Chapter_14_DNA_History__Structure_and_Replication

DNA Structure, Replication & History

Chapter 14

I. History of DNA

• Early Research: DNA's discovery has historical roots in genetics and scientific collaboration.

• Key Figures: James Watson, Francis Crick, and Rosalind Franklin are notable contributors to the understanding of DNA's structure.

II. DNA Structure

• Configuration: DNA is composed of two antiparallel sugar-phosphate backbones with nitrogenous bases paired in the interior.

• Base Pairing: Adenine pairs with Thymine (A-T), while Guanine pairs with Cytosine (G-C).

• Double Helix: Watson and Crick developed the double helical model of DNA.

III. DNA Replication

• Mechanism: DNA replication is described as semi-conservative, meaning each new DNA molecule consists of one old and one new strand.

• Key Enzymes:

  • DNA Polymerases: crucial for synthesizing new DNA strands and involved in repair processes.

  • Helicase: unwinds the DNA.

  • Primase: synthesizes RNA primers.

  • Ligase: seals nicks in the DNA backbone.

• Prokaryotic vs. Eukaryotic:

  • Prokaryotes have a single origin of replication; Eukaryotes have multiple.

  • Speed: Prokaryotes replicate DNA at approx. 1000 nucleotides/sec; Eukaryotes at 50-100 nucleotides/sec.

IV. Human Genome Project (HGP)

• Overview: An international scientific project that began in 1990 and was declared completed in April 2003, with about 92% of the human genome sequenced.

• Purpose: Goals included sequencing all the nucleotides in human DNA and identifying all genes.

V. DNA Packaging

• Levels of Packaging:

  • Nucleosomes: DNA wrapped around histones, forming a bead-like structure.

  • Chromatin: further coiling of nucleosomes into higher-order structures.

  • Metaphase Chromosome: highly condensed state observed just before cell division.

• Types of Chromatin:

  • Euchromatin: loosely packed; often expressed.

  • Heterochromatin: densely packed; generally not expressed.

VI. Chromosomes

• Definition: Structures that contain DNA and protein, found in the nucleus of eukaryotic cells.

• Types:

  • Autosomes: all chromosomes except sex chromosomes.

  • Sex Chromosomes: determine biological sex, represented by X and Y (XX = female; XY = male).

• Ploidy:

  • Diploid (2n): cells with pairs of homologous chromosomes (e.g., human somatic cells).

  • Haploid (1n): cells with a single set of chromosomes (e.g., human gametes).

VII. DNA Repair

• Importance: To ensure the integrity of the genetic material by correcting any errors that occur during replication.

• Mechanisms:

  • DNA Proofreading: performed by DNA polymerases which check and replace incorrect nucleotides.

  • Nucleotide Excision Repair: removes bulky lesions and corrects mismatches.

• Error Types:

  • Point Mutations: changes at a single nucleotide.

  • Frameshift Mutations: insertions or deletions that change reading frame.

  • Chromosomal Mutations: larger-scale changes to chromosome structure.