The Structure of DNA (Ch 12.2)

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

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

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

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

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

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)