2. DNA: Structure and Replication

3. DNA DNA. DNA is often called the blueprint of life. DNA contains the instructions for making proteins within the cell. Nucleic acid

4. Why do we study DNA? We study DNA for many reasons, e.g., its central importance to all life on Earth, medical benefits such as cures for diseases, better food crops. Solves crimes

5. Chromosomes and DNA Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA.

6. The Shape of the Molecule DNA is a very long polymer – made of nucelotides The basic shape is like a twisted ladder This is called a double helix.

7. The Shape of the Molecule Sides - alternating phosphates and sugars Center – made of nitrogen bases

8. Nucleotides CC C O Phosphate O C C O -P O O O One deoxyribose sugar together with its phosphate and base make a nucleotide. Nitrogenous base Deoxyribose

9. Nucleotides CC C O Phosphate O C C O -P O O O Nitrogenous base Deoxyribose

10. Four nitrogenous bases 1. Cytosine C 2. Thymine T 3. Adenine A 4. Guanine G DNA has four different bases:

11. Chargraff’s Rule: Adenine and Thymine always join together A T Cytosine and Guanine always join together C G

12. Three Representations of DNA

13. DNA Replication

14. Process of copying DNA – done by a series of enzymes Process of copying DNA – done by a series of enzymes Essential in all life forms Essential in all life forms Must occur before the cell divides – ensures each cell will have identical DNA Must occur before the cell divides – ensures each cell will have identical DNA Takes place in the nucleus Takes place in the nucleus

15. Steps of DNA Replication 1. Helicase enzyme breaks the H-bonds between bases 2. DNA polymerase adds complementary bases 3. Process continues until entire DNA is copied

16. Steps of DNA Replication Enzymes proofread and correct these mistakes 4. Enzymes proofread and correct these mistakes 5. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

17. Steps of DNA Replication

18. T A C C G A T G G C #1#2 Enzyme Helicase T A C C G #1 A T G G C A T G G C #2 T A C C G New DNA Polymerase DNA Polymerase

19. Semiconservative Model of Replication Idea presented by Watson & Crick Idea presented by Watson & Crick The The two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA

20. Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errors DNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakes Enzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

24. Steps of DNA Replication 1. Helicase enzyme breaks the H-bonds between bases and “unzips” the DNA 2. DNA polymerase adds complementary nucelotides to the exposed bases 3. Process continues until entire DNA is copied

25. Steps of DNA Replication Enzymes proofread and correct these mistakes 4. Enzymes proofread and correct these mistakes 5. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

26. DNA Molecule S P AT S P P P SS SS P P G C C G Nucleotide Hydrogen bonds