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 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 Overview (1 of 3)

Chemical Mutagens- Nucleoside Analogs Compounds that resemble bases closely

Physical Mutagens: Radiation

Repair of thymine dimers

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