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Plasmids are __. Linear descent—__—is not the only way in which bacteria obtain genes.
mobile and can move easily between bacteria, even between members of different species
repeated passage of the genome from parent to daughter cells
Some of the genes in bacterial genomes have been acquired from other bacteria, possibly other species, by __. E.coli is used often for this. It was first isolated in 1885. It also has an uncomplicated genome.
horizontal gene transfer
There are two distinct regions within an E. Coli cell. There is a central area called the __, which takes up about __ of the volume of the cell. It is surrounded by a __. Most of the DNA in an E. Coli cell is contained in the __.
peripheral region that is usually referred to simply as the cytoplasm
The E. Coli __contains __. The E. coli nucleoid is made up of __ and __.
DNA: __ of 4639 kb, corresponding to a contour length of 1.6 mm= __
supercoiled DNA attached to a protein core
DNA and protein
single, circular molecule
Because the DNA chromosome is big and the cell is small, it undergoes __, which occurs when additional turns are introduced into the DNA double helix (__) or if turns are removed (__)
With a linear molecule, the __ introduced by over-or-underwinding is immediately released by __, but a __, having no ends, cannot reduce the strain in this way.
Instead, the circular molecule responds by __. Supercoiling is an ideal way of __
- torsional stress
- rotation of the ends of the DNA molecule
- circular molecule
- winding around itself to form a more compact structure
- packaging a circular molecule into a small space
Supercoiling is generated and controlled by two enzymes, __ and __.
Supercoiling must not hinder __ and __. It must also be possible to __= DNA must be folded up in a very ordered manner.
- DNA gyrase and DNA topoisomerase I
- accessibility and transcription
replicate the DNA and separate the daughter molecules without everything getting tangled up
DNA is attached to a protein core from which supercoiled loops, each containing 10 to 100 kb of DNA, radiate out into the cell
The protein component of the nucleoid includes __ and __, the two enzymes that are primarily responsible for maintaining the __, as well as a set of at least four proteins believed to have a more specific role in packaging the bacterial DNA
DNA gyrase and DNA topoisomerase I
The most abundant of these packaging proteins is __, which forms a tetramer around which approximately 60 bp of DNA becomes wound.
Plasmids are __
The cytoplasm of an E. coli cell might contain other DNA molecules called __, which are __—though some can be __.
independent DNA molecules within a bacterial cell
What do plasmids carry?
Plasmids carry one or more genes, and often tehse genes are responsible for a useful characteristic
All plasmids possess at least one DNA sequence that can act as an __, allowing tehm to __
The smaller plasmids make use of the cell’s __ to make copies of themselves, whereas some of the larger ones carry genes that code for special enzymes that are specific for __.
- origin of replication
- multiply in the cell independently of the bacterial chromosome
- own DNA replicative enzymes
- plasmid replication
A few types of plasmid are also able to replicate by doing what?
These __ may be stably maintained in this form through numerous cell divisions, but always at some stage exist as independent elements
inserting themselves into the chromosome
integrative plasmids or episomes
Plasmids can be classified according to the genes they carry
Virtually all species of bacteria harbor plasmids. Some plasmids are __to just a few related species and are found in no other bacteria, but others have a __and can exist in numerous species though they have preferences.
broad host range
Plasmids are most classified according to __. There are five main types of plasmids.
What are they?
the genes that they carry and the characteristics that those genes confer on the host bacterium
- Fertility F plasmids
- Resistance plasmids
- Col plasmids
- Degradative plasmids
- Virulence plasmids
Most common type: Resistance (R) plasmids.
they carry genes conferring on the host bacterium resistance to one or more antibacterial agents. Important in clinical microbiology
Some species have copies of a second type of plasmid called a fertility or F plasmid: they do what?
direct conjugation between different bacteria of the same, or related, species
Conjugation is a process that __
Conjugation is one of the processes that can lead to __
enabels two bacteria to join together so that plasmids, and possibly parts of the chromosome, can be passed from one cell to another
horizontal gene transfer
Some plasmids carry genes that __. Bacteria carrying these plasmids __. Plasmids of this type are not common in bacterial species as a whole, but are present in some strains of E. coli. They are: __
code for toxic proteins which kill other bacteria
gain an advantage in the competition for scarce resources
- Col plasmids
- Degradative plasmids
- Virulence plasmids
Col plasmids: codes for toxins alled colicins
Degradative plasmids: allow the host bacterium to metabolize unusual molecules such as toluene and salicylic acid
Virulence plasmids confer pathogenicity on the host bacterium
A single bacterium can have __
Each type of plasmid has its own __, this being the __
Some plasmids are __and have a __ of perhaps just one or two per cell
multiple copies of the same or different plasmids
characteristic copy number
number of copies of the plasmid that are present in a single bacterial cell
low copy number
Others, called __, are present in multiple copies of 50 or more
There can also be more than one type of plasmid in a single cell but they must be __.
If they are incompatible, then __
- relaxed plasmids
- one or the other will be lost from the cell
Different types of plasmids can therefore be assigned to __ on the basis of whether or not they can __
Plasmids from a single incompatibility group are often related to one another in various ways
different incompatibility groups
The factors determining the copy number and compatibility relationships between plasmids can be somewhat attributed to __. There are two theories for this. What is the first?
events during replication
- Theory 1: copy number is determined by an inhibitor molecule that prevents further replication of the plasmid once the characteristic value is reached
- -->Replication may even be controlled by the same inhibitor molecule
What is theory two?
Theory 2: plasmid replication requires attachment to a specific binding site on the cell membrane and that incompatible plasmids compete for the same attachment sites
How Daughter cells acquire Copies of the bacterial chromosome and plasmids is not well understood
Unlike chromosomes, inheritance of plasmids is not such an absolute requirement. However, they usually __, revealing the fact that there is a mechanism
become evenly distributed between the daughters
What we know suggests that the __ plays an important role; and, the arrangement is not random.
In different cells, the equivalent parts of the DNA molecule are always located at __.The origin of replication is always __, with the __diametrically opposite, adjacent to the wall on the other side of the cell
arrangement of the chromosome within the nucleoid
adjacent to the cell wall
replication termination region
When the chromosome begins to replicate, the origins of replication of the two daughter DNA molecules do what?
move apart from one another, each toward a different end of the cell.
How this occurs is unknown. But, a __ is required—passive role as it provides a framework along which the origins travel.
One theory for this is that the origins are __
cytoskeletal protein called MreB
pushed apart by RNA polymerase enzymes as these gain access to the newly replicated strands in order to transcribe the genes close to the origin
The replicated DNA molecules therefore take up the __ needed for them to become enclosed within the daughter cells that are formed when __.
two daughter chromosomes move toward either end of the bacterium as the parent molecule is being replicated.
cytokinesis divides the bacterium into two
In a rich growth medium, one in which the bacteria are able to divide every 20 minutes or so, what happens?
The last parts of the replicated DNA molecules are __
the septum begins to form across the middle of the parent cell before chromosome replication has been completed
“pumped” across the septum by special translocase enzymes
Plasmid partitioning: three mechanisms, each using a different plasmid
Type 1 system:__
two members of a pair of daughter plasmids are pushed apart from one another by growth of a microfilament made up of ParM proteins, which attach to each plasmid at a special sequence called parC, which is the binding site for another protein, ParR.
What does ParR do?
The latter forms a platform for attachment of the first of the ParM proteins in the growing microfilament
What does parC do?
parC is sometimes called the plasmid centromere (no similarity to eukaryotic centromere other than it being the attachment site for a microfilament)
What does the parM do?
the ParM filament grows along the axis fo the bacterium, so the replicated plasmids become pushed to opposite ends.
The other systems work in similar ways, the main distinction being in __
the identity of the protein that forms the filament that pushes the daughter plasmids to the poles