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