Chapter 8 Microbial Genetics Flashcards
Two Methods of Genetic Change in Bacteria
1) Mutation
2) Horizontal Gene Transfer
Mutation
- A change in the nucleotide sequence of DNA
- May cause a change in a protein encoded by a gene
Horizontal Gene Transfer
Genetic info is passed from one living cell to another cell of the same generation
Example: Plasmid Transfer
Base Substitution/ Point Mutation (Mutation)
- A single nucleotide is replaced by another nucleotide
- When DNA replicates - results in a substituted base pair
- When DNA is transcribed and translated can result in an incorrect amino acid in the protein - Missense mutation
Frameshift Mutation (Mutation)
- Insertion - one or two nucleotide(s) added to the gene
- Deletion - one or two nucleotides removed from the gene
- Changes the reading frame of the mRNA
- Sequence of amino acids is changed “downstream” of mutation site
- Almost always results in a non-functional protein
Spontaneous Mutation (How mutations occur)
Occur in absence of mutagens, due to occasional mistakes during DNA replication
Induced Mutation (How mutations occur)
Occur when DNA damaging agents cause changes in DNA sequence - mutagens
-Example: Radiation, some chemicals
Regardless of Origin, Mutations can result in
1) No effect on the protein (remains functional) - silent mutation
2) A protein with a different amino acid sequence that may have an altered function - missense mutation
3) Premature stop codon- Incomplete (truncated) protein, usually nonfunctional - nonsense mutation
Plasmids
- Self-replicating, DNA molecules
- Contain non-essential genes
Example. Genes for toxin production
F Plasmids
- Fertility Factors
- Carry genes to make F pills (or sex pilus)
- Involved in conjugation (transfer of genetic material between bacteria)
R Plasmids
- Resistance Factors
- Carry genes for antibiotic resistance
Vir Plasmids
- Virulence factors
- Carry genes for toxin production
Horizontal Gene Transfer in Bacteria (Basic Methods)
1) Transformation
2) Transduction
3) Conjugation
Transformation
- Pieces of “naked” DNA are taken up by bacterial cells
- ie. From dead cells, or from released plasmids
- These pieces can be integrated into the chromosomes
1) Recombination
2) Can then be passed to progeny, and become a stable part of the genome
Transduction (Part 1)
- Small fragments of DNA transferred between bacteria by bacteriophage (viruses that infect bacteria)
- Phage attaches to bacterial cell wall - injects its DNA into the cell
- Phage DNA is replicated inside bacterial cell
- Phage DNA also encodes enzymes that cut the bacterium’s DNA into fragments