Mutation, Transposition, and DNA Repair: Genetics Study Notes

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Mutation, Transposition, and Repair

Introduction

This study guide covers the fundamental concepts of genetic mutation, mechanisms of transposition, and cellular processes for DNA repair. These topics are central to understanding genetic variation, disease, and molecular genetics research.

General Mutation Classifications

How Mutation Happens

Spontaneous mutation: Occurs naturally due to errors in DNA replication or spontaneous chemical changes in DNA bases.
Induced mutation: Results from exposure to external agents (mutagens) such as chemicals or radiation.

Cells Affected

Somatic mutation: Occurs in non-reproductive cells; affects only the individual and is not inherited.
Germline mutation: Occurs in reproductive cells; can be passed to offspring.

Affected Chromosome

Autosomal mutation: Occurs on non-sex chromosomes (autosomes).
X-linked mutation: Occurs on the X chromosome; often shows distinct inheritance patterns.

Classification: Molecular Change

Point Mutation/Base Substitution

Transition: Substitution of a purine for another purine (A <-> G) or a pyrimidine for another pyrimidine (C <-> T).
Transversion: Substitution of a purine for a pyrimidine or vice versa (A or G <-> C or T).

Result of Mutation

Missense mutation: Changes one amino acid to another, potentially altering protein function.
Nonsense mutation: Converts an amino acid codon to a STOP codon, truncating the protein.
Silent mutation: Alters a codon but does not change the amino acid due to genetic code redundancy.

Frameshift Mutation

Insertion or deletion of nucleotides not in multiples of three, shifting the reading frame and altering all downstream amino acids.

Regulatory Mutation

Occurs in non-coding regions affecting gene expression (e.g., promoters, enhancers).

Classification: Phenotypic Change

Functional Effects

Loss-of-function mutation: Reduces or eliminates gene product activity.
Gain-of-function mutation: Increases or changes gene product activity.
Null mutation: Complete loss of gene function.

Other Phenotypic Classifications

Visible mutations: Affect observable traits (e.g., eye color).
Nutritional mutation: Alters ability to synthesize essential compounds (e.g., prototroph to auxotroph).
Biochemical mutation: Disrupts metabolic pathways (e.g., Sickle cell anemia, Phenylketonuria (PKU)).
Behavioral mutation: Alters organism behavior.
Lethal mutation: Causes death of the organism.
Conditional mutation: Expressed only under certain conditions (e.g., temperature-sensitive mutation).

What Causes Mutations?

Errors in Replication

DNA polymerases can make mistakes during replication, even with proofreading activity.
Replication slippage: Occurs at highly repetitive sequences, leading to insertions or deletions. Example: Huntington disease is caused by expansion of CAG repeats (>36 glutamines).

DNA Damage

Double strand breaks: Can occur during recombination, translocation, replication, or transposition.

Mutagens

Chemicals and naturally occurring substances that increase mutation rates.

Replication Errors

Mechanisms

Despite 3'-5' exonuclease proofreading, DNA polymerases still make errors.
Replication slippage is common in regions with repetitive DNA sequences.

Example: Huntington Disease

Caused by expansion of CAG repeats (codes for glutamine) in the HTT gene.
>36 glutamines leads to disease phenotype.

Tautomeric Shifts

Definition and Impact

Tautomeric shifts: Temporary changes in base structure (e.g., keto to enol, amino to imino) that alter base-pairing properties.
Can result in mispairing during replication, leading to point mutations.

Standard vs. Anomalous Base Pairing

Standard: Thymine (keto) pairs with Adenine (amino); Cytosine (amino) pairs with Guanine (keto).
Anomalous: Thymine (enol) can pair with Guanine (keto); Cytosine (imino) can pair with Adenine (amino).

Summary Table: Mutation Classifications

Classification	Type	Example
By Origin	Spontaneous, Induced	Replication error, chemical mutagen
By Cell Type	Somatic, Germline	Cancer (somatic), inherited disease (germline)
By Chromosome	Autosomal, X-linked	Cystic fibrosis (autosomal), hemophilia (X-linked)
By Molecular Change	Transition, Transversion, Frameshift	Sickle cell anemia (missense), Duchenne muscular dystrophy (frameshift)
By Phenotype	Loss-of-function, Gain-of-function, Conditional	PKU (loss), Huntington disease (gain), temperature-sensitive mutation

Key Equations

Mutation Rate:

Example: If DNA polymerase error rate is 1 in 10,000,000 bases, and the human diploid genome is ~6 billion base pairs, then:

Additional info:

Further topics such as DNA repair mechanisms, transposable elements, and genetic screens are covered in subsequent sections of the original notes.