The Genetics of Bacteria and Their Viruses PDF

The Genetics of Bacteria and Their Viruses Ch. 8; pp. 163-180; Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012  For a research scientist, bacteria and viruses have several advantages compared to creatures like maize or Drosophila. First, they...

The Genetics of Bacteria and Their Viruses Ch. 8; pp. 163-180; Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012  For a research scientist, bacteria and viruses have several advantages compared to creatures like maize or Drosophila. First, they are small, reproduce quickly, and form large populations in just a matter of days. An experimenter can grow 1010 bacteria in a small culture tube; 1010 Drosophila, by contrast, would fill a 14 ft x14 ft x 14 ft room. Second, bacteria and viruses can be grown on biochemically defined culture media. Because the constituents of the culture medium can be changed as desired, a researcher can identify the chemical needs of the organism and investigate how it processes these chemicals during its metabolism. Drugs such as antibiotics can also be added to the medium to kill bacteria selectively. This type of treatment allows a researcher to identify resistant and sensitive strains of a bacterial species—for example, to determine M. tuberculosis cultured from a patient is resistant to a particular antibiotic. Third, bacteria and viruses have relatively simple structures and physiology. They are therefore ideal for studying fundamental biological processes. Finally, genetic variability is easy to detect among these tiny microorganisms. If we examine bacteria or viruses, we almost always find that they manifest different phenotypes and that these differences are heritable. For example, some strains of a bacterial species can grow on a biochemically defined medium containing lactose as the only energy source, whereas other strains cannot. Strains that are not able to grow on this type of medium are mutant.  Bacteriophage T4 (a) Diagram showing the structure of bacteriophage T4 and (b) electron micrograph of a T4 bacteriophage (center) from which the DNA has been released by osmotic shock. Both ends of the linear DNA molecule are visible. The T4 chromosome is approximately 168,800 base pairs long and contains about 150 characterized genes and an equal number of uncharacterized sequences thought to be genes.  The life cycle of bacteriophage T4 Bacteriophage T4 is a lytic phage; when it infects a bacterium, it replicates and kills the host, producing about 300 progeny viruses per infected cell T4 DNA contains an unusual base—5-hydroxymethylcytosine (HMC; cytosine with a OCH2 OH group attached to one of the atoms in the cytosine molecule)—instead of cytosine  Bacteriophage λ Electron micrograph (a) and diagram (b) showing the structure of bacteriophage λ  The life cycle of bacteriophage λ The two intracellular states of bacteriophage lambda: lytic growth and lysogeny  Integration of the DNA molecule into the chromosome of E. coli. Inside the cell, the circular chromosome can proceed down either of two pathways. It can enter a lytic cycle, during which it reproduces and encodes enzymes that lyse the host cell, just like phage T4. Or, it can enter laysogenic pathway, during which it is inserted into the chromosome of the host bacterium and thereafter is replicated along with that chromosome. In this integrated state, the chromosome is called a prophage. For this state to continue, the genes of the prophage that encode products involved in the lytic pathway—for example, enzymes involved in the replication of phage DNA, structural proteins required for phage morphogenesis, and the lysozyme that catalyzes cell lysis—must not be expressed. Integration of the chromosome occurs by a site-specific recombination event between the circular DNA and the circular E. coli Chromosome. This recombination occurs at specific attachment sites—attP on the chromosome and attB on the bacterial chromosome—and is mediated by the product of the integrase. It covalently inserts the DNA into the chromosome of the host cell. The site-specific recombination occurs in the central region of the attachment sites where both attP and attB have the same sequence of 15 nucleotide pairs: GCTTTTTTATACTAA CGAAAAAATATGATT  Bacterial colonies showing colonies of the bacterium Serratia marcescens growing on agar-containing medium. The distinctive color of the colonies results from the red pigment produced by this species UNIDIRECTIONAL GENE TRANSFER IN BACTERIA Recombination in bacteria. The three types of gene transfer in bacteria  The U-tube experiment with bacteria The U-tube is used to determine whether or not cell contact is required for recombination to occur. Bacteria of different genotypes are placed in separate arms of the tube, separated by a glass filter that prevents contact between them. If recombination occurs, it cannot be due to conjugation. Thank You