Gene Regulation, DNA Replication, Genetic Mutations, DNA Repair Study Notes

Overview of Gene Regulation, DNA Replication, Genetic Mutations, and DNA Repair

Lecture Introduction

  • Course: BIO 181: General Biology I

  • Instructor: Dr. Hart

  • Date: 11/4/25

  • Lecture 10 covers:

    • Gene Regulation (Chapter 12)

    • Genetic Mutations (Chapter 13)

    • DNA Repair (Chapter 13)

1. Gene Regulation

Definition and Importance

  • Gene Regulation: The ability of cells to control the level of gene expression.

  • Genotype: The genetic information for a specific trait, e.g., the gene for hair color.

  • Phenotype: The observable characteristics of a trait, e.g., the observed hair color.

  • Most genes are regulated to ensure proteins are produced in appropriate amounts and at the correct times.

    • This regulation saves energy by producing products only when needed.

  • Constitutive Genes: Genes that maintain constant expression levels under all conditions over time.

Bacterial Gene Regulation

  • Bacteria respond to environmental changes for gene regulation.

  • Example: Escherichia coli (E. coli) and Lactose Regulation:

    • When lactose is available, E. coli produces:

    • Lactose Permease: Protein that transports lactose into cells.

    • β-Galactosidase: Protein that breaks down lactose.

    • When lactose levels decrease, the synthesis of these proteins stops.

Regulation of Transcription in Bacteria

  • Regulatory Transcription Factors: Bind to DNA near promoters and influence the transcription of nearby genes.

    • Repressors: Factors that decrease the rate of transcription (Negative Control).

    • Activators: Factors that increase the rate of transcription (Positive Control).

Small Effector Molecules

  • Play a role in transcriptional regulation by binding to transcription factors and causing conformational changes, altering their ability to bind to DNA.

  • Repressors and activators that respond to small effector molecules have:

    • Two Domains:

    1. DNA Binding Site

    2. Small Effector Molecule Binding Site.

Operons

  • A Bacterial Operon: A cluster of genes under control of a single promoter, transcribed into polycistronic mRNA, encoding multiple polypeptides.

    • Allows coordinated regulation of a group of genes with shared functions.

Lac Operon

  • In E. coli, the lac operon regulates lactose metabolism:

    • Components:

    • lacP: Lac promoter

    • Three Structural Genes:

      • lacZ: Encodes β-galactosidase

      • lacY: Encodes lactose permease

      • lacA: Encodes galactosidase transacetylase.

lac Operon Regulation

  • Regulatory Sites Near lac Promoter:

    • lacO: Operator, provides binding site for repressor protein.

    • CAP Site: Binding site for activator protein.

  • lacI Gene: Codes for the lac repressor, which is critical for lac operon regulation and has its own promoter.

Negative Control of lac Operon

  • The lac operon is regulated negatively by a repressor protein that binds to the operator, blocking transcription.

  • In the 1950s, Jacob and Monod discovered the lac repressor which binds the operator, preventing transcription of lac genes in the absence of lactose.

    • When exposed to lactose, it binds to a byproduct and prevents binding to DNA, allowing transcription.

Induction Process (Lactose Present)

  • When lactose is present, the byproduct Allolactose binds to the lac repressor, preventing it from binding to the operator, which allows for transcription (inducible operon).

Positive Control of lac Operon

  • CAP (Catabolite Activator Protein): Acts as an activator by binding to cAMP, forming a cAMP-CAP complex that binds to the CAP site, enhancing RNA polymerase's ability to bind to the promoter.

  • Glucose Influence: High glucose levels decrease cAMP production, preventing CAP activation and consequently turning off the lac operon.

Summary of Conditions for lac Operon Activation

  • Lactose High, Glucose Low: Activated due to Allolactose's inhibition of repressor.

  • Lactose High, Glucose High: Off, as high glucose inhibits cAMP and CAP activity.

  • Lactose Low, Glucose Low or High: Off, due to repressor binding preventing transcription.

2. Genetic Mutations

Definition and Importance

  • Mutations: Heritable changes in genetic material, essential for evolution as they provide variation for natural selection.

  • New mutations typically pose a higher risk for harm than benefit.

  • DNA repair systems exist to reverse DNA damage, and mutations can lead to diseases such as cancer.

Types of Mutations and Consequences

  • Gene Mutations: Alters DNA sequence, creating either:

    • Changes in base sequences

    • Additions or deletions of base pairs.

Point Mutations

  • Point Mutation: Affects only a single base pair in DNA.

    • SNP/SNV: Single Nucleotide Polymorphism/Single Nucleotide Variant.

Effects of Point Mutations

  • Types of Point Mutations:

    • Silent Mutations: No change in protein function.

    • Missense Mutations: Changes one amino acid in the protein.

    • Nonsense Mutations: Converts a codon to a stop codon, shortening the protein.

    • Frameshift Mutations: Result from addition or deletion of nucleotides, altering the reading frame.

Gene Mutations Outside Coding Sequences

  • Mutations may also affect:

    • Promoters: Alter transcription rates, enhancing or inhibiting transcription.

    • Regulatory Elements/Operator Sites: Affect transcription regulation, e.g., by preventing repressor binding.

    • Splice Sites: Affect mRNA splicing efficiency.

Germ-line vs Somatic Mutations

  • Germ-line Mutations: Occur in gametes; if these participate in fertilization, they propagate through all body cells of offspring.

  • Somatic Mutations: Occur in non-germ-line cells, leading to genetic mosaics in tissues.

Causes of Mutations

  • Mutations can be classified as:

    • Spontaneous Mutations: Occur naturally as a result of random errors in biological processes.

    • Induced Mutations: Caused by environmental agents, typically at higher rates than spontaneous mutations.

Types of Common Causes of Mutations

  • Common Causes: As shown in Table 13.3:

    • Errors in DNA replication (Spontaneous)

    • Toxic metabolic products (Spontaneous)

    • Physical and chemical mutagens (Induced, such as UV light, chemical agents)

Mutagen Mechanisms

  • Mutagens: Substances that alter DNA structure, e.g.,

    • Covalent Modification: Modifies nucleotide structure, such as nitrous acid deaminating bases.

    • Base Analogues: Substitutes that cause replication errors.

    • Distortion Agents: Insert between bases and induce single-nucleotide insertions/deletions, e.g., benzopyrene.

3. DNA Repair

Types of Repair Mechanisms

  • Direct Repair: Recognizes and directly fixes incorrect structures within DNA.

  • Nucleotide Excision Repair (NER): Removes the segment of DNA containing damage and synthesizes a new segment using the undamaged strand as a template.

  • Mismatch Repair: Detects and repairs mismatched base pairs during DNA replication.

Nucleotide Excision Repair (NER)

  • NER Process: The most common DNA repair system utilized by both prokaryotes and eukaryotes. It includes:

    • Removal of damaged DNA.

    • Resynthesis of a normal strand using an intact strand as a template.