Week 6 Lecture 2

Recombination: Eukaryotes = reciprocal  Bacterial = nonreciprocal (one way transfer of DNA from donor to recipient) Previously we discussed recombination in eukaryotes. Recombination is the exchange of chromosome segments between homologous chromosomes. Reciprocal recombination is the equal exchange between chromosomes. The paternal chromosomes can give up a segment of DNA to the maternal chromosome and vice versa. Eukaryotic recombination is characterized by reciprocal recombination. Recombination occurs in every organism. Species benefit from recombination because it introduces variability into the gene pool which may better equip the species for adapting to environmental changes, an introduction of a predator, etc. Individuals within a species are able to "try out" new combinations of alleles. Most new combinations of alleles are neutral. Prokaryotic (Bacterial) recombination: Prokaryotic mechanisms of recombination are different from eukaryotic. The life cycle of bacterial cells and virus are very different from that of yeast and human cells, for example. Bacteria have evolved mechanisms to allow for recombination of their single circular chromosome. Non reciprocal recombination occurs in bacteria. It is a one-way transfer of DNA from the "donor" cell to the "recipient" cell, followed by recombination. The donor in all cases is dying or dead and DNA from the donor has the potential to be picked up by another bacterial cell and incorporated into its own DNA. Recall that prokaryotes have no nucleus and no nuclear membrane. Bacteria have evolved three different mechanisms to allow for this one way transfer of alleles:     1) Conjugation is one transfer mechanism that does not involve dead cells. Conjugation is analogous to mating between two bacteria followed by DNA transfer and the potential for recombination.      2) Transformation is the uptake of naked DNA from the bacterial cell's surroundings. If the naked DNA is homologous to the cell's DNA it can be incorporated into the genome.      3) Transduction is virus mediated. In the aftermath of a viral infection in which infection has lead to the death of the host, the virus can pick up host DNA and transfer it into a second, new host. In all three cases the donor cell receives no DNA from the recipient cell. Conjugation: Conjugation was first discovered in 1946 by Lederberg and Tatum. They were able to determine that bacteria in solution could exchange genes. DNA is not reciprocally exchanged in conjugation, a male (donor) strain and a recipient (female) strain can be identified by the proteins on their cell surface. These strains were identified as F+ and F- strains, respectively. F indicates "fertility". Mating can only occur between two opposite strains. Two F+ strains or two F- strains cannot mate. The F+ strain has an extra chromosome in its cytoplasm called a "plasmid" or "F factor." It is a small, double-stranded mini chromosome that is circular. Its genes have the ability to direct DNA transfer into a recipient cell. The figure below illustrates F+ and F- strains. INCLUDEPICTURE "http://lifesciences.asu.edu/bio340/goldstein/strains.jpg" \* MERGEFORMATINET INCLUDEPICTURE "http://lifesciences.asu.edu/bio340/goldstein/convrsn.gif" \* MERGEFORMATINET       When an F+ cell bumps into an F- cell it recognizes it by its cell surface markers (proteins). During conjugation, It constructs a "pilus" (a protein bridge, encoded by genes on the F plasmid). The transfer of DNA occurs across this bridge. One of the F+ plasmids becomes incorporated into the F- cell. After the transfer of a plasmid from the F+ to the F- cell, the F- cell becomes F+. In crosses of F+ and F- cells all of the cells become F+. This results in a one-way DNA transfer. F+ x F- -> All F+ cells Properties of the plasmid: The first property of the "F factor" or "fertility factor" is that it is autotransmissable. It contains the genes that encode for the pilus and its mobilization across to the F- cell. Most plasmids are autotransmissable. Its second property is that it is autonomous for replication. It strictly controls the number of replications per cell cycle on its own. The bacterial chromosome is strictly constrained to one replication per cell cycle. These plasmids are under no such constraint. Its replication cycle is separate from that of the bacterial chromosome. Since the plasmid is a fully independent chromosome, the bacterial cell can lose the plasmid. The most common plasmids are the drug resistance plasmids. They are also small mini chromosomes like those discussed above. They are autotransmissable and autonomous. They also retain genes which enable them to destroy or evade antibacterial drugs. They can introduce drug resistance into bacteria or can pass it between bacterial species. A New Bacterial Strain In 1950 a new bacterial strain was discovered called Hfr - High frequency recombination. The Hfr strain transfers genes at a 1000 times higher rate than F+. However, only a limited number of genes can be transferred. Hfr cells result when a plasmid in a F+ cell integrates into the cell chromosome forming a main chromosome with a minichromosome integrated. Integration is accomplished by crossing over between the plasmid and the chromosome. Within the cell chromosome, there are multiple possible integration sites for the plasmid; an episome is a plasmid that can integrate.  Hfr cells do not have free plasmids. The process of the minichromosome being taken out of the Hfr chromosome is called excision.  INCLUDEPICTURE "http://lifesciences.asu.edu/bio340/goldstein/Hfr.GIF" \* MERGEFORMATINET