W4 Microbial Genetics Flashcards
how do organisms evolve
drift (diversification) > selection (fixation and loss)
heritable changes in genetic code:
- drift: mutation, chromosomal rearrangement
- horizontal gene exchange: plasmids, transposons, DNA uptake and recombination, viruses and bacteriophage
genetics of prokaryotes
circular chromosome (+plasmids sometimes)
single chromosome per genome
haploid and asexual reproduction through binary fission
DNA in different bacteria and eukaryotes
dna in most bacteria circular (bidirectional replication from single origin)
exception for bacteria: borrelia burgdorferi, streptomyces and coxiella burnetii (all carry single linear chromosome)
archaea also circular but may have more than one origin > more than 2 replication forks
eukaryotic: linear chromosomes with many replication forks
difference between horizontal and vertical gene transfer
horizontal: peer to peer (can be different species)
vertical: parent to child
definitions of exogenote, endogenote and merozygote
exogenote: dna of donor that is transferred into recipient cell
endogenote: genome of recipient cell
merozygote: recipient cell that is temporarily diploid as result of transfer process
different methods of horizontal gene transfer
conjugation: transfer of plasmid directly from donor
transduction: transfer of dna fragment by viral delivery (bacteriophage)
transformation: transfer of free dna (which ma be linear fragments or in plasmid forms)
potential outcomes after dna uptake in horizontal gene transfer
integration of donor dna > stable recombinants
donor dna self replicates > stable recombinants
donor dna cannot see replicate > just becomes a loose piece of dna > no stable recombinants
host restriction > dna gets disintegrated > no stable recombinants
3 types of recombination at molecular level
homologous recombination
site specific recombination
transposition
what is homologous recombination
usually involves a reciprocal exchange between pair or dna molecules with same nucleotide sequence
initiates often with double strand breaks
complementary to a certain extent
what is nonreciprocal homologous recombination
incorporation of single strand of dna into chromosome > form stretch of heteroduplex dna
occur during bacteria transformation
difference between reciprocal and nonreciprocal recombination
reciprocal: both recombinants gets new exchanged parts from partner
nonreciprocal: only one of them gets the dna part from partner > partner simple gives part of itself away and gets discarded
what is site specific recombination
involves exchange or integration of dna at defined, short recognition sequences
important in insertion of viral genome into host chromosomes
only one small region of homology between inserted genetic material and host chromosome
what are transposable elements
segments of dna that move about in the genome during transposition > can be integrated into different sites in chromosome
transposition may be cut and paste mode or copy and paste mode
definition of composite transposons
transposable elements which contain genes other than those used for transposition
difference between cut and paste transposition and copy and paste transposition
cut and paste: transposable element cut from original location > inserted into new target site > does not leave copy of original element in old location
copy and paste: new copy of transposable element made and inserted into target site > original element remains in its original site
process of conjugation
F+ cell forms sex pilus > pulls F- cell closer
relaxosome makes cut at origin of transfer and separate dna > transfer dna into F- cell via mating bridge > replicated via rolling circle replication so that both F- and F+ will be double stranded
what is high frequency recombination (Hfr) conjugation
when donor cell has F factor integrated into its chromosome
what is F’ conjugation
results when the F factor incorrectly leaves the host chromosome
some of the F factor is left behind in the host chromosome
some host genes have been removed along with some of the F factor > can be transferred to a second host cell by conjugation
what is bacteria transformation
uptake of naked dna by competent cell > incorporation of the dna into recipient cell’s genome
dna made by linear or intact plasmid
process of transformation
dna fragment binds to cell surface receptor > cut by extracellular endonuclease into smaller fragments > one strand degraded and single strand transported into cell > dna strand aligns itself with homologous region on bacterial chromosome > incorporated via homologous recombination
heteroduplex dna repaired in a way that changes lac- strand to create lac+ gene
what is generalised transduction
any part of bacterial genome can be transferred
occurs during lytic cycle of virulent phage
during host assembly, fragments of host dna mistakenly packaged into phage head
what is specialised transduction
carried out only by temperate phages that have established lysogen
only genes located near the phage’s integration site are transferred
occurs when prophage is incorrectly excised
