Inheritance of Genes in Bacteria: 3
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Horizontal gene transfer is when __. It was originally thought that __. This assumption was overturned in the 1990s when complete genome sequences were obtained for a variety of bacterial species.
- genes are acquired directly from other bacteria
- transfer of plasmids and chromosomal genes could take place only between members of the same species, or occasionally between closely related species
Comparisons of these sequences revealed that the __, suggesting that the barriers to __ are much less rigid than previously thought. It also became clear that although we know a great deal about the processes by which plasmids and chromosomal genes move within species, we know much less about how genes are transferred from one species to another
same genes are present in quite distantly related species
horizontal gene transfer
Plasmids can be transferred between bacteria by __
Bacteria can exchange genes by __; it is __.
The donor cells are referred to as __ and the recipient cells are __
unidirectional from donor to recipient cells
Conjugation involves a __, which__
- tubelike pilus
- F+ cells are able to construct
Explain the fertility factor and formation of the tubelike pilus.
An F+ cell forms a connection with an F- cell via its sex pilus, and because the pilus is hollow, DNA goes through it (this is suggested). It is also suggested that the pilus brings the F+ and F- cells into close contact
What is the difference between F+ and F-?
that the former contains the fertility plasmid. In E. coli, it contains the tra (for transfer) genes, which code for proteins involved in synthesis and assembly of the pilus and in the DNA transfer process itself
Conjugation is always between __. The F+ cell, the one that __, initiates __by __, which __. The two cells become __, and what happens?
- F+ and F-
- contains an F plasmid
- making a pilus
- attaches to an F- cell
- drawn together
- a copy of the F plasmid is transferred from the F+ to the F- bacterium
Replication is by the __, 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 __
Theoreticalyl, this should mean that, over time, all E. coli bacterial will become F+. Why is this not the case?
This is not the case, so presumably the F plasmid is occasionally lost from an F+ bacterium, so there are always F- cells in a population
A large number of bacterial species are able to __in the same way as E. coli, and in all of these the process is controlled by a __. Fertility plasmids of this type are called __, which means that they can __ and __
plasmid analogous to the F plasmid
set up conjugation and mobilize themselves into the recipient cell
__ and __are two distinct functions, and only self-transmissible plasmids are able to direct both
A few other plasmids can only __ and not __, and some can __
Conjugation and mobilization (passage of plasmid DNA from donor to recipient)
set up the conjugation contact between cells
mobilize on their own
mobilize but only if they are coexisting in the cell with a second plasmid that can set up the initial contact
Other plasmids are totally __and can neither __ nor __
- conjugate nor mobilize
Chromosomal genes can also be transferred during __
Bacterial conjugation is important because it does what?
both enables plasmids to be transferred between bacteria and they can also be passed from donor cell to recipient
There are several ways, the first being simply that __. This occurs fairly infrequently.
More important are the __ properties of two special types of donor cell, called __.
- a small random piece of the donor cell’s genome is transferred along with the F plasmid
- gene transfer
- Hfr and F’.
In Hfr cells, the F plasmid has done what?
The integrated form of the F plasmid can still do what, but in this case, in a ddition to transferring itself, it also does what?
become integrated into the E.coli chromosomal DNA
direct conjugal transfer
carries into the F- cell a copy of at least of the E. coli DNA molecule to which it is attached
It takes approximately 100 minutes for the entire E. coli chromosome to be transferred in this way, but conjugation rarely continues for this long. Termination of conjugation interrupts the DNA transfer, so usually only what happens?
a part of the chromosomal DNA is passed to the recipient cell
F’ cells, the second type of donor cell regularly assoiated with transfer of chromosomal genes, occasionally arise from __ when the __ becomes excised from the genome
Normally this event results in an__, but sometimes __ is not entirely accurate, and a small segment of the adjacent bacterial DNA is also snipped out
- Hfr cells
- integrated F plasmid
- F+ cell
- excision of the F plasmid
This leads to an __ that carries a segment of the bacterium’s genome, possibly including a few genes
Conjugation involving an F’ cell always results in __
transfer of these plasmid-borne bacterial genes
Bacterial genes can also be transferred __ between donor and recipient
Bacteria can also acquire segments of chromosomal DNA from other cells by processes that do not involve __ via __ and __
direct cell-to-cell contact via transformation and transduction
Some bacteria have efficient mechanisms for __
The outer membranes of these species contain __
Other species, including E. coli, do not have __, but some DNA can still penetrate the outer surface of the bacterium and enter the cell
A bacterium takes up DNA that it encounters in its local environment
proteins that bind DNA and transfer it into the cell
DNA uptake protiens
Some bacteriophages do not immediately __. Instead, the virus genome becomes __, just like an F plasmid in an Hfr cell
The integrated virus genome is called a __
The transfer is mediated by a bacteriophage, one of the viruses that infect bacterial cells
kill the host cell
integrated into the host bacterium’s chromosome
At some stage the prophage will __
This is accompanied by __, some of which, by chance, will be about the same size as the bacteriophage genome
become excised from the host chromosome and replicate, and new virus particles will be constructed around the replicated virus genomes
breakdown of the bacterial chromosome into small fragments
By mistake, one of these small chromosome fragments might be __. The resulting particle is still __, as infection is solely a function of the proteins within which the DNA is enclosed. The fragment of bacterial DNA can therefore be transferred to a new cell when that recipient becomes “infected” with the __
packaged into a virus particle
transducing virus particle
Transferred bacterial DNA can become a permanent feature of the recipient’s genome If the DNA that is acquired by __, __, or __, it will be able to __, adding to the __ This is the outcome of most __events and might also arise from __, as it is quite possible for plasmid DNA to be taken up from the environment in this way
conjugation, transformation, and transduction
replicate in its new host
plasmid complement of the recipient cell
__ is immensely important in microbiology because it is responsible for the __
This brought about the evolution of __
- Horizontal transfer of plasmids
- spread of antibiotic resistance genes
- strains of pathogenic bacteria that can no longer be controlled with the antibiotics that previously were effective against them
Chromosomal DNA that is acquired might be __ for the __
Sometimes DNA can __. For this to happen, the DNA must __
broken down and used as a source of C, N, and P
construction of new molecules within the recipient bacterium
survive in the cell
become integrated into the bacterial chromosome, or into a chromid
This is possible if the transferred DNA has the __, as will be the case if it comes from the same species.
The segment of transferred DNA might then __
same or a similar sequence to a segment of the recipient cell’s chromosome
replace the equivalent stretch of DNA in the chromosome
If this happens then the recipient bacterium will __. As the genes are the same as the ones that it had before, the replacement might appear to be immaterial.
But, bacterial genes, just like eukaryotic genes, have variant forms called __, each one that it had before, the replacement might appear to be immaterial
inherit the genes from the donor
But bacterial genes, just like eukaryotic genes, have variant forms called __, each one with a slightly different DNA sequence Transfer might therefore result in the __. Its biological characteristics might therefore change in a small way
recipient acquiring a different allele for one or more of the genes that it obtains
Genome sequencing projects have told us that __.
Suggested mechanism: __
transfer of DNA also occurs between quite different species
transfer is by transformation because unrelated specieis are unable to conjugate, and transduction can take place only within the host range of the transducing bacteriophage, which is always quite narrow
The incoming DNA is unlikely to have __ with any part of the recipient’s chromosome, so __is assumed to occur __
extensive sequence similarity
Once it becomes a part of the chromosome, the new DNA, and any genes it contains, will __
become a permanent part of the recipient’s genome and passed onto daughter cells along with the rest of the genome when the parent cell divides
__ might not happen very frequently, but it occurs often enough for bacteria to acquire, from time to time, new genes from other species
Transfer across a species boundary
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