L8 - Horizontal Gene Transfer Flashcards
(30 cards)
What are the sources of genetic diversity?
- spontaneuous mutagenesis = DNA polymerase has proofreading properties but its not completely perfect
- errors in DNA repair
- homologous recombination = deletion/duplication/inversion
- transposons = jumping genes, affects functions, occurs naturally (transposable elements)
- exposure to mutagens (chemical agents and radiation)
- horizontal gene transfer
What is horizontal gene transfer?
movement of genetic material between organisms other than by the transmission of DNA from parent to offspring
What does HGT do?
- genome diversification
- evolution
- adaptation
- survivability
- selection = antibiotic resistance
How can HGT be advantageous to human?
pros to human :
1. evolution = discovered bacterias that breaks down plastic
2. genomes in japan: gut biome have acquired genes that help them break down
seafood (marine bacteria), agliases
In terms of HGT definition (mobilome extracellular DNA), What is a mobilome?
mobile genetic elements e.g. lysogenic phages, transposons, plasmids
- chromosomal rearrangements, chromosomal islands
extracellular DNA (eDNA)
What are lysogenic phages?
phages that become incorporated into bacteria
What are the 4 mechanisms of HGT
transformation, conjugation, transduction, vesiduction
What is transformation?
when bacteria die their DNA is released into the environment for other bacterias to take up. it is taken up by recipient (incorporated into chromosome) by homologous recombination. recipient has to be “competent” (have ability to take up eDNA)
How is competency achieved in labs?
cold shock to make membranes permeable
How is transformation achieved in gram positive organisms?
- have membrane proteins that are specific for identifying extracellular dna
- bound to site, nuclease switched on to convert dsDNA to ssDNA
- ssDNA binds to diff proteins
- eclipse phase bc the ssDNA cant be seen by nucleases that breaks down free roaming DNA (covered and protected)
- rec-A = if there are any homology, the foreign strand incorporates and recombines into chromosome
usually more competnet bc only one membrane
Transformation in gram negative organism
- membrane proteins identifies extracellular DNA
- needs to have these signatures T tract (AT rich) to be able to bind to gram negative membrane proteins (gram negative is less competent usually)
- as the DNA gets pulled and tract, it enteres the periplasmic space
- translocated into cytoplasm
- Rec2 makes competency proteins and works together to replicate
- if there are any homology, the foreign strand incorporates and recombines into chromosome
What is conjugation?
transfer of DNA mediated by conjugative plasmid in donor. requires cell to cell contact , cytoplasmic bridges passes plasmid from donor to recipient (mating). recipient can receive conjugative plasmid.
Describe the conjugative plasmid in conjugation
- F plasmid (e. coli)
- tra genes allow the conjugation of the pills (F pills assembly and F plasmid transfer
- insertion sequences (IS) and Tn1000 = integration with chromosome
- oriV = origin of replication
- oriT = origin of transfer
Conjugation step by step using F plasmid
- F positive cell = carries conjugative plasmid
- has to take place with F- cell
- pillus connects cytoplasm of 2 cells
- pillus retracts so they become closer
- F posoitive cell knicks the DNA of one strand and DNA gets transferred by rolling motion
- receipient cell starts replicating as info gets exchanged until a whole plasid is replicated
Conjugation step by step using Hfr strain (integrated F plasmid)
- The plasmid can become integrated into the bacterial chromosome.
- F+ bacteria have insertion sequences that allow integration at different sites of the chromosome.
- oriT (origin of transfer)
- When the plasmid integrates into the cell, it is no longer called an F+ cell but becomes an Hfr strain.
- During conjugation, single-stranded DNA gets pulled into the recipient cell.
- The recipient does not become F+ because the complete plasmid is not incorporated.
What are integrative conjugative elements
alternative mechanism (not conjugation machinery)
- ICEs are integrated into chromosome and is commonly through the type IV secretion syste,
- transferred like conjugative plasmids
- carries AMR, virulence, metabolic genes that enhance survival
What is staphylococcus aureus pathogenicity islands (SaPIs)
- ICE like elements
- have integration and excision genes
- do not have machinery for transfer so uses prophage as helpers
- when stress initiates SOS response, phage DNA pops out and gets carried along with it in phage particles and enters recipient
What is transduction
transfer of DNA via bacterial virus (bacteriophage- virus that affects bacteria)
What are the 2 types of transduction
- generalised = carried out by phages that infect bacterial cell, destroys it and uses it as its own machinary (lyric/viorulent phages)
- specialised = phages that gets incorporated into bacterial chromosome (by lysogenic/temperate phages)
What is generalised transduction?
any gene from donor chromosome -> transductant by lytic/virulent phage
How does generalised transduction work?
- After transduction, the resulting cell is called a transductant.
- A bacterial cell is infected by a phage particle.
- The phage inserts its DNA into the bacterial cell and uses the bacterial machinery to make more copies of its genome and proteins needed for new phage particles.
- The host chromosome is broken into pieces.
- At the end of infection, new phage particles form and exit the cell.
- Occasionally, at low frequency, when these phage particles are being assembled, DNA pieces get packed into the phage heads along with viral DNA - bacterial DNA can get incorporated with viral DNA.
- This unusual phage is called a transducing particle (contains bacterial chromosomal DNA).
- While it remains infectious like a normal phage, it now carries bacterial DNA. When it infects a new bacterial cell and injects this DNA, the bacterial DNA can be replicated. However, since the viral DNA is incomplete, it cannot hijack the host’s machinery.
What is specialised transduction and how does it work ?
- selected region of donor chromosome becomes transductant by lysogenic/temperate phage
- phage is integrated at specific site in chromosome
- phage replication is controlled by bacterial chromosome
- lamda phage
- when replicated along w bacterial chromosome, normally, phage gets circularised and goes out making new phages (normal infection)
- sometimes, bacterial chromosome gets incorporated to phage (not random it is specific)
What is vesiduction?
vesicle driven HGT
- gram negative lets out vesicles filled with protein and linear DNA
- vesicles picked up and is incorporated through fusion
