BIOC0005 - BACTERIAL PLASMIDS AND CONJUGATION
EXAM QUESTIONS AND ANSWERS WITH COMPLETE
SOLUTIONS VERIFIED LATEST UPDATE
What is a plasmid?
Non-chromosomal DNA
Usually circular, but sometimes are linear
Autonomous replication
Stable inheritance
What are some general differences between plasmids and chromosomes?
Plasmids are generally smaller
Plasmids are generally not essential for “normal” growth
Plasmids are generally transferable
What is the purpose of the origin of replication?
Site where DNA replication is initiated
Also called a "replicator" sequence
Determines how many copies of the plasmid will be present in a cell
Can allow for conditional replication (temperature-sensitive, host-dependent)
Can determine plasmid compatibility
Describe the different copy numbers of plasmids.
,High copy number: ~500-700 copies/cell
Medium copy number: ~40 copies/cell
Low copy number: ~10-15
Ultra-low: ~1-2
How does replication of a plasmid initiate?
Replication requires initiators to bind which drives unwinding and unzipping of the DNA.
For unzipping to occur, a critical threshold amount of initiator needs to bind
For bacterial chromosome and many plasmids, the initiator is a protein
For some plasmids (e.g. pUC origin), the initiator is an RNA that inserts itself into the
DNA
How is copy number controlled by the origin?
Regulating the amount of initiator present and how the initiator binds to the origin.
How do incompatibility groups work?
Two plasmids from the same incompatibility group will not be stably maintained.
One of the plasmids will eventually be lost after a few generations.
This is because when different plasmids have the same origin, they compete for
replication initiation machinery.
Statistically over time, bacteria will lose plasmids.
Explain how plasmid maintenance works in a cell.
Plasmids can be a significant metabolic burden, so cells "need a reason" to keep them.
Individual cells do not make a decision, but population of cells use natural selection to
decide whether plasmids are maintained.
, If a plasmid makes a bacterium grow slightly slower, then after enough generations, any
cells that lose the plasmid will eventually out-compete the plasmid-containing cells.
Plasmid maintenance mechanisms are in place to make it more difficult to lose the
plasmid and provide a selective advantage to cells that keep the plasmid.
Why does plasmid partitioning need to occur in bacteria?
Replicated plasmids need to be distributed into daughter cells.
High, medium, and even low copy number plasmids can be partitioned randomly.
Ultra-low copy number plasmids need special machinery to ensure that both daughter
cells receive a copy of the plasmid.
What are the two most common mechanisms for plasmid partitioning?
Filament formation
Brownian rachet
Explain how filament formation is a mechanism for plasmid partitioning,
including the genes involved.
Plasmid R1 uses “actin-like” filament to push plasmids to opposite sides of the cell.
•parC = sequence on plasmid
•ParR = protein that binds parC
•ParM = protein that forms filaments; also binds ParR
Explain how Brownian rachet is a mechanism for plasmid partitioning, including
the genes involved.
parC = sequence on plasmid
EXAM QUESTIONS AND ANSWERS WITH COMPLETE
SOLUTIONS VERIFIED LATEST UPDATE
What is a plasmid?
Non-chromosomal DNA
Usually circular, but sometimes are linear
Autonomous replication
Stable inheritance
What are some general differences between plasmids and chromosomes?
Plasmids are generally smaller
Plasmids are generally not essential for “normal” growth
Plasmids are generally transferable
What is the purpose of the origin of replication?
Site where DNA replication is initiated
Also called a "replicator" sequence
Determines how many copies of the plasmid will be present in a cell
Can allow for conditional replication (temperature-sensitive, host-dependent)
Can determine plasmid compatibility
Describe the different copy numbers of plasmids.
,High copy number: ~500-700 copies/cell
Medium copy number: ~40 copies/cell
Low copy number: ~10-15
Ultra-low: ~1-2
How does replication of a plasmid initiate?
Replication requires initiators to bind which drives unwinding and unzipping of the DNA.
For unzipping to occur, a critical threshold amount of initiator needs to bind
For bacterial chromosome and many plasmids, the initiator is a protein
For some plasmids (e.g. pUC origin), the initiator is an RNA that inserts itself into the
DNA
How is copy number controlled by the origin?
Regulating the amount of initiator present and how the initiator binds to the origin.
How do incompatibility groups work?
Two plasmids from the same incompatibility group will not be stably maintained.
One of the plasmids will eventually be lost after a few generations.
This is because when different plasmids have the same origin, they compete for
replication initiation machinery.
Statistically over time, bacteria will lose plasmids.
Explain how plasmid maintenance works in a cell.
Plasmids can be a significant metabolic burden, so cells "need a reason" to keep them.
Individual cells do not make a decision, but population of cells use natural selection to
decide whether plasmids are maintained.
, If a plasmid makes a bacterium grow slightly slower, then after enough generations, any
cells that lose the plasmid will eventually out-compete the plasmid-containing cells.
Plasmid maintenance mechanisms are in place to make it more difficult to lose the
plasmid and provide a selective advantage to cells that keep the plasmid.
Why does plasmid partitioning need to occur in bacteria?
Replicated plasmids need to be distributed into daughter cells.
High, medium, and even low copy number plasmids can be partitioned randomly.
Ultra-low copy number plasmids need special machinery to ensure that both daughter
cells receive a copy of the plasmid.
What are the two most common mechanisms for plasmid partitioning?
Filament formation
Brownian rachet
Explain how filament formation is a mechanism for plasmid partitioning,
including the genes involved.
Plasmid R1 uses “actin-like” filament to push plasmids to opposite sides of the cell.
•parC = sequence on plasmid
•ParR = protein that binds parC
•ParM = protein that forms filaments; also binds ParR
Explain how Brownian rachet is a mechanism for plasmid partitioning, including
the genes involved.
parC = sequence on plasmid
