Chapter 12: DNA

1. Chapter 12: DNA The Blueprint of Life

2. DNA Video Importance of DNA video clip

3. DNA = Deoxyribonucleic Acid • Role of DNA • Molecule that stores genetic information in living cells • Genetic information can be copied for new cells • Information is transmitted to new offspring

4. 12.2 Structure of DNA Objectives • Identify the chemical components of DNA. • Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code. • Describe the steps leading to the development of the double-helix model of DNA.

5. Components of DNA • DNA is a polymer of nucleotides • These nucleotides have three parts: • Phosphate group • 5 carbon sugar: deoxyribose • Nitrogen base: A, G, C, T • A = adenine G = guanine • C = cytosine T = thymine • Covalent bonds join nucleotides

6. Check-in • What are the three roles of DNA? • DNA is a polymer of _______________. • A nucleotide is made of what three things? • What type of bond joins nucleotides together? Storing, copying and transmitting genetic information nucleotides Phosphate group, deoxyribose sugar, nitrogenous base Covalent bond

7. Solving the Structure of DNA • Chargaff’s Rule – true for all organisms • Amount of A = Amount of T • Amount of C = Amount of G • Rosalind Franklin • Used X-ray diffraction • Took picture of DNA in 1952 • Picture gave clues about DNA structure: • Strands twisted into helix (like coiled spring) • Two strands in DNA • Nitrogenous bases in center

8. Solving the Structure of DNA • James Watson and Francis Crick • Built 3-D models of DNA • Shown Franklin’s Photo 51 • Led to discovery of double helix shape in 1953

9. Check-in • Describe Chargaff’s rule? • What clues did Rosalind Franklin’s picture give about the structure of DNA? • What method did she use to take the picture? Looking at DNA in all organisms, amount of adenine equals amount of thymine and amount of guanine equals amount of cytosine. DNA was helix shaped and contained two strands; bases are in the center of DNA molecule. X-ray diffraction

10. Check-in • Who is credited with the discovery of the structure of DNA? • What method did they use to discover its structure? James Watson and Francis Crick Built three dimensional models.

11. Double-Helix Model • DNA is shaped like a double helix • Looks like: spiral staircase or twisted ladder • Sides = Phosphate groups and sugars • Steps or rungs = Nitrogen bases • Bases are held together by weak hydrogen bonds

12. Double-Helix Model • Stands are antiparallel – run in opposite directions • Which bases are complementary to each other? • A with T • G with C • Called base pairing • Explains Chargaff’s rule

13. Structure of DNA Chromatin, Chromosomes and DNA subunits video clip

14. Check-in • What does a double helix look like? • What forms the backbone of DNA or sides of the ladder? • What forms the rungs of the ladder? • What type of bond holds the bases together? Twisted ladder or spiral staircase Sugar (deoxyribose) and phosphate groups. Nitrogenous bases Hydrogen bonds

15. Check-in • What base bonds with A (adenine)? • What base bonds with C (cytosine)? • What is the complementary DNA strand for the following: AGGTCT T (thymine) G (guanine) TCCAGA

16. 12.3 DNA Replication Objectives • Summarize the events of DNA replication. • Compare DNA replication in prokaryotes with that of eukaryotes.

17. DNA Replication • What is it? Process by which chromosomal DNA is copied before mitosis and meiosis • When does it happen? Interphase – S phase • Why does it happen? So each new cell gets a complete copy of DNA

18. Steps of DNA Replication • Strands of DNA Separate: Hydrogen bonds between bases are broken by an enzyme and the strands separate • Base Pairing: Free-floating nucleotides pair up with bases on the exposed strand (matches with the complementary base – AT & CG)

19. Steps of DNA Replication • Bonding Nucleotides Backbone (sugar/phosphates) of the once-free nucleotides are bonded together by an enzyme (DNA polymerase) to form a new complementary strand • Result of Replication Two DNA molecules, each with a new strand and an old strand Animation of DNA Replication

20. DNA Replication • Role of enzymes • One enzyme unzips the strands (breaks hydrogen bonds) • DNA polymerase creates sugar-phosphate bonds and “proofreads” new DNA strand • Telomerase adds short DNA sequences to telomeres Telomere

21. Prokaryotes vs. Eukaryotes • Prokaryotes • Single circular DNA molecule in cytoplasm • Replication starts at onepointand continues in both directions

22. Prokaryotes vs. Eukaryotes • Eukaryotes • Chromosomes much bigger than prokaryotes • Replication starts at many places and continues in both directions

23. Check-in • Describe the three steps of DNA replication? • What is the result of DNA replication? • What are the two functions of DNA polymerase? • DNA strands separate • Free-floating nucleotides pair with exposed bases • Sugar-phosphate backbone bonds together Two DNA molecules, each with one new and one old strand • Bond sugar and phosphate together • Proofread new DNA strand

24. Check-in • What is the function of telomerase? • What is the major difference in DNA replication between prokaryotes and eukaryotes? Add DNA sequences to tips of chromosomes to prevent genes from being lost or damaged during replication In prokaryotic cells, replication starts in one spot on the chromosome, while in eukaryotic cells it starts in multiple spots.