chapter 6 part 1 Flashcards
in what ways are bacteria useful genetic models
- genome simplicity (fewer genes/bases)
- haploid genomes
- short generation times (min)
- enormous numbers of progeny
- ease of propagation
- numerous heritable differences
what are bacterial genomes usually composed of
single chromosome, which carries mostly essential genes
bacterial chromosome structure
covalently closed circular molecule of double-stranded DNA
how many base pairs do bacterial genomes usually have
few thousand to few million
plasmids
small double-stranded circular DNA molecules containing non-essential genes
are plasmids smaller or larger than bacterial chromosomes
smaller
plasmids are used in a variety of ____________ _____ ______________
recombinant DNA applications
2 types of plasmids
- R (resistance) plasmid
- F (fertility) plasmid
R plasmid
carries antibiotic resistance genes that can be transferred to recipient cells
fertility plasmid
contains genes that promote its own transfer from donors to recipients
high-copy number plasmids
plasmids that can replicate independently of the bacterial chromosome so that the number of plasmids per cell can increase rapidly
low-copy number plasmids
present in 1/2 copies per bacterial cell and usually cannot replicate independently of the bacterial chromosome
3 methods of recombination in prokaryotes
- conjugation
- transformation
- transduction
conjugation overview
transfer of replicated DNA from a donor to a recipient
transformation overview
uptake of DNA from the environment
transduction overview
transfer of DNA from one bacterium to another by a viral vector
each method of recombination involves what
one-way transfer of genetic material from a donor bacterial cell to a recipient cell
what is the hollow tube called where genetic information is transferred in conjugation
conjugation pilus/tube
which recombination method involves the direct physical union of two bacteria
conjugation
what is the ability to act as a donor determined by
fertility factor (F factor)
does the donor or recipient possess an F factor
donor - F+ cells
does the donor or recipient lack an F factor
recipient - F- cells
what is conjugation controlled by
genes carried on the F plasmid
what does the F factor plasmid control
40 genes that control conjugation
what do the F factor genes direct formation of
exported structure that will move the donor DNA into the recipient cell
relaxosome
cuts one strand of F factor DNA, triggering DNA replication, and movement of F factor DNA to the recipient cell where its own DNA replication occurs
exoconjugant cell
term for the recipient cell after conjugation
- recipient cell with its genetic information modified by receiving DNA from the donor cell
mechanism of conjugation 1
- conjugation plus forms between donor/recipient cells
- gene expression from F factor produces protein complex called relaxosome
- relaxosome binds to origin of transfer (ori) on F factor and cleaves one phosphodiester bond on T (transfer) strand of F factor
- relaxosome partially degenerates, leaving relaxase attached to free 5’end of T strand
mechanism of conjugation II
- relaxase facilitates movement of t strand through conjugation pilus into recipient cell
- T strand transfer across pilus is accompanies by a specialized process called rolling circle replication
- rolling circle replication uses non-transferred DNA strand as a template and displaces the 5’ end of T strand, freeing it for transfer into recipient
- recipient cell uses imported DNA as template for replication
at the end of conjugation, which cell contains a complete double-stranded F factor
both cells
how many insertion sequences does a large F factor contain
4
insertion sequence (IS) elements
mobile segments of bacterial NA that can transpose themselves to new locations (like transposons)
episome
when circular elements like the F factor or plasmid integrate into the bacterial chromosome
Hfr cells (high frequency recombination)
when an F factor forms an episome in bacteria
how does gene transfer to recipient cells from Her strains occur
rolling circle replication (F+ x F- conjugation)
what is not accomplished in gene transfer from Hfr cell
complete transfer of bacterial chromosome - it is too long
what happens when the segment of T strand DNA enters the recipient in transfer from Hfr cell
used to generate a double-stranded linear fragment
can linear DNA in the recipient cell re-circularize?
no
where does homologous recombination occur?
between transferred linear DNA and circular chromosome of recipient
how may the new exconjugant cell acquire 1+ donor genes?
homologous recombination between linear DNA and circular chromosome
F’ cells
donor cells carrying and F factor derived from excision of F factor from Hfr chromosome
F’ donor bacterium contains
functional F factor derived from aberrant (imperfect) excision of F factor from an Hfr chromosome)
what does resulting F’ factor contain
all its own DNA plus a segment of the bacterial chromosome
what are exconjugants that contain a complete F’ factor called
partial diploids
partial diploids
contain 2 copies of bacterial chromosome genes found on the F’ factor
how is partial diploidy retained
as a characteristic of the exconjugants and their descendants
merozygote
a state when a partially diploid bacterial cell is produced
