DNA Biology Flashcards

DNA and RNA Structure

• Friedrich Miescher (1844-1895): Discovered nucleic acids, isolating a phosphate-rich chemical from cell nuclei called "Nuclein."
• Mendel (1843): Proposed the particulate theory of inheritance based on minute particles (genes) found on chromosomes.

Key Experiments in DNA Discovery

• Fredrich Griffith (1928): Reported transformation in bacteria.
  • Transformation: External DNA is taken up by a cell, changing its characteristics.
  • Experimented with Streptococcus pneumoniae (rough and smooth strains).
  • S strain is pathogenic, with a capsule, while R strain is non-pathogenic.
  • Mixture of heat-killed S cells and living R cells killed mice, with living S cells recovered.
• Oswald Avery, Colin McLeod, and Maclyn McCarty (1944): Determined DNA as the transforming principle.
  • Isolated proteins and nucleic acids from S strain.
  • Used enzymes to degrade each component and tested for transformation.
  • Only when DNA was degraded, the mixture failed to transform bacteria.
• Alfred Hershey and Martha Chase (1952): Confirmed DNA is the genetic material using bacteriophage T2.
  • Tracked which component (DNA or protein) entered bacteria during infection.
  • Radioactive phosphorus (32P) labeled DNA was found inside the cell and in new viruses.

The Structure of DNA

• DNA (Deoxyribonucleic Acid) is the genetic material; RNA (Ribonucleic Acid).
• Nucleotide components: Phosphate, five-carbon sugar, and a nitrogenous base.
• Erwin Chargaff:
  • Discovered that the amount of A, T, G, and C in DNA varies among species.
  • Established Chargaff’s rules: A = T and G = C in each species.
• Maurice Wilkins and Rosalind Franklin: Used X-ray crystallography to study DNA structure, revealing its helical nature with repeating portions.
• James Watson and Francis Crick (1951): Developed the double helix model of DNA.
  • DNA is a double helix, like a twisted ladder, with deoxyribose sugar and phosphate forming the sides.
  • Complementary base pairing: A with T, and G with C.
  • Hydrogen bonds hold the helix together.

DNA has Directionality

• Because of the directionality of DNA, new nucleotides can only be added in the 5’ to 3’ direction.

DNA Replication

• Process of copying DNA during interphase.
  • Strands separate, and each serves as a template for a new strand.
  • Semiconservative: Each new DNA molecule contains one parent strand and one new strand.
• Requires:
  • Unwinding (helicase).
  • Complementary base pairing.
  • Joining (DNA polymerase and DNA ligase).
  • New DNA molecule is identical to the original.
• Process:
  • Parent strand unwinds and separates via helicase.
  • New strands form through complementary base pairing via DNA polymerase.
  • DNA ligase seals breaks in the sugar-phosphate backbone.
• In eukaryotes, replication starts at multiple origins forming replication bubbles that spread until they meet.
• Key Replication Proteins and Functions:
  • Helicase: Unwinds the double helix.
  • Single-strand binding protein: Stabilizes single-stranded DNA.
  • Topoisomerase: Relieves overwinding strain.
  • Primase: Synthesizes RNA primer.
  • DNA pol III: Synthesizes new DNA strand.
  • DNA pol I: Removes RNA primer and replaces it with DNA.
  • DNA ligase: Joins Okazaki fragments.