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What is horizontal gene transfer?
It is another form of gene transfer in which genes in bacterial genomes are acquired from other bacteria as opposed to being passed on through generations. It can occur among a variety of bacterial species. This can occur by conjugation, transduction, and transformation.
How does the bacterial chromosome fit into the cell?
The DNA is folded up tightly by supercoiling, which occurs when additional turns are introduced into the DNA double helix (+) or if turns are removed (-). Because there are no free ends, the bacterial chromosome winds around itself to form a more compact structure. Supercoiling occurs by DNA gyrase and DNA topoisomerase I. Supercoiling doesn’t prevent transcription, replication, and separation of daughter molecules. The folding is very ordered.
The DNA is attached to a protein core from which supercoiled loops radiate out into the cell. The protein component includes DNA gyrase and DNA topoisomerase I, as well as at least four proteins believed to have a more specific role in packaging the bacterial DNA. The most abundant is HU, which forms a tetramer around which approximately 60 bp of DNA becomes wound.
- They are circular; some can be linear.
- They carry one or more genes, which are responsible for a useful characteristic.
- They possess at least one DNA sequence that can act as an origin of replication, allowing for independent multiplication.
- Smaller plasmids make use of the cell’s own DNA replicative enzymes to make copies of themselves, whereas some of the larger ones carry genes that code for special enzymes that are specific for plasmid replication.
- Some plasmids replicate by inserting themselves into the chromosome, becoming episomes.
What are the types of plasmid?
Resistance, or R plasmids: carry genes giving resistance to one or more antibacterial agents
Fertility, or F plasmids, are able to direct conjugation (the process that enables two bacteria to join together so that plasmids, and possibly parts of the chromosome, can be passed from one cell to another) between different bacteria
Col plasmids code for toxins called colicins
Degradative plasmids allow the host bacterium to metabolize unusual molecules such as toluene or salicylic acid
Virulence plasmids confer pathogenicity on the host bacterium
Explain the characteristics of plasmids.
They have a copy number, the number of copies of the plasmid that are present in a single bacterial cell. Some are stringent and have a low copy number. Others are relaxed and can be present in multiple copies of 50 or more
There is also a compatibility component that allows certain plasmids to coexist in the same cell or restrict coexistence
What is the arrangement of the chromosome within the nucleoid and the mechanism of its replication?
The equivalent parts of a DNA molecule are always located at equivalent parts in different cells. When the chromosome begins to replicate, the origins of the two daughters move apart from one another, each toward a different end of the cell. A cytoskeletal protein called MreB is required, but may play a passive role, simply providing a framework along which the origins travel. The outcome is that the two daughter chromosomes move toward either end of the bacterium as the parent molecule is being replicated. The replicated DNA molecules therefore take up the appropriate positions needed for them to become enclosed within the daughter cells that are formed when cytokinesis divides the bacterium into two. The septum eventually begins to form across the middle of the parent cell before chromosome replication has been completed and the last parts of the replicated DNA molecule are pumped across the septum by special translocase enzymes.
What is the mechanism of plasmid replication?
- There are three systems for partitioning.
- The type I 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, each of which attaches to each plasmid at a special sequence called parC, which is the bidning site for another protein, ParR. ParR forms a platform for attachment of the first of the ParM proteins in the growing microfilament. The ParM filament grows along the axis of the bacterium, so the replicated plasmids become pushed to opposite ends
What happens with the ends of linear genomes of bacteria?
So as not to mistaken them for broken pieces of DNA, they require terminal structures equivalent to telomeres in eukaryotes, such as covalent linkages between the 5’ and 3’ ends of the polynucleotides in the DNA double helix, or markings by special proteins.
What is another variation in bacterial/ E. coli?
The presence of multipartite genomes, genomes that are divided into two or more DNA molecules, is a complicated feature. To establish whether this exists, one must establish that the DNA molecule present is not a plasmid.
What are the three types of DNA molecules that may be found in a bacterium?
- There is the bacterial chromosome, carrying essential genes and located in the nucleoid.
- There are genuine plasmids, which are distinct from a bacterial chromosome because of their special plasmid partitioning system and whose genes are nonessential to the bacterium.
- There are chromids, which use a plasmid partitioning system but which carry genes that the bacterium needs to survive.
The transfer of genes is unidirectional from donor to recipient cells. It involves a tube-like pilus, which F+ cells can construct. The F+ cell forms the connection with the F- cell via its sex pilus. The pilus brings the cells into close contact, allowing the DNA (fertility plasmid) to be transferred from one cell to another. The F+ cell has the fertility plasmid, which has a tra gene, which codes for proteins involved in synthesis and assembly of the pilus and in the DNA transfer process itself.
Replication is by the rolling-circle mechanism, with the parent plasmid remaining in the F+ cell and the copy transferring to the recipient F- cell as it is rolled off of the parent. The F- cell therefore becomes F+
This is conjugation (set up of initial contact) and mobilization (passage of plasmid DNA from donor to recipient)
How is it possible for chromosomal genes to be transferred from donor to recipient?
The first way is the transfer of a small random piece of the donor cell’s genome with the F plasmid.
Another way is with the Hfr and F’ cells. In Hfr cells, the F plasmid integrates into the chromosomal DNA. Therefore, in addition to transferring itself, it also carries into the F= cell a copy of at least some of the E. coli DNA molecule to which it is attached
F’ cells occasionally arise from Hfr cells when the integrated F plasmid becomes excised from the genome. Sometimes excision of the F+ plasmid is not accurate and a small segment of the adjacent bacterial DNA is also snipped out. This leads to an F’ plasmid that carries a segment of the bacterium’s genome, possibly including only a few genes.
How else, besides conjugation, can horizontal gene transfer occur?
There is transformation and transduction.
During transformation, a bacterium takes up DNA that it encounters in its local environment. The outer membranes have proteins that bind DNA and transfer it into the cell.
In transduction, the transfer is mediated by a bacteriophage, some of which don’t kill the host cell but instead integrate into the host bacterium’s chromosome, becoming a prophage. Eventually, the prophage gets excised and the new virus particles will be constructed around the replicated virus genome. The bacterial chromosome is also broken down. Because of this, one of the small chromosome fragments/ bacteriophages might be packaged into a virus particle. The DNA is still infective.
What happens to the DNA once inside a recipient cell?
If DNA is an intact plasmid, it can replicate. Chromosomal DNA might be broken down and used as a source of carbon, nitrogen, and phosphorus for construction of new molecules within the recipient bacterium. Sometimes, the DNA can also survive, becoming integrated into the bacterial chromosome or chromid. This is possible if the same or similar sequence to a segment is present. This can lead to inheritance of genes.