Defined: Molecule that stores genetic information Nucleotides: Three repeating units of nucleic acids –1) Sugar –2) Phosphate Group –3) Nitrogen Base Millions of nucleotides per strand of DNA

Sugar (Deoxyribose) connects with the phosphate group 4 Different Bases: –Adenine (A) –Thymine (T) –Guanine (G) –Cytosine (C) Rule: A always joins with T Rule: C always joins with G Human DNA: ~3 billion pairs

James Watson and Francis Crick identified DNA’s structure in 1953 Double Helix: spiral shape of DNA Left and right nucleotide strands are connected by a hydrogen bond

Defined: Process of copying DNA –During interphase (S-stage) Step 1: DNA Helicase “unzips” the two DNA strands (breaks the H bonds) Step 2: Free floating nucleotides (A, T, C, G) match up Step 3: DNA Polymerase reconnects the two strands

Video Clip

Mutation: change in DNA sequence Mutations can be corrected –Enzyme “proofreads” DNA and check/correct errors Pre-repair: 1 in 10,000 nucleotides has an error Post-repair: 1 in 1 billion nucleotides has an error

Quick Review DNA nucleotides form long chains DNA nucleotides have 4 different bases (A, T, C, G) DNA makes exact copies of itself