1. The Structure of DNA (Ch 12.2)

2. The Components of DNA
DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.
Nucleic Acids
macromolecules made of smaller units (nucleotides) linked together
Nucleotides: 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base

3. The Components of DNA Nitrogenous Bases and Covalent Bonds
They contain nitrogen
Four bases:
adenine (A) cytosine (C)
guanine (G) thymine (T)
Nucleotides are joined to the phosphate groups of the next nucleotides by a covalent bond.
Covalent bond: sharing electrons to create stability in the molecule
Nucleotides can be joined in any order -- the bases allow for them to absorb UV light
Pyrimidine: 1 carbon-nitrogen ring and 2 nitrogen atoms
Purine: 2 carbon-nitrogen rings and 4 nitrogen atoms

4. The Structure of DNA Chargoff’s Rule: A = T and G = C
Adenine and Thymine are in equal quantities in the DNA molecule
Cytosine and Guanine are in equal quantities in the DNA molecule
Rosalind Franklin
She used X-ray diffraction to get a sense of what the DNA molecule looked like
Results:
“X” shaped pattern suggesting DNA is twisted like the coil of a spring -- helix
there are two strands in the structure -- the bases are in the center of the molecule

5. The Structure of DNA Watson and Crick
They attempted to construct a DNA molecule
Watson saw a picture of Franklin’s X-ray
Watson and Crick built a model explaining the specific structure and properties of DNA
DNA = double helix where two strands of nucleotides are wound around each other

6. The Double-Helix Model
Double-Helix = two strands twisted like a staircase
Chargoff’s rule of base pairing is how the two strands are held together -- the bases are in the middle of the two strands
Antiparallel Strands
the two strands of the double helix run in opposite directions (anti-)
Hydrogen Bonding
hydrogen bonds hold together the nitrogenous bases (A, T, C, G) together
hydrogen bonds are weak so the molecule is able to separate

7. The Double-Helix Model
Base Pairing
Nitrogenous bases fit together in the middle of two
double-helix strands
Adenine (A) always pairs with thymine (T)
Cytosine (C) always pairs with guanine (G)
Base pairing explains Chargoff’s rule: A = T and C = G
Nitrogenous bases are paired together with hydrogen bonds (weak bonds)