Chapter 16

• DNA


  A double helix composed of two antiparallel strands
• DNA strand


  • Consists of a sugar-phosphate backbone and nitrogenous bases
  • Adenine (A) pairs with thymine (T)
  • Guanine (G) pairs with cytosine (C)
  • Strands run in opposite directions, affecting replication
• DNA Replication


  1. Leading Strand: Synthesized continuously

  2. Lagging Strand: Synthesized in segments (Okazaki fragments)

  3. Primase: Creates RNA primers for DNA polymerase to start replication

  4. Helicase: Unwinds the DNA helix

  5. Topoisomerase: Relieves tension from unwinding

  6. Single-Strand Binding Proteins: Stabilize single-stranded DNA

  7. DNA Ligase: Joins Okazaki fragments on the lagging strand
• More than a dozen enzymes and other proteins are involved in DNA replication
• Telomeres


  • Repetitive sequences at the ends of chromosomes
  • Protect chromosomes from degradation during replication
  • Shorten with each cell division, which is linked to aging
• Telomerase


  Enzyme that lengthens telomeres in germ cells and some cancer cells
• Bacterial Chromosome


  • Circular DNA, fewer proteins, located in the nucleoid region
• Eukaryotic Chromosome


  • Linear, with histone proteins, located in the nucleus
  • Chromatin can be loosely packed (euchromatin) or highly condensed (heterochromatin)
• Transformation and Bacteriophages


  Provided evidence that DNA can carry genetic information
• Chargaff's Rules


  DNA composition varies by species; A = T and G = C
• Watson and Crick's Model


  Established double-helical structure of DNA, explaining Chargaff's rules