DNA: The Code of Life

Components of DNA

  • Nucleic Acids: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid).

  • Monomers: Nucleotides consist of three components:

    • Sugar: Deoxyribose for DNA.

    • Phosphate group.

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

  • Structure:

    • DNA forms a double helix with a sugar-phosphate backbone and complementary nitrogenous bases.

    • Base pairing rules: A-T and G-C.

  • Types of Bases:

    • Purine: Adenine (A) and Guanine (G).

    • Pyrimidine: Cytosine (C), Thymine (T), and Uracil (U, in RNA).

DNA Structure and Features

  • Hydrogen Bonds: Bases are held together by hydrogen bonds - 2 hydrogen bonds between A and T, and 3 hydrogen bonds between G and C.

  • Nucleotide Arrangement: Equal amounts of A-T and G-C in DNA strands.

  • DNA Organization:

    • Genome: Complete set of genetic material.

    • Gene: DNA segment coding for a specific protein.

Discovery of DNA Structure

  • Rosalind Franklin: Used X-Ray crystallography to determine that DNA is a helix (Photo 51).

  • Wilkins' Contribution: Helped produce key X-ray images that led Watson and Crick to propose the double helix model.

  • Watson and Crick: Developed the double helix model based on Franklin's work.

DNA Replication

  • Process:

    1. Unwinds and Unzips: The double helix unwinds, separating the two strands.

    2. Template Strand: Each original strand serves as a template for new strands.

    3. Complementary Base Pairing: Free nucleotides attach to their complementary bases (A-T, G-C).

    4. Formation of New Strands: Creates two identical DNA molecules, each with one original and one new strand.

  • Significance: Essential for cell division, ensuring that daughter cells receive identical genetic material.

DNA vs RNA

  • Differences:

    • DNA:

    • Double-stranded structure.

    • Contains deoxyribose sugar.

    • Bases: A, T, C, G.

    • RNA:

    • Single-stranded structure.

    • Contains ribose sugar.

    • Bases: A, U (instead of T), C, G.

Protein Synthesis

  • Process Overview:

    1. Transcription: DNA unwinds and unzips, and mRNA is synthesized from the DNA template.

    2. Translation: mRNA is translated into a polypeptide chain at the ribosome using tRNA.

  • Role of mRNA: Carries the genetic code from DNA to ribosomes for protein synthesis.

  • tRNA Function: Transfers suitable amino acids to the ribosome to form proteins based on mRNA sequence.

DNA Profiling

  • Applications:

    • Forensic identification, genetic disorder identification, paternity testing, and organ transplant matching.

  • Process: Comparison of DNA bands; each match confirms genetic relationship or identity.

  • Paternity Testing: Bands from the child must match those of both parents to confirm paternity. If not, the man is excluded as a father.