CH 9 Microbial Genetics

Section 9.5 Mutations: Changes in the Genetic Code

What is a mutation?

any change to the nucleotide sequence in the genome

What are wild type mutations

• microorganism exhibits a natural, nonmutated characteristic

• the trait present in the highest numbers in a population

What are mutant-strain mutations

• microorganism has a mutation

• useful for:

  • tracking genetic events

  • detecting nutritional needs for mutants

What can mutant-strain mutations affect?

1. morphology

2. nutritional needs

3. genetic controls

4. resistance to chemicals

5. enzyme functions

What are the two causes of mutations?

1. spontaneous mutations

2. induced mutations

What are spontaneous mutations

• a random change in DNA that arises from errors in replication

• occurs randomly (1 to 10^5 to 10^10)

What are induced mutations

a change in DNA due to the exposure to known mutagens

What are mutagens

an agent, such as radiation or a chemical substance, which causes genetic mutation

Nitrous Acid and Its Effect

• a chemical mutagenic agent

• removes an amino group from some bases

Ethidium Bromide and its Effect

• a chemical mutagenic agent

• inserts between the paired bases

Acridine Dyes and its Effect

• a chemical mutagenic agent

• causes frameshift mutations due to insertion between base pairs

Nitrogen Base Analogs and its Effect

• a chemical mutagenic agent

• compete with natural bases for sites on replicating DNA

Ionizing and its Effect

• a radiation mutagenic agent

• includes gamma rays and X-rays

• forms free radicals that cause single or double breaks in DNA

Ultraviolent and its Effect

• a radiation mutagenic agent

• causes cross-links between adjacent pyrimidines

What are the categories of mutations? (7)

1. point mutations

2. lethal mutations

3. neutral mutations

4. missense mutation

5. nonsense mutation

6. silent mutation

7. back-mutation

What are point mutations

• small mutations that affect only a single base on a gene

• involves addition, deletion, or substitution of single bases

Categories of point mutations

1. missense mutations

2. silent mutations

3. nonsense mutations

What are missense mutations

• an amino acid is switched

• may or may not be functional

What are silent mutations

• AKA base substitutions

• alters a base, but does not change the amino acid

• no change in function

What are nonsense mutations

• this mutation stops the protein from being made

What are frameshift mutations

• a base can either be deleted or inserted

• both can lead to premature stop codons and/or poorly functioning proteins

What are lethal mutations

mutations that lead to cell dysfunction or death

What are neutral mutations

mutations that produce neither adverse nor helpful changes

What are back-mutations

a gene that has already undergone a mutation reverses back to its original base composition

Photoactivation Repair of Mutations

• photoactivation - visible light and light-sensitive enzymes

• light repair of damage caused by ultraviolent radiation

Excision Repair of Mutations

• mutations are excised by a series of enzymes that removes the incorrect bases and add the correct ones

Mismatch Repair of Mutations

• this repair system locates mismatched bases that were missed during proofreading

• the base must be replaced soon after the mismatch is made, or it will not be recognized by the repair enzymes

The Ames Test

• uses Histidine-free Salmonella bacteria

• is susceptible to back mutation and lack of DNA repair mechanisms

• allows easy observation and monitoring of gene expression and mutation rate

T/F: The Ames Test is commonly used to rapidly detect chemicals with carcinogenic potential. Why?

• True

• Any chemical capable of mutating bacterial DNA could similarly mutate mammalian DNA and is potentially hazardous

Positive Effects of Mutations

• Contributes to the success of the individual and the population

  • Variant strains can more readily adapt, survive, and reproduce

Negative Effects of Mutations

many mutations are not repaired

Differentiate among frameshift, nonsense, silent, and missense mutations. (Definition and Effect)

1. Frameshift Mutation:

- Definition: Frameshift mutations occur when nucleotides are inserted into or deleted from the DNA sequence, causing a shift in the reading frame during translation.

- Effect: This alteration disrupts the triplet reading frame, leading to the translation machinery interpreting the codons incorrectly. As a result, the protein sequence produced downstream from the mutation is entirely different from the original, often resulting in a non-functional or truncated protein.

2. Nonsense Mutation:

- Definition: Nonsense mutations involve a change in a single nucleotide that results in the creation of a premature stop codon within the mRNA sequence.

- Effect: It prematurely terminates protein synthesis. As a consequence, the resulting protein is typically incomplete and often non-functional.

3. Silent Mutation:

- Definition: Silent mutations are those mutations that occur within the DNA sequence but do not result in any change to the amino acid sequence of the protein.

- Effect: The same amino acid is still incorporated into the protein during translation, and the protein's function remains unchanged.

4. Missense Mutation:

- Definition: Missense mutations are single nucleotide substitutions that lead to the replacement of one amino acid with another in the resulting protein sequence.

- Effect: Missense mutations can lead to altered protein structure and function. They can be benign, meaning they have little or no effect on protein function, or they can be deleterious, resulting in impaired protein function or even loss of function.

In summary, frameshift mutations and nonsense mutations often result in non-functional or premature proteins, silent mutations have no effect on protein function, and missense mutations can have variable effects depending on the specific amino acid change and its location within the protein.

T/F: A spontaneous mutation arises from exposure to chemicals or physical agents

False

A ______ mutation results in a stop codon

nonsense

A _____ mutation nearly always results in a nonfunctional protein

frameshift

The ____ is a first-line screening technique to detect mutagenic chemicals

Ames Test

T/F: Mutations are always harmful to organisms.

False