chapter 6 part 1 Flashcards
genome simplicity
fewer genes & fewer bases than other organisms
haploid genomes
mutations can be observed directly because there i one copy of each gene
short term generation times
can be measured in minutes
enormous numbers of progeny
allow detection of rare events
ease of propagation
bacterial culture is easy & inexpensive & takes up very little space
numerous heritable differences
mutations are easily created, identified, isolated, & manipulated for study
why are bacteria useful genetic models?
-genome simplicity
-haploid genomes
-short generation times
-enormous numbers of progeny
-ease of propagation
-numerous heritable differences
how many chromosomes are bacterial genomes usually composed of?
a single chromosome that carries mostly essential genes
is the bacterial chromosome usually circular or linear?
it is a usually circular model of double stranded DNA
is a bacterial chromosome big or small compared to eukaryotic genomes?
small
it only contains a few thousand to a few million base pairs
how many plasmids do bacteria usually contain?
one or more
what are plasmids
small double-stranded circular DNA molecules containing non-essential genes
are plasmids usually bigger or smaller than bacterial chromosomes?
smaller
how many different kinds of plasmids are found in bacteria?
many different kinds of naturally occurring plasmids are found in bacteria
R (resistance) plasmid
- carries antibiotic resistance genes that can be transferred to recipient cells
F (fertility) plasmid
- contains genes that promote its own transfer from donors to recipients
what are the two types of plasmids
- R (resistance) plasmid & F (fertility) plasmid
can plasmids replicate independently of the bacterial chromosome?
yes, so that the number of plasmids per cell can increase rapidly
high copy number plasmids
plasmids that replicate independently where there are 50 or more of that plasmid per cell
low-copy number plasmids
present in one or two copes per bacterial cell & usually cannot replicate independently of the bacterial chromosome
do low-copy number plasmids replicate independently of the bacterial chromosome?
no
what are the three ways of recombination in prokaryotes?
conjugation
transformation
transduction
conjugation
the transfer of replicated DNA from a donor to a recepient
transformation
the uptake of DNA from the environment
transduction
the transfer of DNA from one bacterium to another by a viral vector
what does each method of recombination involve?
a one-way transfer of genetic material from a donor bacterial cell to a recipient cell
which form of recombination involves physical contact transfer?
conjugation
conjugation in detail
-the donor cell transfers some of its genetic info to a recipient cell
in conjugation, what is the genetic info transferred through?
a conjugation pilus or conjugation tube
what takes place between the donors & recipients
plasmid that facilitates a one-way-transfer
what determines the ability ot act is a donor?
the ‘fertility factor’ or (F) factor
do donor cells have the F factor?
they have the F factor so they are F+, while the recipient cells are called F- and lack an F factor
what is conjugation controlled by?
genes carried on the F plasmid
how many genes does the F factor (plasmid) encode for?
40 genes that control conjugation
what do the F factor genes direct formation of and what does it do?
an exporter structure that will move the donor DNA into the recipient cell
what does the relaxosome do?
cuts one strand of F factor DNA, which triggers DNA replication, and movement of F factor DNA to the recipient cel where its own DNA replication occurs
what is the recipient cell called after conjugation?
the exoconjugant cell
it is the recipient cell with its genetic info modified by receiving DNA from the donor cell
mechanism of conjugation 1
- a conjugation pilus forms between donor & recipient cells
- gene expression from the F factor produces a protein complex called the relaxosome
- relaxosome binds the origin of transfer (OriT) on the F factor & cleaves one phosphodiester bond on the T (transfer) strand of the F factor
- relaxosome partially degenerates, leaving relaxase attached to the free 5’ end of the T strand
mechanism of conjugation 2
-t strand transfer across the pilus is accompanied by a process called rolling circle replication, which starts at OriT
-rolling circle replication uses non-transferred DNA strand as a template & displaces the 5’ end of the T strand, freeing it for transfer into the recipient
-the recipient cell uses the imported DNA as a template for replication, by the end of the process, both cells contain complete double-stranded F factor
how many insertion sequence (IS) elements are there?
4
A large component of the F factor consists of four _______ ________ elements
insertion sequence
what are IS elements
moble segments of bacterial DNA that can transpose themsleves to new locations (like transposons)
when does an episome occur?
when a plasmid integrates into the bacterial chromosome
what forms an episome?
when circular elements like the F factor can integrate into the bacterial chromosome
what do we call it when the plasmid e=integrates into the bacterial chromosome?
Hfr cells
what does Hfr stand for
high frequency recombination
is complete transfer of the bacterial chromosome in Hfr accomplished?
no
what does gene transfer to recipient cells from Hfr strains occur by?
rolling circle replication
can linear dna in the recipient cell re-circularize in Hfr gene transfer?
no
where does homologous recombination occur?
between the transferred linear DNA & the circular chromosome of the recipient (Hfr gene transfer)
what produces partial diploids?
F’
what is derived from the abberant excision of the F factor from an Hfr chromosome?
an F’ donor bacterium that contains a functional F factor
what does the F’ factor contain
all its own DNA plus a segment of the bacterial chromosome
what are donor cells carrying an F factor called?
F’ cells
exconjugants that contain a complete F’ factor called what & why?
partial diploids
since they contain two copies of the bacterial chromosome genes found on the F’ factor
is the partial diploidy retained as a characteristc of the exconjugants & their descendants?
`yes
what is a merozygote
a state when a partially diploid bacterial cell is produced
what can partial diploids be used to do?
examine the mode of action of bacterial genes & their regulation
