DNA Structure and Function

1. DNA Structure and Function AP Biology: Chapter 13

2. Key Concepts: • DNA contains the information of heritable traits in all cells • Each DNA strand consists of two strands of nucleotides twisted together • Hereditary information is encoded in the sequence of nucleotides • Nitrogen bases have specific pairing arrangements • DNA is replicated before a cell divides

3. Discovery of DNA Function • Miescher (1868) – isolated DNA from nucleus • Fred Griffith (1928) – discovered hereditary transformation • Oswald Avery (1944) – DNAase blocks transformation • Hershey and Chase (1952) – bacteriophage injects DNA--not protein--into bacterium

4. Mystery of Hereditary Material • Originally believed to be an unknown class of proteins • Experiments in the 1950 showed that DNA, not protein, was the hereditary material. • The race was on to determine the structure of DNA.

5. Discovery of DNA • Griffith’s experiments – 1928 • Trying to develop a pneumonia vaccine • Transfer of hereditary material from dead S cells to living R cells (transformation) S = pathogenic; smooth protective capsule covers the bacteria R = non-pathogenic; the rough cell is exposed to attack by host’s immune system.

6. Discovery of DNA FunctionHershey-Chase virusparticle labeled with 32P virus particle labeled with 35S • 1952 • Radioisotope label protein and DNA of bacteriophage • Radioactive phosphorus is detected inside of bacteria bacterial cell label outside cell label inside cell

7. DNA Structure • Pairing Arrangement • A - T • C – G • Chargaff’s rules • Nucleotides • Deoxyribose (5 carbon sugar) • Phosphate Group • Nitrogen base • Adenine A • Guanine G • Thymine T • Cytosine C

8. Nucleotide Bases ADENINE (A) GUANINE (G) phosphate group adenine (A) base with a double-ring structure guanine (G) base with a double-ring structure deoxyribose THYMINE (T) CYTOSINE (C) cytosine (C) base with a single-ring structure thymine (T) base with a single-ring structure

9. one base pair Components of DNA • Four types of nucleotides • Adenine (A) (base with double ring) • Guanine (G) (base with double ring) • Thymine (T) (base with single ring) • Cytosine (C) (base with single ring • Amount of A=T and C=G (Chargaff) • Pyrimidines (T & C) form hydrogen bonds with purines (A & G).

10. Which bases pair? • Rosalind Franklinand Maurice Wilkinsused X-ray diffraction (crystallography) techniques to produce images of DNA molecules. • DNA is helical • DNA had a uniform width (2 nm) • 1 purine + 1 pyrimidine per “rung” • “double helix”

11. Patterns of Base Pairing Thymine pairs with Adenine, forming 2 hydrogen bonds. Cytosine pairs with Guanine, forming 3 hydrogen bonds. Watson and Crick: • 1954 – put the pieces together • DNA consists of two strands of nucleotides held together at bases by hydrogen bonds • Two kinds of base pairs form: • A-T and C-G • Base with double-ring structure always binds with base with single-ring structure

12. Watson-Crick Model 2-nanometer diameter overall 0.34-nanometer distance between each pair of bases 3.4-nanometer length of each full twist of the double helix In all respects shown here, the Watson–Crick model for DNA structure is consistent with the known biochemical and x-ray diffraction data. The pattern of base pairing (A only with T, and G only with C) is consistent with the known composition of DNA (A = T, and G = C).

13. DNA strands are anti-parallel – run in opposite directions (b) Partial chemical structure (c) Space-filling model (a) Key features of DNA structure

14. Detailed Look at Replication • Hydrogen bonds between two strand easily break • Each single strand then serves as template for new strand • Strand assembly is continuous on one strand, discontinuous on other • Nucleotides can only be added in the 5’ to 3’ direction

15. DNA is highly condensed • DNA is wrapped tightly around proteins & folded. • DNA must unwind for replication to occur.

16. Kinetochore One nucleosome DNA DNA histone core

17. DNA — A 2-Way Street Figure 10-5

18. DNA replicates differently depending upon strand Replication can only occur in the 5’ to 3’ direction right side is discontinuous forming Okazaki fragments as replication proceeds.

19. DNA ligase joins the fragments together continuous assembly on one strand discontinuous assembly on other strand newly forming DNA strand one parent DNA strand

20. DNA Replication DNA Replication and Repair • Enzyme regulated • Hydrogen bonds break • Attachment of nucleotides to new strands • DNA polymerases DNA ligases • New strand is half old, half new Stepped Art

21. Details, details…  Helicase breaks hydrogen bonds.  Binding proteins stabilize strands; prevent them from rejoining.  Free nucleotides move in & H-bond; DNA polymerase links nucleotides to each other starting at primer & working in the 5’ to 3’ direction.  DNA polymerase “proofreads” new strand (replaces incorrect bases). • Repair enzymes remove RNA primers; ligase connects Okazaki fragments

22. DNA replication is semi-conservative

23. In Conclusion • Hereditary information is located in DNA • DNA consists of nucleotides • DNA molecule consists of two nucleotide strands twisted into a double helix • The bases of DNA strands pair in a constant fashion • DNA of one species has specific nucleotide sequences

24. In Conclusion • Enzymes unwind the two strands of DNA and assemble a new strand during DNA replication • Resulting new DNA molecule has an old strand and a new strand • Some of the enzymes in DNA replication also repair DNA