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Biology Chapter 9

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Biology 1201A

Biology Notes Chapter 9 Genetic Recombination Pages 181- 210 Why It Matters Mutations in genetics are what causes the variation seen in life, it is important and makes organisms "sexual" (inserting for variation) rather than "asexual" (clones) GeneticRecombination:Literally cutting and pasting DNA backbones into new combinations. Allows "jumping" genes to move, insert some viruses into the chromosome of their hosts, underlies the spread of antibiotic resistance among bacteria and archaea and is at the heart of meiosis in eukaryotic organisms. 9.1 Mechanism of Genetic Recombination There are two requirements for genetic recombination, 1. two homologous chromosomes or regions, 2. a mechanism for bringing the DNA molecules close together and the enzymes that "cut", "exchange", and "paste" the DNA back together. Homologous:two things that appear the same but aren't when looked at microscopically, the two chromosome regions are homologous because they are vary similar but different for the sake of variation. Once the homologous regions of DNA are paired, enzymes break the 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 molecules. Cutting and pasting 4 DNA backbones results in 1 recombination event. Bases of DNA molecules are held together by relatively weak hydrogen bonds. 9.2 Genetic Recombination in Bacteria Genetic Recombination was for a long time only associated with meiosis in sexually reproducing eukaryotes. However genetic recombination can be found in bacteria and other non sexual organisms. It can bring two bacteria together and the two DNA molecules can recombine and make different offspring. 9.2A Genetic Recombination Occurs in E. coli In 1946, at Yale University, Lederberg and Tatum set out to determine if genetic recombination occurs in bacteria to see if they had a kind of sexuality in their reproduction process. They used E. coli. Bacteria are grown in a minimalmediumcontaining water, an organic carbon source such as glucose, and a selection of inorganic salts, including one that provides nitrogen. This can be a liquid or made into a gel by adding agar. Clones: in this sense scientists clone bacteria to form a culture in the agar gel, this collection is called a colony. Lederberg and Tatum mixed together to types of bacteria, one that could grow with biotin and methionine in the medium, the other could only grow with leucine, threonine, and thiamine. They were identified as bio+met+leu-thr-thi- and bio-met- leu+thr+thi+ as it follows genetic notation. They were placed in a medium and colonies grew, therefore there must have been a form of DNA recombination to allow for all 5 genes to be expressed. 9.2B Bacterial Conjugation Brings DNA of Two Cells into Close Proximity DNA recombination occurs between bacteria cells by a matter of forming a long tubular structure called the sexpilus, forming a cytoplasmic bridge, and continuing to conjugation. Conjugation is where a copy part of the DNA of one cell moves to the other through the sex pilus. DNA recombination follows. FFactorandConjugation: Conjugation is initiated by a bacterial cell that contains a small circle of DNA, called a plasmid, in addition to the main circular chromosomal DNA. The plasmid that initiates fertility, is known as the F Factor. The 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 usual process of bacterial cell division. VerticalInheritanceis from one generation to the next. However, during conjugation, the F factor also has the ability to be copied and passed directly from the donor cell to the recipient cell, this is HorizontalInheritance. Donor cells are called F+ cells because they contain the F factor and are able to mate with recipients but not other donor cellos. Recipients are called F- cells due to the lack of the F factor. The F factor carries about 20 genes, several for the sex pills (F pilus). During conjugation F plasmid replicates using a special DNA replication called rollingcircle. There is a site called the origin of transfer on the F plasmid, then there is a break in just one strand of the double helix of this site. It is then pulled and the remaining strand, still in a circle,e "rolls" like a spool of tape. DNA synthesis fills in the complementary bases to ensure that the F facto is double stranded in both donor and the recipient cells. Once the F factor has been transferred and replicated, it circularizes again. /although the recipient becomes F+, no chromosomal DNA is transferred between cells in this process, that is, no genetic recombination occurs between the DNA of two different cells in such a mating. HfrCellsandGeneticRecombination: When transferring the F factor into another bacteria, it becomes part of the whole chromosome, and these are known as Hfr cells (High frequency recombination). Hfr cells can promote genetic recombination between two different bacteria cells. This is as the section of the chromosome that includes the F factor integrated, where the origin of transfer is right in the middle of the F factor. So that when the bacteria makes its sex pili with the F- cell, the F factor (half because it starts in the middle) enters the cytoplasmic bridge and recombines with the host cell. The chromosome is fragile and generally breaks, leaving only several genes recombined, coining this cell a PartialDiploidas it contains two of some genes. The extra genes are lost, and the cell is then able to reproduce normally, producing new combinations of alleles. MappingGenesbyConjugation: There are two scientists responsible for genetic mapping discovery Jacob and Wollman, from Paris. Thye mated Hfr and F cells that differed in a number of alleles. At regular intervals after conjugation commenced, they removed some of the cells and broke them apart and separated and analyzed. They found that the longer they allowed cells to conjugate, the greater number of donor genes that entered the recipient. They were able to map relative positions of genes in the E. coli chromosome. 9.2C Transformation and Transduction Provide Additional Sources of DNA for Recombination The discovery of conjugation and genetic recombination in E. coli showed that genetic recombination is not restricted to eukaryotes. Also, DNA can transfer by two other mechanisms that direct one way and create partial diploids in which recombination can occur between alleles in the homologous DNA regions. This DNA used in the mechanisms are obtained from dead donors. Transformation: is the process by which bacteria pick up pieces of DNA that are released into the environment as other cells disintegrate. -Fred Griffith, in 1928, observed in his experiment with the virus pneumonia and mice, he used two strains of Streptococcus pneumoniae, one that was virulent and had a polysaccharide capsule, and the other nonvirulent, one did not. He saw that a mixture of previously heated and dead virulent strains and living nonvirulent strains still caused a dangerous virus. Clearly the dead cells released DNA that was transformed to other hosts. This is a demonstration of transformation for DNA recombination where the introduction of the normal allele for capsule formation is combined to the DNA structure and expresses the formation, making the cell virulent. In 1944 Oswald Avery found out that the substance that affected Griffiths experiments was DNA. Transduction:DNA is transferred from donor to recipient cells inside the head of an infecting bacterial virus, called a bacteriophage. Transduction begins when new phages are assembled within an infected bacterial cell. They sometimes incorporate fragments of the host cell DNA along with, or instead of, the viral DNA. After the host bursts, the new phages infect other hosts with either bacterial DNA and/or viral DNA. This new DNA makes the cell partial diploid also and allows for DNA recombination to take place. GeneralizedTransduction: all donor genes are equally likely to be transferred Some virulentbacteriophagesundergo generalized transduction as they kill their host cells during each cycle of infection. The phage develops around the virulent DNA, and the bacterial chromosome is degraded and used for synthesis of new phage chromosomes. However, sometimes a fragment of the host DNA is packaged within a new phage by mistake. This is called a transducingphageand delivers DNA linearly. This host cell will survive, incoming DNA may then pair, and recombine with the host chromosome. SpecializedTransduction: Lambda is a temperatebacteriophage. That is, when a lambda first infects a new host cell, it first determines whether it is damaged/healthy/good enough. The lambda chromosome then lines up with a small region of homology on the bacterial chromosome, and a phage-coated enzyme catalyzes a single recombination event (four DNA backbones). This phage is integrated into the host can called a prophage. (Similar to the F factor) The prophage is replicated and passed to daughter cells as long as conditions remain favourable. However, if there is damage the prophage activates several genes, releases itself from the chromeosome by a recombination event, and proceeds to manufacture new phages. They are released when the host bursts as a result of lyticgrowth. A mistake can occur in specialized transduction when the prophage is excised or partially excised from the host. As a result, this bacterial DNA is packaged into new phages and carried to recipient cells. Since the transducing phage is defective, having left some of its genes behind in the host, it does not kill the new host. Generally only bacterial genes that lay close to the integration site of the phage will undergo a recombination mistake. 9.3 Genetic Recombination in Eukaryotes: Meiosis SexualReproduction:
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