Ch25All-module 9

Chapter 25: DNA Structure and Gene Expression

Overview

• Main Topics:

  • DNA, RNA, and DNA Replication

  • From DNA to RNA to Protein

  • Control of Gene Expression

  • Mutations

DNA as Genetic Material

• Key Points:

  • Chromosomes carry genes.

  • Initial debate: Is protein or DNA the genetic material?

    • Protein was thought to be the more complex and likely candidate.

• Pivotal Experiments:

  • Griffith's Experiment:

    • Observed transformation from dead virulent bacterium to non-virulent strain.

  • Avery's Findings:

    • Transformation did not occur with DNA-digesting enzymes.

    • Transformation persisted with protein or RNA-digesting enzymes.

  • Hershey-Chase Experiment:

    • Explored the role of bacteriophages in confirming DNA as genetic material.

The Hershey-Chase Experiment

• Procedure:

  1. Mix radioactive-labeled phages with bacteria; phages infect bacterial cells.

  2. Blend to separate external phages from bacterial contents.

  3. Centrifuge to form a pellet from bacteria.

  4. Measure radioactivity in the pellet (inside bacteria) versus the liquid (outside).

• Results:

  • Conclusively showed DNA, not protein, was the genetic material injected into bacteria.

The DNA Structure and Composition

Components of Nucleotides

• Nucleotides comprise:

  • Nitrogenous base: A, T, C, G (in DNA) / A, U, C, G (in RNA)

  • 5-carbon sugar: Ribose (in RNA) or Deoxyribose (in DNA)

  • Phosphate group

Polynucleotide Structure

• Backbone:

  • Covalent bonds between sugar and phosphate groups create a sugar-phosphate backbone.

  • DNA and RNA can have long chains of nucleotides.

Nitrogenous Bases

• Types of Bases:

  • Pyrimidines: Single-ring structures (T, C, U)

  • Purines: Double-ring structures (A, G)

DNA Double Helix

• Chargaff's Rule: A = T and C = G in any DNA sample.

• Rosalind Franklin: Provided critical X-ray crystallography images of DNA.

• Watson and Crick: Determined the double helix structure using existing data.

  • Visual analogy: Twisted ladder; sugars/phosphates are the sides, bases are the rungs.

DNA Replication

• Mechanism:

  • Parental strands separate and serve as templates for new strands.

  • Free nucleotides align complementary to template bases.

  • Enzymes link nucleotides, creating two new strands (semiconservative mechanism).

• Enzymatic Action:

  • DNA helicase unwinds the double helix.

  • DNA polymerase adds nucleotides; can only synthesize in one direction.

  • DNA ligase links short segments of DNA (Okazaki fragments).

RNA Characteristics and Types

• Differences from DNA:

  • Ribose sugar instead of deoxyribose.

  • Uracil (U) replaces thymine (T).

  • Generally single-stranded.

• Types of RNA:

  • Messenger RNA (mRNA): Copy of genetic instructions; formed during transcription.

  • Transfer RNA (tRNA): Brings amino acids to the ribosome.

  • Ribosomal RNA (rRNA): Combines with proteins to form ribosome subunits.

Central Dogma of Molecular Biology

• Process Overview:

  • Genetic information flow: DNA → RNA → Protein.

  • Phases:

    • Transcription: DNA information transferred to RNA.

    • Translation: RNA information converted into protein.

Gene Expression and its Regulation

Gene Expression Variability

• All cells have the same DNA yet differ in function.

• Housekeeping Genes: Commonly expressed in all cells (e.g., glycolysis genes).

• Mechanisms exist for activating/deactivating specific genes based on the cell's needs.

Operons in Prokaryotic Gene Control: The Lac Operon

• Function: Allows E. coli to regulate enzyme production for lactose digestion when needed.

• Structure of Operons:

  • Clustered genes, a promoter, and an operator (binding site for repressors).

• Repression Mechanism:

  • When lactose is absent, the repressor protein binds to the operator, blocking RNA polymerase.

• Induction Mechanism:

  • Presence of lactose leads to its binding to the repressor, allowing transcription of lactose-digesting enzymes.

Eukaryotic Gene Regulation

• Regulatory Sequences: Most eukaryotic genes are not clustered.

• Levels of Control:

  • Pretranscriptional

  • Transcriptional

  • Posttranscriptional

  • Translational

  • Posttranslational

Types of Control Mechanisms

Pretranscriptional Control

• DNA structure changes (histones, heterochromatin, euchromatin).

Transcriptional Control

• Involves promoters and enhancers; transcription factors play a role.

Posttranscriptional Control

• Involves processing of the primary mRNA to mature mRNA (capping, tailing, splicing).

Translational Control

• Rates of mRNA breakdown influence protein synthesis levels.

Posttranslational Control

• Modifications may be required for full protein activity; stability impacts functionality.

Mutations

• Permanent changes in DNA sequence affecting heritability.

• Types:

  • Point Mutation: Single nucleotide change.

    • Silent, missense, or nonsense mutations.

  • Frame Shift Mutation: Caused by insertions or deletions, altering downstream codons.

Cascade Effects of Mutations

• Influence on pathways and function, possibly leading to disorders or diseases (e.g., cancer).

• Examples include the p53 gene affecting the cell cycle regulation.