Transcript Lecture 6 S
Lecture 7 Microbial Genetics: Genetic Mutations Gene Transfer Genetic Mutations Mutations: Changes in DNA • Why are mutations in DNA important to humans? • 2 types of mutations: • Spontaneous Mutations: – occur in the natural environment without the addition of mutagens (agents that cause mutations) – Occur randomly and spontaneously • Induced Mutations: – Mutations that are created by the addition of mutagens Spontaneous Mutations • Two types: 1. Base substitutions 2. Frameshift mutations Spontaneous Mutations: Base substitutions • • • Most common type of substitution Mistake during DNA replication, incorrect base incorporated into DNA Three types: 1. Silent mutation: no effect on protein (remember- several codons code for the same amino acid) 2. Missense mutation: codon has changed and different amino acid is incorporated 3. Nonsense mutation: codon has changed to a stop codon Figure 8.15 - Overview Spontaneous Mutation: Base-pair deletion or insertion • Insert or delete a nucleotide- very disastrous • Shifts codons of DNA when transcribed into RNA (also called frameshift mutation) • All nucleotides downstream of mutation will be grouped into improper codons, and wrong amino acids will be added • Protein will be non-functional Figure 8.16a Figure 8.16b Figure 8.16c Figure 8.16d Induced Mutation • Mutations are induced by either certain chemical mutagens or physical mutagens • Sometimes scientists intentionally mutate DNA to study it Chemical Mutagens- Chemical Modification • Example: nitrous acid • Converts adenine so it no longer pairs with thymine • Instead pairs with cytosine Figure 8.17 - Overview (1 of 3) Chemical Mutagens- Nucleoside Analogs • Compounds that resemble bases closely Physical Mutagens: Radiation Repair of thymine dimers Gene Transfer Gene Transfer • Gene Transfer= • Vertical Gene Transfer= When genes are passed from an organism to it’s offspring • Horizontal Gene Transfer= Occurs between bacteria Horizontal Gene Transfer • Two types of cells: 1.Donor: transfers DNA to recipient 2.Recipient: receives the DNA Gene Transfer • Three methods of horizontal gene transfer: 1. Transformation 2. Transduction 3. Conjugation Transformation • Donor cell wall can rupture, and DNA that is normally tightly packed can break up into pieces and explode through cell wall • Naked DNA= • These pieces can then pass through cell walls and cytoplasmic membranes of recipient cells and get integrated into their chromosomes Figure 8.15 Transduction • DNA is transferred from one bacteria to another by virus that infects bacteria (bacteriophage) Conjugation • Transfer of DNA by contact of two bacterial cells • Can transfer plasmid or chromosome Conjugation • Two types of cells: 1. Donor cell- contains fertility plasmid and is designated F+ 2. Recipient cell- does not contain fertility plasmid and is designated F• DNA transferred from F+ to F- Conjugation • In some cells carrying F factors, the F factor integrates into the host chromosome • Now called Hfr cell • Conjugation between Hfr and F– Chromosome replicates – Transferred to F- cell – Usually chromosome breaks off before completely transferred – Generally remains F- because does not receive F factor R plasmid • Resistance plasmid- confer antibiotic resistance • Two parts: 1. Resistance genes (R genes) 2. Resistance transfer factor (RTF) Figure 8.22