W4 Microbial Genetics

Question 1

how do organisms evolve

Answer 1

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

Question 2

genetics of prokaryotes

Answer 2

circular chromosome (+plasmids sometimes)

single chromosome per genome

haploid and asexual reproduction through binary fission

Question 3

DNA in different bacteria and eukaryotes

Answer 3

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

Question 4

difference between horizontal and vertical gene transfer

Answer 4

horizontal: peer to peer (can be different species)

vertical: parent to child

Question 5

definitions of exogenote, endogenote and merozygote

Answer 5

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

Question 6

different methods of horizontal gene transfer

Answer 6

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)

Question 7

potential outcomes after dna uptake in horizontal gene transfer

Answer 7

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

Question 8

3 types of recombination at molecular level

Answer 8

homologous recombination

site specific recombination

transposition

Question 9

what is homologous recombination

Answer 9

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

Question 10

what is nonreciprocal homologous recombination

Answer 10

incorporation of single strand of dna into chromosome > form stretch of heteroduplex dna

occur during bacteria transformation

Question 11

difference between reciprocal and nonreciprocal recombination

Answer 11

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

Question 12

what is site specific recombination

Answer 12

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

Question 13

what are transposable elements

Answer 13

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

Question 14

definition of composite transposons

Answer 14

transposable elements which contain genes other than those used for transposition

Question 15

difference between cut and paste transposition and copy and paste transposition

Answer 15

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

Question 16

process of conjugation

Answer 16

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

Question 17

what is high frequency recombination (Hfr) conjugation

Answer 17

when donor cell has F factor integrated into its chromosome

Question 18

what is F’ conjugation

Answer 18

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

Question 19

what is bacteria transformation

Answer 19

uptake of naked dna by competent cell > incorporation of the dna into recipient cell’s genome

dna made by linear or intact plasmid

Question 20

process of transformation

Answer 20

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

Question 21

what is generalised transduction

Answer 21

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

Question 22

what is specialised transduction

Answer 22

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