Unit 16 Bio 1201

DNA Structure and Replication

Space-Filling Model of DNA

  • Figure 16.7b illustrates a space-filling model of DNA showing its three-dimensional structure.

  • Source: ©2011 Pearson Education, Inc.

Griffith's Experiment and Transformation

  • In the early studies of genetics and DNA, Frederick Griffith conducted experiments on the transformation of bacteria.

  • Experiment Setup:

    • Control Groups:

    • Living S cells (pathogenic control): Mice injected with these cells died.

    • Living R cells (nonpathogenic control): Mice injected with these cells remained healthy.

    • Heat-killed S cells (nonpathogenic control): Mice injected with these cells remained healthy.

    • Experimental Group:

    • Mixture of heat-killed S cells and living R cells: Mice injected with this mixture died, indicating transformation.

    • Result: Transformation of R cells into pathogenic S cells.

  • Source: ©2017 Pearson Education, Inc.

Bacteriophage Structure

  • Bacteriophage (phage) is a virus that infects bacteria.

  • Components:

    • Head: Contains genetic material.

    • DNA: The genetic material of the phage.

    • Tail Sheath and Tail Fibers: Facilitate infection of bacterial cells.

  • Size: Approximately 100 nm in length.

  • Source: ©2017 Pearson Education, Inc.

Hershey-Chase Experiment

  • Alfred Hershey and Martha Chase conducted experiments to determine whether DNA or protein is the genetic material.

  • Batch 1: Included radioactive sulfur (35S) to label protein.

  • Batch 2: Included radioactive phosphorus (32P) to label DNA.

  • Experiment Results: Analysis showed that radioactive DNA entered bacterial cells, supporting that DNA is the genetic material.

  • Source: ©2017 Pearson Education, Inc.

Structure of DNA

  • Components of DNA:

    • Sugar-Phosphate Backbone:

    • Composed of deoxyribose sugar and phosphate groups.

    • Nitrogenous Bases:

    • Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

  • Nucleotide Composition: Each DNA nucleotide comprises a sugar, a phosphate group, and a nitrogenous base.

  • Figures Description:

    • Figure 16.5: Depicts the sugar-phosphate backbone and nitrogenous bases.

    • Nitrogenous bases pair specifically: A with T and C with G.

  • Base Pairing: Hydrogen bonds form between paired bases, stabilizing the double helix structure.

X-ray Diffraction and DNA Structure

  • Rosalind Franklin's X-ray Diffraction:

    • Figure 16.6 displays Franklin's famous photograph of DNA, which led to the discovery of its helical structure.

  • Evidence indicated that DNA is made up of pairs of purines and pyrimidines that maintain a consistent width.

    • Purine + Purine: Too wide

    • Pyrimidine + Pyrimidine: Too narrow

    • Purine + Pyrimidine: Consistent with X-ray data

  • Source: ©2011 Pearson Education, Inc.

DNA Replication

  • Key Features of DNA Replication:

    • Origin of Replication: A specific location where replication begins.

    • Parental Strand: Original template strands used for copying.

    • Daughter Strands: Newly synthesized strands built complementary to the parental strands.

  • DNA replication is semiconservative, meaning each resulting DNA molecule consists of one original and one new strand.

  • Meselson and Stahl Experiments:

    • Bacteria were cultured in media containing heavy isotope nitrogen (15N) and then switched to lighter isotope nitrogen (14N).

    • Following replication, the density of the extracted DNA was analyzed using centrifugation.

    • Predictions:

    • Conservative Model: Original DNA molecule remains intact.

    • Semiconservative Model: Each new DNA molecule contains one old strand and one new strand.

    • Dispersive Model: Parental and new DNA are mixed in both strands.

    • Conclusion: The results supported the semiconservative model of DNA replication.

DNA Replication Mechanics

  • Enzymes Involved:

    • Helicase: Unwinds the DNA double helix.

    • DNA Polymerase III: Synthesizes new DNA strands by adding nucleotides complementary to the template strand.

    • Primase: Synthesizes RNA primer that provides a starting point for DNA polymerase.

    • DNA Ligase: Joins Okazaki fragments on the lagging strand.

  • Okazaki Fragments: Short strands synthesized on the lagging strand during replication due to the opposite direction of synthesis relative to the replication fork.

Lagging vs. Leading Strand

  • Leading Strand: Synthesized continuously in the direction of the replication fork.

  • Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the replication fork.

  • The synthesis of the lagging strand requires multiple RNA primers and involves several steps (priming, elongation, and joining of fragments).

DNA Repair

  • Types of DNA Repair Mechanisms:

    • Mismatch Repair: Corrects errors in base pairing; occurs directly following DNA replication.

    • Nucleotide Excision Repair: Removes damaged DNA segments and replaces them with newly synthesized strands.

Telomeres and DNA Shortening

  • Telomeres: Repetitive DNA sequences at the ends of linear chromosomes that protect against degradation during replication.

  • Each round of replication may result in the gradual shortening of chromosomes, which is significant in cellular aging.

DNA Packaging

  • DNA Condensation: DNA is packaged into a compact, organized form for cell division:

    • Nucleosome: DNA wrapped around histone proteins forming “beads” on a string (10 nm diameter).

    • 30-nm Fiber: Coiled nucleosomes forming a thicker fiber.

    • Looped Domains: Further folding and coiling into larger loops, facilitating the organization of DNA within the nucleus.

  • Sizes:

    • DNA double helix: 2 nm

    • Nucleosome: 10 nm

    • Chromatid fibers: 700 nm

    • Replicated chromosome: 1,400 nm.