Bacteria reproduce by the process of binary fission. In this process, the chromosome in the mother cell is replicated and a copy is allocated to each of the daughter cells. As a result, the two daughter cells are genetically identical. If the daughter cells are always identical to the mother, how are different strains of the same bacterial species created? The answer lies in certain events that change the bacterial chromosome and then these changes are passed on to future generations by binary fission. In this chapter, you will explore some of the events that result in heritable changes in the genome: genetic transfer and recombination, plasmids and transposons.
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- Genetic recombination refers to the exchange between two DNA molecules.
- It results in new combinations of genes on the chromosome.
- You are probably most familiar with the recombination event known as crossing over.
- In crossing over, two homologous chromosomes (chromosomes that contain the same sequence of genes but can have different alleles) break at corresponding points, switch fragments and rejoin.
- The result is two recombinant chromosomes.
- In bacteria, crossing over involves a chromosome segment entering the cell and aligning with its homologous segment on the bacterial chromosome.
- The two break at corresponding point, switch fragments and rejoin.
- The result, as before, is two recombinant chromosomes and the bacteria can be called a recombinant cell.
- The recombinant pieces left outside the chromosome will eventually be degraded or lost in cell division.
- But one question still remains...how did the chromosome segment get in to the cell?
- The answer is Genetic Transfer!
- Genetic transfer is the mechanism by which DNA is transferred from a donar to a recipient.
- Once donar DNA is inside the recipient, crossing over can occur.
- The result is a recombinant cell that has a genome different from either the donar or the recipient.
- In bacteria genetic transfer can happen three ways:
- Remember that a recombination event must occur after transfer in order that the change in the genome be heritable(passed on to the next generation).
- After death or cell lyses, some bacteria release their DNA into the environment.
- Other bacteria, generally of the same species, can come into contact with these fragments, take them up and incorporate them into their DNA by recombination.
- This method of transfer is the process of transformation.
- Any DNA that is not integrated into he chromosome will be degraded.
- The genetically transformed cell is called a recombinant cell because it has a different genetic makeup than the donar and the recipient.
- All of the descendants of the recombinant cell will be identical to it.
- In this way, recombination can give rise to genetic diversity in the population.
- The transformation process was first demonstrated in 1928 by Frederick Griffith.
- Griffith experimented on Streptococcus pneumoniae, a bacteria that causes pneumonia in mammals.
- When he examined colonies of the bacteria on petri plates, he could tell that there were two different strains.
- The colonies of one strain appeared smooth.
- Later analysis revealed that this strain has a polysaccharide capsule and is virulent, that it, it causes pneumonia.
- The colonies of the other strain appeared rough.
- This strain has no capsules and is avirulent.
- When Griffith injected living encapsulated cells into a mouse, the mouse died of pneumonia and the colonies of encapsulated cells were isolated from the blood of the mouse.
- When living nonencapsulated cells were injected into a mouse, the mouse remained healthy and the colonies of nonencapsulated cells were isolated from the blood of the mouse.
- Griffith then heat killed the encapsulated cells and injected them into a mouse.
- The mouse remained healthy and no colonies were isolated.
- The encapsulated cells lost the ability to cause the disease.
- However, a combination of heat-killed encapsulated cells and living nonencapsulated cells did cause pneumonia and colonies of living encapsulated cells were isolated from the mouse.
- How can a combination of these two strains cause pneumonia when either strand alone does not cause the disease?
- If you guessed the process of transformation you are right!
- The living nonencapsulated cells came into contact with DNA fragments of the dead capsulated cells.
- The genes that code for thr capsule entered some of the living cells and a crossing over event occurred.
- The recombinant cell now has the ability to form a capsule and cause pneumonia.
- All of the recombinant's offspring have the same ability.
- That is why the mouse developed pneumonia and died.
- Transposons (Transposable Genetic Elements) are pieces of DNA that can move from one location on the chromosome another, from plasmid to chromosome or vice versa or from one plasmid to another.
- The simplest transposon is an insertion sequence.
- An insertion sequence contains only one gene that codes frotransposase, the enzyme that catalyzes transposition.
- The transposase gene is flanked by two DNA sequences called inverted repeats because that two regions are upside-down and backward to each other.
- Transposase binds to these regions and cuts DNA to remove the gene.
- Yhe transposon can enter a number of locations.
- When it invades a gene it usually inactivates the gene by interrupting the coding sequence and the protein that the gene codes for.
- Luckil, transposition occurs rarely and is comparable to spontaneous mutation rates in bacteria.
- Complex transposons consist of one or more genes between two insertion sequences.
- The gene, coding for antibiotic resistance, for example, is carried along with the transposon as it inserts elsewhere.
- It could insert in a plasmid and be passed on to other bacteria by conjugation.