DNA

DNA Discovery and Structure

Key Scientists and Experiments

• Frederick Griffith: Discovered the theory of "transformation" when harmless bacteria became disease-causing after exposure to heat-killed virulent bacteria.

• Oswald Avery: Determined that DNA was the molecule responsible for transformation by testing various cell extracts.

• Hershey and Chase: Used radioactive phosphorus ($^{32}P$) and sulfur ($^{35}S$) in bacteriophages to prove that DNA, not protein, is the genetic material.

• Rosalind Franklin: Used X-ray diffraction (Photo $51$) to discover that DNA is a double-stranded helix.

• Watson and Crick: Built the first comprehensive double-helix model of DNA using Franklin's data.

DNA Composition

• Functions: DNA stores genetic information, expresses inherited traits, and allows for replication during $Mitosis$.

• The Nucleotide: The building block of DNA, consisting of:

  1. $5$-Carbon Sugar: Deoxyribose.

  2. Phosphate Group: Forms the structural backbone with the sugar.

  3. Nitrogenous Base: Categorized into Purines and Pyrimidines.

Nitrogenous Bases and Rules

• Purines: Adenine ($A$) and Guanine ($G$). They are larger molecules with $2$ rings (Mnemonic: "A PuG").

• Pyrimidines: Cytosine ($C$) and Thymine ($T$). They are smaller molecules with $1$ ring (Mnemonic: "PyCuT").

• Chargaff’s Rule: The percentage of Adenine equals Thymine ($A = T$), and the percentage of Cytosine equals Guanine ($C = G$).

Structural Orientation

• Antiparallel Strands: The two strands of DNA run in opposite directions, designated by the $3'$ and $5'$ ends.

• Hydrogen Bonds: These bonds connect the nitrogenous base pairs to hold the two strands together.