Chapter 17 - Microbial Genetics Flashcards
Horizontal Gene Transfer
process in which bacteria use to acquire new genes
Conjugation
process by which donor bacteria transfers copy o a plasmid to a recipient bacterium through a pilus.
conjugative plasmid
Located on donor cell, an extrachromosomal piece of dsRNA that codes for proteins necessary to make a threadlike filament known as a pilus
pilus
Threadlike filament used to bind the recipient cell to the donor cell and brings them in close proximity.
after pilus attachment?
Believed that a channel is then opened, allowing for a ssDNA copy of the plasmid to enter the recipient cell. Both cells make the complementary copy to the ssDNA, resulting in two new cells capable of conjugation.
Transformation
Allows bacterial cell to acquire new genes, but it does not require cell-to-cell contact. Process in which genes acquired directly from environment.
What is required for transformation?
A donor cell: lysed and released DNA into environment.
A recipient cell: takes up the DNA and incorporates into its own genome.
When does transformation usually occur?
at the end of exponential phase of growth or beginning of the stationary phase, in the presence of high cell density and limited nutrients.
What proteins are manufactured for transformation
DNA binding protein (DNA translocase), endonucleases, transmembrane channel proteins
What do Gram Negative cells make for transformation?
Autolysin, to transport DNA across the outer membrane
Translocase
Pull DNA binded to receptors on the outside of the cell into the cell, through the transmembrane channel
transmembrane channel
a large structure often involving numerous different proteins
endonuclease
used to degrade one strand of dsDNA when ssDNA can only pass, or cleave DNA fragments into smaller sizes.
RecA
incorporates DNA into the bacterial chromosome so the genes can be expressed
Transduction
Involves virus, a bacteriophage, to act as a conduit for shuttling bacteria genes from one cell to another. Negates cell-to-cell contact. 2 types of transduction (Generalized and Specialized)
Generalized Transduction
Bacterial host infected w/ virulent or temperate bacteriophage engaging in lytic cycle.
Generalized Transduction steps
- Absorption 2. penetration 3. synthesis. When virus enters assembly stage, random pieces of bacterial DnA are mistakenly packaged into the phage head - results in transducing particle.
What is the importance of a transducing particle?
Though not capable of infecting a cell conventionally, they can bind to a new bacterial host and inject their DNA inside. If this DNA is incorporated into recipients DNA, then the genes can be expressed.
Specialized Transduction
Can only occur with temperate bacteriophage, involved lysogenic cycle of replication.
Specialized Transduction steps
- Bacteriophage attaches and injects viral DNA inside
- DNA then integrates into chromosome of host cell (forms prophage)
- Induction occurs, where prophage is excised from bacterial chromosome. Excision is incorrectly performed and portion of bacterial genes immediately adjacent to viral genes are excised too.
- This means that all copies contain both viral and bacterial DNA.
- Cell lyses and releases virions. Virions then inject into other bacteria.
What’s unique about specialized transduction?
Since it contains both DNA from the virus and the bacteria, the second bacteria host that receives DNA from the virion has the ability to receive its own DNA, DNA from previous bacterial host and the viral DNA.
What is homologous recombination
involves RecA protein. DNA from 2 sources are paired, based on similar nucleotide sequence in one area.
Homologous Recombination - Strand Invasion
endonuclease nicks one strand, allowing RecA to pair up bases from different stands.
Homologous Recombination - resolvase
cuts and rejoins DNA into two separate dsDNA molecules
Site-specific recombination
process used by viruses to insert genome into chromosome of their host.
Transposable elements - “jumping genes”
Responsible for the activation or inactivation of genes w/in an organism. Simple in structure, designed to move from one location to another w/in DNA (transposition)
transposition
moving from one location to another w/in a DNA molecule
transposase
enzyme responsible for allowing transposition to occur - all transposable elements code for this - short inverted repeats (IR’s) at each end
Inverted repeat
Located at the ends of transposase
insertion sequence
simplest transposable element - contains transposase and IR’s of varying lengths.
transposon
contains additional genes, with the exact type varying widely from transposon to transposon - can be removed from 1 location and relocated to another
cut-and-paste model (conservative transposition)
moving a transposon from 1 location and moving it to another location
replicative transposition
a copy of a transposon that is inserted at a second site
