Plasmids Flashcards
What are plasmids
Circular genetic elements that replicate independently of the chromosome that encodes non-essential genes
2 things needed to be a replicon
a) the DNA molecule must have at least one origin of replication (ori) and b) the cell must contain proteins that allow replication to initiate at the ori site
encodes a few proteins for their replication (initiator protein)
oriV vs OriT
For normal growth the origin is often called oriV for vegetative origin – as opposed to oriT for origin of transfer used for conjugation
Theta uni-directional vs Bidirectional modes of plasmid replication
Some plasmids initiate replication by opening the strands at the ori and sending one replication fork out from the ori (unidirectional replication)
Also possible but very rare is initiation at the ori by opening the strands at the ori and sending two replication forks out from the ori in opposite directions (bidirectional replication)
What is rolling circle replication? Describe mechanism
Used by plasmids with a wide host range
2 stages that use 2 ori’s
Stage 1 – a dsDNA circle is replicated to produce a dsDNA circle and an ssDNA circle – Stage 1 is initiated at a double-strand origin (DSO)
Stage 2 – the ssDNA circle is replicated to produce another dsDNA circle – Stage 2 is initiated at a single-strand origin (SSO)
Plasmid properties determined by the ori
Host range
plasmid copy number- number of copies of a plasmid in the cell after cell division
Incompatibility- Refers to the inability of two plasmids to stably coexist in the same cell
narrow vs broad host range plasmids
Narrow: plasmids with a ColE1-type origin, replicate in E. coli and similar species
Broad: Most rolling circle plasmids, RK2, RSF1010, ecnode all their own proteins for initiating replication
Can broad range host plasmids cross the gram negative vs. gram positive divide
generally no
What is plasmid curing
the loss of a plasmid from a species or strain
What are plasmid partitioning systems
Mechanism to prevent plasmid curing
Ensures each daughter cell receives at least one copy of the plasmid
what are the commonalities of the partitioning systems
both system help localize the plasmid to the site of replication which is usually midcell and promote the pairing of daughter plasmids after replication
R1-type plasmid partitioning system
R1-type plasmid partitioning system involves site-specific binding of ParR to a centromere
ParM forms a complex with ATP to push the plasmids to opposing poles
ATP then hydrolyzes
ParM then degrades leaving the plasmids on opposite sides of the cell
P1-type plasmid partitioning model
P1-type plasmid partitioning systems involves site-specific binding of ParB to a centromere
ParA forms filament with ATP at a random position, Filament grows and ParB forms a complex with ParS
ParB-ParS interact with ParA-ATP
ParA-ATP hydrolyzes to ParA-ADP and starts to dissociate
Association of ParB-parS and hydrolysis drives movement of plasmid
What is Bacillus cereus
ubiquitous (found everywhere) soil organism that can also become a source of mild food poisoning, forms spores
What is Bacillus thuringiensis
ubiquitous soil organism that does not cause human disease but is a source of insecticidal toxins that are used in agriculture, forms spores
