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Chapter 9

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Biological Sciences

 Chapter 9: Genetic Recombination  9.1 Mechanisms of Genetic Recombination: - Genetic recombination (diagram p.182 figure 9.1) requires the following:  two DNA molecules that differ from one another, (2) a mechanism for bringing the DNA molecules into close proximity, (3) and a collection of enzymes to “cut”, “exchange” and “paste” the DNA back together - the sugar-phosphate backbone of DNA is held together by strong covalent bonds whereas the base pairs are bonded by relatively weak hydrogen bonds - Most of the recombination occurs between regions of DNA that are very similar but not identical homologous   9.2 Genetic Recombination in Bacteria: - Escherichia coli (E. coli) extracted from a diaper during a diarrhea outbreak in 1885 named after Viennese pediatrician Theodore Escherich - 1946 Joshua Lederberg and Edward Tatum set out to determine if genetic recombination occurred in bacteria using E. coli as their experimental organism - E. coli and many other bacteria can be grown in a minimal medium (containing water, organic carbon source and an inorganic salt; this medium can be a liquid or a gel) - In order for Lederberg and Tatum to detect genetic recombination, they needed detectable differences this difference was related to nutrition - Strains that were able to synthesize necessary amino acids prototrophs; strains unable to synthesize amino acids auxotrophs (auxotrophs can only grow if the required amino acid is provided in the growth medium) - A strain that cannot manufacture its own arginine is represented as: argA - (a form of mutation) + - A strain that can manufacture its own arginine is written as argA (this naming can be used for any other amino acids or mutations present in an organism)  -Lederbeg-Tatum experiment further detail (p. 184 figures 9.2-9.3)  -Bacteria haploid organisms, has its own single circular chromosome  -Transfer of genetic information in bacteria unidirectional (going from one donor cell to the recipient cell) - Bacteria cells conjugate cells contact each other through the sex pilus that forms a cytoplasmic bridge between the two bacteria - During conjugation, a copy of the part of DNA of one cell moves through the cytoplasmic bridge into the other cell. Once DNA from one cell enters the other, genetic recombination can occur - Through the unidirectional transfer of part of the chromosome, conjugation facilitates a kind of sexual reproduction in prokaryotic organisms  F Factor Conjugation  -Conjugation is initiated by a bacterial cell that contains a small circle of DNA in addition to the main circular chromosomal DNA - Small circle DNA plasmids  -Fertility plasmid or F factor  initiates conjugation - F factor carries several genes as well as a replication origin that permits a copy to be passed on to each daughter cell during the process of bacterial cell division (vertical inheritance) - F factor that is passed from one bacterial donor cell to another recipient cell during conjugation (horizontal inheritance) - REFER TO DIAGRAM 9.6 ON PAGE 186 - Donor cells F cells because they contain the F factor - - Recipient cells F cells because they lack the F factor - F pilus sex pilus - When the entire F factor strand has transferred and replicated, it circularizes again. Although the recipient cell becomes F no chromosomal DNA is transferred between cells in this process  Hfr Cells and Genetic Recombination - When two circular DNA molecules recombine they simply fuse together into one larger circle  -Hfr(high-frequency recombination) donor cells  bacterial chromosome mixed with the F factor - Hfr cells are called “high-frequency recombination” cells because they can promote recombination between DNA of different cells by “exporting” copies of chromosomal genes in to another cell - When the F factor in integrated into the bacterial chromosome, its genes are still available for expression. Therefore, Hfr cells make sex pili conjugate with an F cell - The conjugation bridge is fragile, therefore it is rare for the entire donor chromosome to be transferred due to the breaking of the conjugation bridge - The recipient cell will become a partial diploid (A bacterial cell having a second copy of a particular chromosomal region)   Transformation and Transduction  Transformation: - In transformation, bacteria simply take up pieces of DNA that are released into the environments as cells disintegrate  -Only some species of bacteria can take up DNA from the surrounding medium by natural mechanisms  Transduction: - In transduction, DNA is transferred from donor to recipient cells inside the head of an infecting bacterial virus “by mistake” - Bacteriophages: viruses that infect bacteria - REFER TO FIGURE 9.7 PAGE 189  -Transduction begins when new phages assemble within an infected bacterial cell (they sometimes incorporate fragments of the host cell DNA along with or instead of viral DNA) - After the host cell is killed, the new phages that are released may then attach to another cell and inject bacterial DNA into that recipient cell  -The introduction of this DNA makes the recipient cell a partial diploid and allows recombination to take place - There are two different types of transduction: (a) generalized and (b) specialized  -Generalized: all donor genes are equally likely to be transferred, is associated with some virulent bacteriophages, which kill their host cells during each cycle of infection (lytic cycle). Generalized transduction transfers random fragments of the host chromosome and all host genes are transferred equally  -Specialized: when a bacteriophage first infects a new host, it determines whether this cell is likely to be a robust and long-lived host. The phage is thus integrated into the host chromosomal DNA and called a prophage. The prophage is then replicated and passed to daughter cells along with the rest of the bacterial chromosome as long as conditions remain favourable (lysogenic cycle). - REFER TO FIGURE 9.8 PAGE 190 - Conjugation, transformation, and transduction are all ways in which DNA from two different bacterial cells brought into close proximity. Homologous regions may then pair and recombine to give rise to a recipient cell that carries a different collection of alleles.  -Overall these processes create more diversity in the DNA sequence among members of a population than would arise by mutation and binary fission alone  -More diversity higher likelihood that individuals will be well suited to survival in a changing environment  9.3 Genetic Recombination in Eukaryotes: Meiosis - Sexual reproduction: combination of male and female gametes (sex cells) - Meiosis: specialized process of cell division that recombines DNA sequences and produces cells with half the number of chromosomes present in the somatic cells (body cells) of a species - At fertilization, the nuclei of an egg and the sperm cell fuse making a zygote  -Without halving of chromosome number by meiotic divisions, fertilization would double the number of chromo
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