
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