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Chapter 10


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BIOL 1000
Julie Clark

Tanya Sivamanoaharan Fri, Dec, 9/11 Biology- chap# 10 textbook notes GENETIC RECOMBINATION -ultimate source of genetic diversity occurs by mutations of the DNA sequence, made by errors during DNA replication -genetic recombination cutting and pasting DNA backbones into new combinations -genetic recombination ensure that offspring are not identical to parent cell during sexual reproduction -genetic recombination is crucial during meiosis 10.1- Mechanism of Genetic Recombination -genetic recombination requires: 2 DNA molec. that differ from each other at atleast 2 diff. places -mechanism to bring DNA to close proximity, collection of enzymes cut, exchange, and paste DNA back together -homologous regions of DNA may be short as a few base pairs or long as a chromosome -homology allows diff. DNA molec. to line up and recombine -homologous DNA are paired and enzymes break covalent bond of backbone of DNA and the free ends of the backbone exchange and reattach to other DNA molecules -the result is 2 recombined molecules with parts from 2 diff. DNA -cutting and pasting of 4 DNA backbones results in 1 recombination event 10.2- Genetic Recombination in Bacteria -genetic recombination historically associated to meiosis and sexual reproduction -it was later proven that bacteria reproduce by recombination as well 10.2b- Bacterial Conjugation Brings DNA of Two Cells into Close Proximity -in eukaryotes genetic recombination occurs in diploid cells by exchanging segments b/w a pair of chromosomes -bacteria, however are haploid cells that cannot recombine by exchanging b/w 2 chromosomes -bacteria transfer genetic info. from one cell to another -bacterial cells are conjugate cells contact each other by long tubular structure called sex pilus, then form a cytoplasmic bridge -conjugation a copy of part of the DNA of one cell move through the cytoplasmic bridge into the other cell -once DNA from 1 bacteria cell reaches another bacteria cell, recombination can occur -bacterial conjugation, thus facilitates a kind of sexual reproduction in prokaryotes -the F factor and conjugation: conjugation is initiated by bacterial cell that contains small circular DNA called plasmids in addition to main chromosomal DNA -plasmids small circles of DNA, specific plasmid called fertility plasmid/F factor -the F factor plasmids carries genes and a replication origin that permits a copy to be passed on to each daughter cell during bacterial cell division -horizontal inheritance F factor has ability to be copied and passed directly from 1 cell called donor to another cell called recipient -donor cells are called F+ cells b/c they contain F factor plasmids -recipient cells are called f- cells b/c thy lack F factor plasmids -rolling circletype of DNA replication where F plasmid replicates -the recipient cell becomes F+ at the end of the transfer but no chromosomal DNA is transferred b/w the cells -no genetic recombination occurs b/w the DNA of 2 diff. cells mating -Hfr cells and genetic recombination: in some F+ cells, the F factor comes in close proximity with main chromosome of bacteria, and goes through a recombination event -the F+ plasmids and the chromosomal DNA simply fuse into 1 large circle, where F factor becomes part of chromosomal DNA -Hfr (high frequency recombination) cellsspecial donor cells with F+ plasmids and chromosomal DNA fused -Hfr cells make sex pili and conjugate with F- cells -although several genes of DNA are transferred to recipient cell, the conjugate cytoplasmic bridge breaks and the entire donor chromosomes are not copied -in this case the recipient cell does not get a full copy of the F factor nor the chromosomal DNA therefore it is called a partial diploid -mapping genes by conjugation: the longer Hfr cells and F- cells were conjugated before separation, the greater the number of donor genes that entered the recipient and produced recombinants 10.2c- Transformation and Transduction Provide Additional Sources of DNA for Recombination -discovery of conjugation and genetic recombination in E.coli proved that they were not exclusive to eukaryotes -DNA can be transferred from 1 bacteria to another by 2 other mechanisms called transformation and transduction -transformation and transduction enable recipient cells to recombine with DNA obtained from dead donors -transformation: bacteria take up pieces of DNA released into environment as other cells disintegrate -living nonvirulent cells had been transformed to virulence by something released from dead cells -the substance derived from killed virulent cells capable of transforming nonvirulent bacteria to the virulent form was DNA -artificial transformation is used to insert recombinant DNA plasmids into E.coli cells as part of cloning or genetic engineering techniques -transduction: DNA is transferred from donor to recipient cells inside the head of infecting bacterial virus -transduction begins when new phages assemble in an infected bacterial cell -after host cell is killed the new phages that are released can attach to another host cell and inject the bacterial DNA into recipient cell -there are 2 diff. types of transduction: generalized and specialized -generalized transduction in which all donor genes are equally likely to be transferred -1. the phage attaches to host cell -2. the phage injects its DNA into the host cell. Expression of phage genes produces an enzyme that breaks down bacterial chromosome -3. the phage DNA is replicated inside the host cell -4. viral head and tail units are synthesized -5. the phage DNA head, and tail units assemble into complete phage particles. Note that a piece of undegraded bacterial DNA has been packaged by mistake. -6. the cell ruptures, releasing the new phage particles. One progeny phage carries bacterial DNA instead of phage DNA -7. bacterial DNA is injected into the next host where it can recombine with similar regions on the host chromosome -specialized transductiontransfer of bacterial genes b/w 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 -1.lambda viral particle binds to the wall of host cell, and linear viral DNA enters the cell’s cytoplasm -2. the viral DNA forms a circ
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