S

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.