prokaryotic gene transfer Flashcards
(24 cards)
what are the three types of genetic transfer
transformation, transduction, conjugation
what is transformation
uptake of naked DNA molecule/fragment by a competent cell from the environment, and the DNA is incorporated into the recipient chromosome in a heritable form. free dna is found when bacteria lyse and the fragments can be large and contain many genes
describe competency when it comes to transformation
bacteria must be in certain pahse of growth- natural transformation only occurs in certian G pos and G neg genera i.e streptococcus (g pos) haemophilus (g neg)
describe transformation mechanism
in s. pneumoniae a competent cell binds to DNA molecule. the molecule is cleaved by endonuclease to small fragments 5-15kb
one strand is hydrolysed by exonuclease., other moves through plasma membrane + integrates by recombination
how does transformation differ in haemophilus
no production of competence factor, DNA uptake is only from closely related species. DNA is taken in by membrane vesicles. transferred DNA must have a special 11bp sequence
what are the limitations of transformation
- limits on host range of the transfer and maintenance mechanism
- limited abiloty of foreign DNA to integrate dur to a lack of sequence similarity
- recipient restriction enzyme activity
how can artificial transformation take place
two methods
- heat shock with CaCl2 treatment or electroporation
- high concentration of DNA used
- DNA inserted into plasmid
what is conjugation
transfer of DNA by direct cell to cell contact
how was conjugation doiscovered
discovered in lederberd and tatum in 1946 via plate method (like we did in lab)
was also demonstrated in 1950 by Davis in a U tube experiment
describe fertility factor plasmids
aka F factor- tend to be 100kb long. have genes for pilus formation and gene transformation.
three types
F+: F factor alone autonomous (only has the tra gene)
F’: Factor autonomou with additional genes (has tra genes and special genes i.e antibiotic resistance)
HFr: integrated F factor plasmid (is inside the chromosome and is carried with the bacterial cell)
what is a criteria of recipients
must lack the F plasmid- the donors will not conjugate with other donors
describe F+ and F- matings
pilus forms between the two cells, extending from F+ cell. a single strand of the F factor moves through the pilus. each strand us replicated. F- becomes F+
describe HFr and F- mating
F factor is integrated into the plasmid. Transfer begins with a nick in F factor at the origin of transfer. the chromosome moves through while replicating. since only part of F factor is transferred, the F- does not become a donor
F’ and F- mating
F’ occurs due to imprecise excision of integated F plasmid from the chromosome- this mating is virtually identical to F+ and F-
what happens during interrupted mating exps
the chromosome moves from donor to recipient at a constant rate, and in an interrupted mating exp the conjugation bridge is broken at various intervals by mixing the culture.
the order and timing of the gene transfer can be determined becuase they are directly reflected in the genes on the chromosome
what is transduction
is a transfer of genes via bacterial viruses
what are the two replications of a phage
reproductive lytic cycle
dormancy lysogenic cycle
what happens in lytic cycle
absorption, penetration, DNA replication, protein synth, assembly, release and is usually caused by a virulent phage
what happens in dormancy lysogenic cycle
absorption, penetrationm integration into bacterial chromosome, multiplication along with chromosome. usually happens with a temperate phage
what are the two types of transduction
generalised and specialised
describe generalised transduction
occurs as result of lytic cycle- any random segment of DNA can transfer
describe specialised transduction
occurs due to lysogenic cycel- only genes adjacent to integrated virus are transferred
describe the process of genealised transduction
the bacteriophages can pick up any portion of the hosts genome.
bacteriophages pick up fragments of donor bacteria degraded DNA. the bacteriophages are released and can infiltrate a recipient bacteria where it donates the donor bacterium’s DNA to the recipient bacterium
describe specialised transduction
during spontaneuous excision, small piece of the donor bacteriums DNA near the bacteriophage insertion site is picked up and replaces some of the phage genome.
these bacteriophages adsorb to a recipient bacterium and injects its genome. the foreign DNA is inserted into the recipient bacterium’s nucleoid
