Chapter 7: Microbial Genetics
The Structure and Replication of Genomes
The Structure of Nucleic Acids
-DNA and RNA are polymers of repeating nucleotides
-nucleotides are linked through their sugars and phosphates to form backbones.
-the phosphate on the 5’ carbon of the sugar is linked to the 3’ carbon of the next nucleotide. In DNA these strands run antiparallel
The Structure of Prokaryotic Genomes
-Prokaryotic Chromosomes-
-Circular chromosome is concentrated in an area called the nucleoid
-Plasmids are small circular molecules of DNA that replicate independently of the chromosome
-genes on plasmids are generally not essential for normal metabolism, growth, or reproduction.
-the genes on plasmids can confer advantages to the cells that carry them
Gene Function
Regulation of Genetic Expression-
The Nature of Prokaryotic Operons- Operons were first proposed in 1961, the operon consists of
1. promoter:
2. operator:
3. structural genes:
-Inducible operons-
-Repressible operons-
-The Lactose Operon is an example of an Inducible Operon
-it includes a promoter, an operator, and three genes that code for enzymes that are involved in catabolism of lactose.
-The Tryptophan Operon is an example of a Repressible Operon
-it includes a promoter, an operator, and five genes that code for enzymes involved in tryptophan synthesis.
-Research has shown that multiple operons can be controlled by a single repressor, or that transcription rates can vary with the concentrations of corepressors.
Mutations of Genes
Types of Mutations
-point mutations (substitutions, insertions, deletions)
-frameshift mutations
Effects of Mutations
-silent mutation-
-missense mutation-
-nonsense mutation-
Mutagens
-Radiation- can be ionizing or nonionizing.
-Chemical mutagens, three general types
-nucleotide analogs-
-nucleotide altering chemicals-
-frameshift mutagens-
Frequency of Mutation- mutations are rare events.
- one in every 107 genes contains an error.
-mutagens increase mutation rate to one in 104 to 106
DNA Repair- bacterial cells have mechanisms to repair DNA damage which may generate mutations.
-Repair of pyrimidine-dimers: thymine dimers result from exposure to UV light, two adjacent thymines bond together, instead of adenine.
-Base-Excision Repair: a wrong base is inserted during DNA replication
-Mismatch Repair: similar to base-excision, an incorrect base is incorporated into the new DNA strand.
-SOS Response: acts when major damage occurs to DNA that cannot be handled by the regular repair systems. (only in bacteria)
Identifying Mutants, Mutagens, and Carcinogens
-Positive Selection- a mixture of wild type and mutants are grown on a plate that will select against the wild type (Ex. penicillin resistant [mutants] bacteria will grow on a plate with penicillin, wild types will not)
-Negative (indirect) Selection- used for bacteria that have lost the ability to produce a needed metabolite (auxotroph). Follows these steps:
1.
2.
3.
4.
5.
6.
Ames Test for Identifying Carcinogens
-Uses Salmonella that are his-, histidine auxotrophs, follows these steps:
1.
2.
3.
Genetic Recombination and Transfer
Horizontal Gene Transfer among Prokaryotes
-genetic material is passed from a donor cell to a recipient cell of the same generation. (vertical transfer is from parent to offspring)
-recipient and donor can be of the same or difference species or genus.
-the recipient usually incorporates the donor DNA into their chromosome, becoming a recombinant.
-horizontal transfer is rare, occurring in less than 1% of the population, however, there are three types of horizontal gene transfer:
1. Transformation-
2. Transduction-
3. Bacterial Conjugation-
Transposons and Transposition
-transposons are segments of DNA (700-40,000 bp) that move themselves from one location in a DNA molecule to another location in the same or a different molecule.
-transpositions can result in frameshift mutations, rare
-can be simple; containing just the palindromes on either side of the gene for trasposase, or complex; containing two transposons on either side of a non-related gene (kanamycin resistance).
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