DNA Structure

Historical Context of DNA Discovery

Key Figures in DNA Research

  • James D. Watson and Francis H. Crick: Developed the double helix model of DNA in 1962, which earned them the Nobel Prize in Physiology and Medicine.

  • Rosalind Franklin: Her X-ray diffraction images of DNA were crucial in revealing its helical structure, although she did not receive the same recognition during her lifetime.

  • Maurice H. Wilkins: Collaborated with Franklin and Watson & Crick, contributing to the understanding of DNA's structure.

The Significance of the Double Helix Model

  • The double helix model proposed by Watson and Crick explained how genetic information is stored and replicated.

  • The model illustrated the complementary base pairing, which is fundamental for DNA replication and function.

Structure of DNA

Composition of DNA

  • DNA is a nucleic acid composed of nucleotides, which are the monomers of DNA.

  • Each nucleotide consists of three components: a phosphate group, a deoxyribose sugar, and a nitrogenous base.

Types of Nitrogenous Bases

  • Pyrimidines: Single-ring structures including Cytosine (C) and Thymine (T) (only in DNA).

  • Purines: Double-ring structures including Adenine (A) and Guanine (G).

  • Base pairing rules: A pairs with T (2 hydrogen bonds), and C pairs with G (3 hydrogen bonds).

DNA vs. RNA

DNA and RNA differ in several key aspects:

  • DNA is double-stranded, while RNA is single-stranded.

  • DNA contains deoxyribose sugar; RNA contains ribose sugar.

  • DNA has Thymine (T), whereas RNA has Uracil (U).

DNA Replication Process

Overview of DNA Replication

  • DNA replication is a semi-conservative process, meaning each new DNA molecule consists of one old strand and one new strand.

  • The process is initiated at origins of replication, where the double helix unwinds.

Enzymes Involved in DNA Replication

  • Helicase: Unwinds the DNA strands.

  • DNA Polymerase III: Adds nucleotides to the growing DNA strand and proofreads for errors.

  • DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Leading vs. Lagging Strand Synthesis

  • The leading strand is synthesized continuously in the 5’ to 3’ direction.

  • The lagging strand is synthesized discontinuously, forming short segments called Okazaki fragments, which are later joined together.

DNA Repair Mechanisms

Proofreading and Error Correction

  • DNA polymerase has proofreading capabilities, correcting errors during replication at a rate of 1 in 10,000 bases.

  • Mismatch repair enzymes correct incorrectly paired bases after DNA synthesis.

Nucleotide Excision Repair

  • Nucleases cut out damaged DNA segments, which are then replaced by DNA polymerase and ligase.

  • Example: Xeroderma pigmentosum is a genetic disorder caused by mutations in DNA repair enzymes, leading to increased skin cancer risk.

Mutation Rates and Implications

  • Mutations can occur at a rate of 1 per 10,000 bases, with a higher cumulative effect due to the large human genome.

  • Each individual may inherit 3-4 mutations, which can contribute to genetic diversity and disease.