Cells need to copy DNA for cellular divisions like meiosis and mitosis

2. Theories of DNA replication Conservative Original DNA double helix acts as a template for producing a new template The original double helix is conserved

3. Destructive Nucleotides from the original strands end up in all daughter strands The original double helix is destroyed

4. Semi Conservative ... the strands are separated and each daughter double helix inherits one original strand The original DNA strands are conserved, but...

5. adenine guanine cytosine tyrosine How do we know which theory is correct? Note: all bases contain nitrogen The Meselson-Stahl Experiment

7. DNA sample CsCl gradient Low CsCl conc. High CsCl conc. Centrifuge: spin very fast When centrifuged... ... molecules migrate until they reach a layer of the same density

8. Bacteria fed 14 N Bacteria fed 15 N Bacteria fed 15 N then one generation on 14 N Intermediate band implies that DNA replication is not conservative Light band Heavy band

10. DNA replication is semi conservative The two original strands act as templates for the synthesis of two new strands Free nucleotides pair with complementary bases Template strand Pairing of complementary bases preserves the sequence of bases when DNA replicates A C T T A G A C T T A G A C T T A G A C T T A G A C T T A G

11. The enzyme Helicase : - unwinds the double helix - breaks the hydrogen bonds between complementary bases - separates the two strands The Details: A C T T A G A C T T A G A C T T A G A C T T A G A C T T A G

12. Free nucleotides pair up with complementary bases, and are held in place, by hydrogen bonding Helicase A C T T A G A C T T A G A C T A C T A C T T A G T A G T A G

13. DNA Polymerase creates two new strands of DNA by creating covalent bond between adjacent nucleotides A C T T A G A C T T A G A C T A C T A C T T A G T A G T A G T A G T A G T A G A C T A C T A C T

14. DNA Polymerase New DNA strand New covalent bonds A C T T A G A C T T A G A C T A C T A C T T A G T A G T A G T A G T A G T A G A C T A C T A C T