Prokaryotic Gene Structure 3 Flashcards
horizontal gene transfer
how do bacteria replicate?
through process of asexual reproduction process called binary fission
what is DNase I ? (2)
+ what it acts on
- an endonuclease that nonspecifically cleaves DNA,
- acts on single & double stranded DNA, chromatin & RNA:DNA hybrids
what are the 3 types of horizontal gene transfer (lateral gene transfer)?
1. conjugation
2. transduction
3. transformation
how does HGT occur?
through mobile genetic elements (MGEs) (plasmids, transposons, viruses) that enter bacterial cell
what makes up the total genome?
bacterial genome + MGEs
what is the supergenome?
total number of genes available to bacterial community (large number of cells) via MGEs within same environment
what is metagenome?
the collection of supergenomes within the wider environment
what is conjugation?
donor bacterial cells transfers plasmid DNA to a recipient bacterial cell only when both cells are in physical contact through a specialised pilus (sex pilus)
what is transformation?
recipient bacterial cell takes up cell-free/naked DNA from the environment
what is transduction?
transfer of DNA from donor bacterial cell to recipient that is mediated by a bacteriophage
what are de novo mutations? (3)
- single nucleotide polymorphisms (SNPs)
- insertions
- deletions (indels result in frameshift)
how do plasmids vary in size?
< 2000 bp to > 100 000 bp
what are the 2 forms of plasmids?
circular dsDNA (common)
linear dsDNA (rare)
what are the 6 types of plasmids? (12)
- fertility (F) plasmids (direct conjugation)
- resistance (R) plasmids (resistance to antibacterial agents)
- col plasmids **(for colicins, bacteriocins in pneumococcal defense)
- metabolic (degradative) plasmids (for metabolism of unusual molecules)
- virulence plasmids (pathogenicity to host bacterium)
- cryptic (no obvious phenotype)
what was the conclusion of lederberg & tatum’s experiment?
auxotrophic bacteria can be converted to prototrophs through gene transfer & recombination
what was the conclusion of davis’ experiment?
physical contact between 2 bacterial cells is required for conjugation
what did william hayes discover about genetic transfer? (2)
was polar (occurred in one direction), thus not reciprocal (one is a definite donor and definite recipient)
what is F+?
donors that donate fertility factor (F)
what is F- ?
recipient cells of the fertility factor
what is mating pair formation?
conjugation bridge/pore forms between F+ and F- cells, membranes fuse to allow DNA to pass through
what are the steps of conjugation (F+ x F- mating) (4)
- F+ produces sex pilus to contact F- → contact made → pilus contracts to bring F- cell closer
- mating pair formation
- one DNA strand on F plasmid is nicked at oriT & separates → intact F plasmid replicated in F+ cell. 5’ end of nicked DNA passes into F- via pore. as ssDNA of F plasmid enters F- = replicated to become dsDNA producing F plasmid copy (F- becomes F+)
- F+ plasmids produce surface exclusion proteins (prevent host cells from conjugating w other F+ cells)
what are the two surface exclusion proteins ?
and what do they do
- TraT blocks mating pair formation
- TraS blocks DNA transfer at pore
traS: Stops (after connection)
traT: Terminate (before connect)
what are Hfr strains?
bacteria with integrated plasmid that can transfer their chromosome & transfers chromosomal genes with high efficiency
what are the two methods through which plasmids can integrate into chromosomes?
- recombination between shared sequences on plasmid & chromosome
- recombination via insertion elements shared between chromosome & plasmid
what are the 3 steps of episomal insertion?
- association of plasmid & bacterial insertion sequences
- integration of F plasmid in bacterial chromo
- F plasmid intact (with O origin of transfer sequence → directs transfer of genetic material between bacterial cells)
integration is reversible (F plasmid can be excised & exist independently again)
what happens when Hfr cells mate with F+ cells?
Hfr cells revert to F+ cells if F plasmid is excised from chromosome
how is it formed
what is the F’ plasmid
when the F episome excises itself from chromosome and an error occurs, where it picks up a portion of the chromosome, making it distinct from the F plasmid
what is a potential result from the mating of F’ x F-?
can transfer bacterial genes acquired during excision, recipients becomes F’ and is a partial diploid (merozygote)
explain the general concepts of transformation
- bacterial uptake mechanisms allow them to take up DNA
- DNA fragment in medium binds to cell-surface receptor
- extracellular endonuclease cuts dsDNA into smaller fragments
- one DNA strand degraded = remaining strand transported into cell
- single strand aligns itself with homologous region on bacterial chromosome (recombination)
what was the conclusion of hershey & chase’s study
only DNA of virus needs to enter a bacterium to infect it
what was the conclusion of avery, mcleod & mccarthy experiment
DNA carries info responsible for transformation
what are the two types of transformation?
natural & artificial
what are competent cells?
cells that can take up DNA
how can bacteria be artificially induced tp become competent
- chemical treatment (Ca ions makes bacteria competent - increase permeability of membrane)
- electroporation (wash w cold, non ionic solution to prevent arcing = high potential diff is applied across washed bacterial cells mixed with naked DNA = strong electric field creates hydrophilic pores
what was the conclusion of ledeberg & zinder’s experiment?
genetic exchange didn’t take place by conjugation (bacteriophage was agent of transfer)
why can bacterial genes incorporate into a phage capsid
because of errors made during virus life cycle
what are the 2 types of transduction?
- generalised transduction
- specialised transduction
describe the steps of the bacteriophage lytic cycle (4)
- adsoprtion & penetration (phage attaches to specific receptor on cell & injects phage dsDNA)
- phage forces bacteria to make virus DNA & proteins
- daughter phages assembled (virus DNA is packaged into virus protein coat)
- mature viruses released by cell lysis
what are virulent phages ?
phages that reproduce using a lytic cycle
explain generalised transduction
1.phage DNA enters host, bacterial chromosome is degraded
2. packaging of viral chromosomes into capsid = some fragments of degraded bacterial DNA can be packaged → generalised transducing particle
3. bacterial DNA is injected by transducing particle into another bacterial cell (no initiation of lytic cycle because no phage DNA)
4. 3 possible outcomes for injected bacterial dsDNA
occurs during lytic cycle
what are the 3 possible outcomes for injected bacterial dsDNA?
- integration into host genome (stable)
- non-integration but genes expressed (abortive transduction – bacterial cell is a partial diploid)
- degradation of DNA
explain specialised transduction (3)
- carried out only by temperate phages that have established lysogeny
- only specific portion of genome transferred
- when prophage is incorrectly excised
