Microbio Exam 2: Lecture 6: Genetic Change and Genome Evolution

Chapter 9

Mutation

A heritable change in the base sequence of that genome

• A change that is passed from the mother cell to progeny cells

Three subcategories of mutations

1. Detrimental

2. Beneficial

3. Neutral

What two genetic processes fuel the evolutionary process?

Mutation (small changes) and genetic exchange (larger changes)

Wild Type (WT)

Organism or gene as found in nature.

Mutant

Organism or gene with mutations that differ from the wild type (WT)

Point Mutation

Change in a single base, has two subcategories with varying effects on protein production.

Insertion

DNA is flipped in orientation.

Duplication

Produces a second copy of a sequence.

Transposition

Movement of a mobile genetic element within a genome.

Reversion

DNA mutates back to original sequence.

Point Mutation Subcategories

Nucleotide Transitions:

• Purine → Purine (A/G)

• Pyrimidine → Pyrimidine (C/T)

Transversions:

• Purine → Pyrimidine (A/T or G/C)

• Pyrimidine → Purine (T/A or C/G)

Point Mutation: Missense Mutation

Changes one amino acid to another.

Point Mutation: Nonsense Mutation

Changes amino acid to stop codon.

Point Mutation: Silent Mutation

Mutation causes no change in amino acid sequence.

Frame-shift Mutation

Changes the open reading frame of the gene.

What causes Frameshift Mutations?

The insertion or deletion of 1 or 2 nucleotide bases.

Why are Frameshift Mutations Harmful?

Causes profound changes in amino acids being translated, resulting amino acid sequence is now out of frame.

When are frameshift mutations ok?

When the insertion or deletion includes 3 nucleotide bases.

• Still in frame

• Just adds or removes an amino acid from the resulting polypeptide.

What phenomenon can cause abnormal pairing between nucleotides?

Tautomeric shifts

• A spontaneous, temporary change in the chemical structure of a nucleotide base caused by the shifting of a hydrogen atom and a single/double bond

Effect of Hydrolysis (breaking down substances with water) on base pairs

Can cause deamination

• Ex. Groups can be pulled off base pairs, deaminating that base pair into another nucleotide that pairs differently.

• Harmful because it can change the genetic code if the switch is not identified by the cell.

Purines: Depurination

When a purine base (A/G) gets completely removed from the backbone, leaving a blank spot.

• If it’s not caught, there will be no base their to give instruction for amino acids being made, or theres a danger of the wrong base getting popped in there and causing a mutation.

Error-Proof Repair Pathways (prevent mutations)

Methyl mismatch repaid, photoreactivation, nucleotide excision repair, base excision repair and recombinational repair.

Error-Prone Repair Pathways (risk introducing mutations)

Operate only when the damage is so severe that the cell has no other choice but to die.

Methyl-directed mismatch repair

Based on recognition of the methylation pattern in DNA bases.

• Uses the parental strand to discriminate from newly replicated DNA (the parental strand will contain the proper DNA sequence)

• Proteins that carry out this repair called Mut (and mut) because a high mutation rate results in strains that are defective in one of these proteins.

Mutator Strain

A bacterial strain with a high mutation rate.

Nucleotide Excision Repair

Used to repair UV damaged DNA

• Such as pyrimidine dimers

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