mutation of a cell

I. The Genetic Code

Codons

  • A codon is a sequence of three mRNA bases

  • Each codon codes for:

    • One amino acid or

    • A stop signal

  • Because codons are triplets, the genetic code is called a triplet code

๐Ÿ”— Cause โ†’ Effect
Three bases per codon โ†’ 4ยณ = 64 possible codons

Redundancy of the Genetic Code

  • Most amino acids are encoded by more than one codon

  • This is called degeneracy

  • Example:

    • Leucine: CUU, CUC, CUA, CUG

  • Protects against harmful mutations

๐Ÿ”— Cause โ†’ Effect
Different codons โ†’ same amino acid โ†’ mutation may be silent

Special Codons

  • Start codon:

    • AUG

    • Codes for methionine

    • Initiates translation

  • Stop codons:

    • UAA, UAG, UGA

    • Terminate translation

  • Only amino acids with one codon:

    • Methionine (AUG)

    • Tryptophan (UGG)

II. Mutations

Definition

  • A mutation is a permanent change in DNA sequence

  • Occurs when replication errors escape proofreading and repair

  • Can alter protein structure and function

Neutral vs Functional Mutations

  • Neutral mutations:

    • Occur in noncoding DNA

    • Or change codon but not amino acid

  • Functional mutations:

    • Alter amino acid sequence

    • Can be harmful, lethal, or beneficial

๐Ÿ”— Cause โ†’ Effect
Mutation in coding region โ†’ altered protein โ†’ altered phenotype

III. Point Mutations

Point Mutations

  • Affect a single nucleotide

  • Two major types:

    • Substitution

    • Frameshift (insertion or deletion)

Substitution Mutation

  • One base replaces another

  • Effects:

    • Silent: same amino acid

    • Missense: different amino acid

    • Nonsense: stop codon formed

๐Ÿ”— Cause โ†’ Effect
Base substitution โ†’ single amino acid may change

Example disease:

  • Sickle-cell anemia

Frameshift Mutations

  • Caused by:

    • Insertion

    • Deletion

  • Shifts the reading frame

  • Alters all downstream codons

๐Ÿ”— Cause โ†’ Effect
Reading frame shifted โ†’ drastically altered protein

Example disease:

  • Cystic fibrosis

  • Tay-Sachs disease

IV. Chromosomal Mutations

Definition

  • Large-scale changes in chromosome structure or number

  • Often occur during meiosis

Types of Chromosomal Mutations

Type

Description

Deletion

Loss of chromosome segment

Duplication

Extra copy of chromosome segment

Inversion

Segment reversed

Translocation

Segment moves to another chromosome

๐Ÿ”— Cause โ†’ Effect
Altered chromosome structure โ†’ altered gene dosage

Examples:

  • Down syndrome (trisomy 21)

  • Some cancers

V. Effects of Mutations

Protein-Level Effects

  • Substitution โ†’ one amino acid change

  • Frameshift โ†’ completely different protein

  • Stop codon โ†’ truncated protein

Population-Level Effects

  • Mutations accumulate โ†’ genetic load

  • Most are recessive

  • Phenotype expressed only if homozygous

VI. Mutations & Evolution

  • Mutations are the source of genetic variation

  • Natural selection acts on mutations

  • Example:

    • Peppered moth during Industrial Revolution

  • Beneficial mutations increase survival and reproduction

VII. Somatic vs Germline Mutations

Type

Occur In

Inherited?

Somatic

Body cells

โŒ No

Germline

Gametes

โœ… Yes

VIII. Mutagens (Causes of Mutations)

Physical Mutagens

  • UV light โ†’ thymine dimers โ†’ skin cancer

  • Ionizing radiation โ†’ double-strand breaks

Chemical Mutagens

  • Cigarette smoke (benzopyrene)

  • Oxidizing agents

  • Drugs and pesticides

Biological Mutagens

  • Viruses

  • Oncogenic viruses โ†’ cancer

๐Ÿ”— Cause โ†’ Effect
Mutagen exposure โ†’ increased mutation rate