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Chapter 10 Genetic Recombination.docx

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York University
BIOL 1500
Tamara Kelly

Heena Loomba 1 Chapter 10: Genetic Recombination 10.1 Mechanism of Genetic Recombination  Genetic recombination requires two DNA molecules that differ from one another in at least two places, a way to bring the DNA molecules into close proximity, and a collection of enzymes to cut, exchange and paste the DNA back together  Sugar-phosphate backbone of DNA is held by strong covalent bonds  Base pairs in DNA are held by weak hydrogen bonds  Homologous: similar  recombination occurs between similar regions in DNA  Homology allows different DNA molecules to line up and recombine precisely. Once homologous regions of DNA are paired, enzymes break a covalent bond in each of the four sugar-phosphate backbones. The free ends of each backbone are then exchanged and reattached to those of the other DNA molecule. Final result is two recombined molecules.  Cutting and pasting four DNA backbones= one recombination event 10.2 Genetic Recombination in Bacteria  Genetic Recombination occurs in E.Coli  E.coli and other bacteria can be grown in minimal medium. Clones (cluster, identical cells) of E.coli. Detectable differences related to nutrition that occurred during changing combinations were used.  Phototrophs: strains that were able to make the necessary amino acids  Auxotrophs: mutant strains that are unable to make amino acids, the required amino acid needed to be provided  Using mutagens (ex. X-rays), two different strains of E.coli were isolated which carried distinctive combinations of alleles for various metabolic genes  Many cells of the mutant strains were mixed together and placed on minimal medium. Colonies grew, but individually, none of the original cells carried all of the normal alleles needed for growth  Bacterial Conjugation Brings DNA of Two Cells into Close Proximity  Genetic recombination in eukaryotes occurs in diploid cells by an exchange of segments between pairs of chromosomes  Bacterial cells conjugate to bring their DNA together because cells contact each other by a long tubular structure (sex pilus) and then form a cytoplasmic bridge. During conjugation, a copy of part of the DNA of one cell moves through the cytoplasmic bridge into the other cells. Then genetic recombination occurs Heena Loomba 2 Chapter 10: Genetic Recombination  Conjugated starts by a bacterial cell that contains a small circle of DNA (plasmid) and the main circular chromosomal DNA  This particular plasmid is the “fertility” or “F factor”  F factor carries many genes and a replication origin that allows a copy to be passed on to each daughter cell during bacterial cell divisionvertical inheritance  F factor can be copied and passed directly from one cell (donor) to another cell (recipient)  Donor cells: F+ cells  Recipient cells: F- cells  During conjugation, the F plasmid replicated using a “rolling circle”  No genetic recombination occurs between the DNA of two different cells in such mating (recipient cell becomes F+ bit no chromosomal DNA is transferred between the cells in this process)  Hfr Cells and Genetic Recombination  Hfr cells allow for high-frequency recombination because it transfers genes on a bacterial chromosome to a recipient bacterium  When the F factor is integrated into the bacterial chromosome, its genes are still active. The Hfr cells make sex pilli and can conjugate with an F- cell  In Hfr cells, the origin of transfer is near the middle of the integrated F factor. Which is why only half of the F factor DNA is transferred at the front of the chromosomal DNA and the other half can follow only after the rest of the entire chromosome.  It is unusual for a recipient cell to obtain the entire F factor and become Hfr as well. Usually the recipient cell will become a partial diploid, it will have two copies of only those genes that came through the conjugation bridge on the donor chromosomal DNA segment  Mapping Genes by Conjugation  The longer they allowed cells to conjugate before separation, the greater the number of donor genes that entered the recipient and produced recombinants  Transformation  Bacteria take up pieces of DNA that are released into the environment as other cells disintegrate  Ex. In the transformation of Sreptococcus, the linear DNA fragments taken up from disrupted virulent cells recombine with the chromosomal DNA of the nonvirulent cells Heena Loomba 3 Chapter 10: Genetic Recombination  Transduction  DNA is transferred from donor to recipient cells inside the head of an infecting bacterial virus  Bacteriophage: infection cycles of viruses that infect bacteria  Begins when new bacteriophages assemble in an infected bacterial cell, they sometimes incorporate fragments of the host cell DNA along with, or instead of, the viral DNA. After the host cell is killed, the new bacteriophages that are released may then attach to another cell and inject the bacterial DNA (and the viral DNA if it is present) into that recipient cell. The introduction of this DNA, as in conjugation and transformation, makes the recipient cell a partial diploid and allows recombination to take place. Recipients are not killed because the received bacterial DNA (not infective viral DNA)  Generalized transduction: transfer of bacterial genes between bacteria using virulent bacteriophages (which kill their host cell during each cycle of infection) that have incorporated random DNA fragments of the bacterial genome  Specialized transduction: transfer of bacterial genes between bacteria using temperate phages that have incorporated fragments of the bacterial genome as they make the transition from the lysogenic cycle to the lytic cycle  Lytic cycle: events from infection of one bacterial cell by a phage through the release of progeny phages from lysed cells  Lysogenic cycle: cycle in which the DNA of the bacteriophage is integrated into the DNA of those host bacterial cell and may remain for many generations 10.3 Genetic Recombination in Eukaryotes: Meiosis  Sexual reproduction: production of offspring through the union of female and male gametes  Meiosis: specialized process of cell division that recombines DNA sequences and produces cells with half the number of chromsomes present in the somatic cells of a species  Fertilization: nuclei of an egg and sperm cell fuse, producing a cell called the zygote  Meiosis and fertilization mix genetic information into new combinations so none of the offspring of a mating pair will be genetically identical to either their parents or their siblings He
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