CH. 18: THE GENETICS OF VIRUSES & BACTERIA. Well-researched: Pneumococcus, Escherichia coli , bacteriophages, TMV. Bacteria. Bacterial cell shapes. Gram staining. Classification of bacteria according to what color stain they retain.
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Well-researched: Pneumococcus, Escherichia coli, bacteriophages, TMV
Classification of bacteria according to what color stain they retain.
A sample of bacteria (on a slide) is first stained with a violet dye.
The slide is then rinsed with ethanol.
If the violet stain washes off, a red dye is added (“counterstaining”).
Depending on the structure of the cellwalls, some types of bacteria (such as staphylococcus and streptococcus) retain the violet stain and are called Gram-positive.
Other types (such as pseudomonas and salmonella) retain the red, but not the violet, stain and are called Gram-negative.
This technique is named after the Danish bacteriologist Hans C. J. Gram (1853-1938) who invented it in 1884.
Gram + (top), Gram – (bottom)
Prokaryotic Cells Replicate their DNA in a bidirectional fashion (replication)
This is regulated by several means:
The Operon Model (1965 Nobel Prize) François Jacob, André Lwoff, and Jacques Monod "for their discoveries concerning genetic control of enzyme and virus synthesis"
- arose from study of mutant cells
studies done on E. coli cells making the enzyme: beta-galactosidase ; found it was "blocked" by a repressor binding to operator.
Operator – gene that activates transcription of a structural geneRegulator - This gene codes for a repressor proteinRepressor: protein that can bind to the operator gene, thus obstructing* the promotor (blocks the RNA polymerase from moving along ("reading") the molecule no mRNA transcription can occur.*when the repressor is removed, mRNA transcription beginsanimations of lac and tryp operons
Bacterial plasmids are often used in genetic engineering, as a “gene of interest” can be inserted easily into them, then cloned into hundreds of copies when the bacteria multiply.
Recombinant DNA: involves modifying/combining DNA from a variety of different sources and inserting these altered molecules into other cells, in which the "new "genes are expressed.
Hfr Cell: Sometimes the F factor gene can be incorporated within the main bacterial chromosome. Because such strains transfer chromosomal genes very efficiently, they are called Hfr (high frequency of recombination).This can then transfer a portion of bacterial chromosome to a F- cell:
diagram: bacteriophages TMV
1. Contain a varying, but small amount of DNA (or RNA) surrounded by protein
T7 bacteriophage has DNA and ~100 genes;
The poliovirus has 7,600 RNA nucleotides;
the vaccinia (cowpox) virus has 240,000 DNA nucleotide pairs
2. Small viruses that don’t have room for a lot of DNA use overlapping genes
3. Retroviruses (ex: HIV) are RNA viruses that use an enzyme called Reverse Transcriptase to make DNA to replicate itself during infection stage)
1. General Transduction
Transduction is bacterial chromosome DNA transfer mediated by a virus. This is a one way nonreciprocal virus mediated transfer from one bacterial cell to another.
2.Restricted (specialized) Transduction
The situation in which a particular phage will transduce only specific regions of the bacterial chromosome.
* can you think of the implications to genetic engineering? gene therapy?
Lambda plaques on an E.coli culture
Many transposons move by a "cut and paste" process: the transposon is cut out of its location (like command/control-X on your computer) and inserted into a new location (command/control-V).
This process requires an enzyme — a transposase — that is encoded within some of these transposons.