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Bacterial Reproduction | DAT But it is not the bacterium’s chromosome o The plasmid is separate from the chromosome and its presence is not critical for survival Binary Fission • Bacteria do not undergo mitosis or meiosis • Instead, they undergo a process known as binary fission • In the last chapter, we discussed DNA replication in prokaryotes: (a) (a) (b) Prokaryotic DNA Replication Prokaryotic DNA is round. In (a), the origin of replication is shown up top. The origin hosts two replisomes which will (b) work in opposite directions around the circle to replicate the DNA. • • • In binary fission, this method of DNA replication will be used After the DNA has been replicated, the cell splits into two daughter cells o Each daughter gets one copy of the DNA So, binary fission produces two genetically identical daughters Conjugation • Though binary fission is the only means of bacterial cell division, there are 3 ways that bacteria can undergo genetic recombination o In other words, there are 3 ways that bacteria can share their DNA • The first of these is called conjugation • Conjugation requires two bacteria, one of which must have a plasmid o A plasmid is a small ring of bacterial DNA (b) Bacteria With and Without Plasmids (a) Some bacteria have plasmids (shown by the arrow), while (b) other don’t. For conjugation, we will need one bacterium with a plasmid and one bacterium without a plasmid. • • The plasmid must be a conjugative plasmid, or a plasmid that contains the gene for the sex pilus The sex pilus is a tube that extends from one bacterium and permeates another o It allows the transfer of genetic information Conjugation With an assembled sex pilus, the plasmidcontaining cell can replicate its plasmid and transmit the copy to the other cell. 1 © 2017 J Co Review, Inc., Accessed by Guest on 06-16-2017 Bacterial Reproduction | DAT • • • Once the sex pilus is constructed, the plasmid-containing cell can duplicate its plasmid o And that copy can be transmitted to the other cell Two common conjugative plasmids are the F plasmid and the R plasmid o A bacterium containing the F or R plasmids is F+ or R+ respectively o A bacterium lacking those plasmids is F- or RThe F plasmid is capable of linking up with the bacterial chromosome o If this happens before the sex pilus forms, the entire bacterial chromosome may be copied and transmitted o This has made it increasingly difficult for humans to combat certain infections with conventional antibiotics Transformation • Transformation is another means of genetic recombination whereby bacteria take up naked DNA from their environment • Suppose, for instance, a cell bursts open during lysis o Its DNA will be released into the environment • Another cell can uptake that naked DNA via transformation Transformation Transformation is the uptake of naked DNA by a bacterial cell. (a) Transduction • Remember when we discussed bacteriophages, we showed that they latch onto their host and inject their viral genetic code (b) The F Plasmid (a) The F plasmid can open up and integrate itself with the bacterial chromosome. If this happens before the sex pilus forms, then (b), the entire bacterial chromosome can be replicated and transmitted through the sex pilus. • The R plasmid is known to be used by bacteria to share resistances to different antibiotics Bacteriophages Remember that bacteriophages digest a hole in the bacterium, and then inject their viral code. 2 © 2017 J Co Review, Inc., Accessed by Guest on 06-16-2017 Bacterial Reproduction | DAT • • Sometimes there may be a fluke in which the virus actually takes DNA from the bacterium o i.e. the plasmid in the previous illustration might be transmitted up into the virus’ capsid Subsequently, when that virus attacks another bacterium, it will inject the bacterial DNA o Thus, the virus facilitates genetic recombination Transduction (a) A bacteriophage latches onto a bacterial host, but instead of injecting its viral code, it takes up DNA from the bacterium. (b) The bacteriophage departs, carrying the bacterial DNA. (c) The bacteriophage latches onto another bacterium, and injects the bacterial DNA. • Transduction is commonly emulated in the laboratory with modified viruses called vectors o The vectors are modified so that they pose no health threat to the researcher 3 © 2017 J Co Review, Inc., Accessed by Guest on 06-16-2017