Introduction to DNA Mutations

Degree of change: silent << missense < nonsense < frameshift. Single nucleotide substitutions are repaired by DNA polymerase and DNA ligase. Types of single nucleotide (point) mutations: Transition—purine to purine (eg, A to G) or pyrimidine to pyrimidine (eg, C to T). Transversion—purine to pyrimidine (eg, A to T) or pyrimidine to purine (eg, C to G).

Silent and Missense Mutations

Silent mutation: Codes for same (synonymous) amino acid; often involves 3rd position of codon (tRNA wobble).

Missense mutation: Results in changed amino acid (called conservative if new amino acid has similar chemical structure). Examples: sickle cell disease (substitution of glutamic acid with valine).

Nonsense Mutations

Nonsense mutation: Results in early stop codon (UGA, UAA, UAG). Usually generates nonfunctional protein. Stop the nonsense!

Frameshift and Splice Site Mutations

Frameshift mutation: Deletion or insertion of any number of nucleotides not divisible by 3 → misreading of all nucleotides downstream. Protein may be shorter or longer, and its function may be disrupted or altered. Examples: Duchenne muscular dystrophy, Tay-Sachs disease.

Splice site mutation: Retained intron in mRNA → protein with impaired or altered function. Examples: rare causes of cancers, dementia, epilepsy, some types of β-thalassemia, Gaucher disease, Marfan syndrome.

Visualizing DNA Mutations

This diagram illustrates various types of DNA mutations and their effects on mRNA codons and resulting amino acids.

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Observe the original sequence and compare it to silent, missense, nonsense, frameshift insertion, and frameshift deletion mutations.

Introduction to the Lac Operon

Classic example of a genetic response to an environmental change. Glucose is the preferred metabolic substrate in E coli, but when glucose is absent and lactose is available, the lac operon is activated to switch to lactose metabolism. Mechanism of shift: Low glucose → ↑ adenylate cyclase activity → ↑ generation of cAMP from ATP → activation of catabolite activator protein (CAP) → ↑ transcription. High lactose → unbinds repressor protein from repressor/operator site → ↑ transcription.

Lac Operon Structure and Regulation

This diagram details the components of the lac operon and its regulatory elements.

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Key elements include the LacI gene, CAP site, promoter, operator, and structural genes LacZ, LacY, LacA, along with the repressor protein, allolactose, and cAMP-CAP complex.

Lac Operon Gene Expression States

These diagrams show the lac operon’s gene expression under four different conditions of glucose and lactose availability.

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Compare the states for low glucose/lactose unavailable, high glucose/lactose unavailable, low glucose/lactose available, and high glucose/lactose available.

Topics Covered:

• Introduction to DNA Mutations
• Silent and Missense Mutations
• Nonsense Mutations
• Frameshift and Splice Site Mutations
• Visualizing Mutation Types
• Introduction to the Lac Operon
• Lac Operon Structure and Regulation
• Lac Operon Gene Expression States