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Chapter 5-The Genetics of Bacteria and Their viruses-Oct 13.docx

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Department
Biology
Course
BIOL 205
Professor
Kenton Ko
Semester
Fall

Description
CH 5 THE GENETICS OF BACTERIA AND THEIR VIRUSES Oct 13/11- pg 181- - Bacteria often carry extra DNA elements called plasmids that are smaller than the main bacterial genome - Bacteria can be parasitized by viruses called bacteriophages which can have DNA or RNA as genetic material 5.1 Working with Microorganisms - Liquid culture of bacteria is spread onto an agar plate in the process of plating, this forms a clump of bacteria called a colony with members of a colony that have a single ancestor as a cell clone Categories of mutants Prototrophic 1. Wild type bacteria are prototrophic, meaning that they can grow on minimal medium, a substrate containing only inorganic salts Auxotrophic - From this culture, auxotrophic mutants can be obtained, these are cells that won’t grow unless the medium contains one or more specific cellular building blocks such as adenine, threonine or biotin. 2. Another type of mutant can’t use a specific energy source whereby the wild type can. (I.e. lac+ can use lactose and lac- can’t) Resistant mutants 3. Wild types are susceptible to an inhibitor, such as antibiotic streptomycin, resistant mutants can divide and form colonies in the presence of the inhibitor. Genetic markers - These types of mutants are genetic markers to keep track of genomes and cells in experiments. 5.2 Bacterial Conjugation Discovery of Conjugation - Discovered by Lederberg and Tatum who studied 2 strains of E.coli with different auxotrophic mutations. o Strain A: met- bio- thr+ leu+ thi+ o Strain B: met+ bio+ thr- leu- thi- - The strains were mixed together and plated on minimal medium where none of the auxotrophs could grow - A small minority of cells were found to grow as prototrophs (wild type) and grew but the controls had no growth - Some form of recombination of genes had taken place. Could it be that the cells of 2 strains leak substances that the other cells can absorb and use for growing? - Davis constructed a U-tube where the arms were separated by a filter that can’t let bacteria to pass but dissolved substances passed through - Prototrophic cells were searched for but none were found. - Physical contact is needed for these wild type cells to form. Conjugation - The physical union of bacterial cells is conjugation Discovery of the Fertility Factor (F) - Hayes discovered that one parent seemed to transfer some or all of its genome into the other cell – donor into the recipient which is different from eukaryotes that transfer genomes equally. - Donor ability is itself a hereditary state, imposed by a fertility factor F+ - F+ can donate F- - F- can’t donate Process of conjugation: 1. The F plasmid directs the synthesis of a pili, projections that initiate contact with a recipient and pulls it closer Rolling circle replication 2. F DNA makes a single stranded copy of itself using rolling circle replication where the copy is reeled out through a pore into the recipient, where it becomes a double strand. 3. A copy of F remains in the donor and the other appears in the recipient. Hfr Strains - Hfr strains have the F plasmid integrated into bacterial genome - Sforza discovered an F+ strain with 2 unusual properties: 1. When crossed with F- strains, the new strain produced 1000 times as many recombinants as a normal F+ strain 2. In Hfr x F- crosses, none of the F- parents were converted to F+ or into Hfr. This is different because when crossed F+ x F-, infectious transfer of F results in a large proportion of the F- parents being converted into F+. - During conjugation, the F factor inserted in the chromosome efficiently drives part or all of that chromosome into the F- cell, the chromosomal fragment can then engage in recombination with the recipient chromosome = production of a lot of recombinants - The replication of the chromosome ensures a complete chromosome for the donor cell after mating, creating a recombinant cell. If no recombination, transferred fragments of DNA are lost in cell division. Linear transmission of HFR genes from a fixed point
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