DNA Structure & Replication: Facts, Function, and Process

1. DNA Structure & Replication

2. DNA Facts • A single strand of human DNA = about 1 meter long (uncoiled). The total DNA in each cell = about 2m. • The nucleus of a typical cell has a diameter of 0.00034mm = 0.000013in • A single strand of DNA contains about 3 billion base pairs. • If the DNA sequence of a person was compiled in books, it would take the equivalent of 200 telephone books, each 1000 pages.

3. Function of DNA • Stores & transmits genetic info from one organism to next generation • Instructs cells on work to do

4. DNA in other organelles? Why?

5. DNA Structure • Deoxyribonucleic Acid • Building blocks - nucleotides • 5-Carbon Sugar (deoxyribose) • Phosphate Group • Nitrogen Base

6. 4 nitrogen bases • Adenine (A) & Guanine (G)  purines • Cytosine (C) & Thymine (T)  pyrimidines

7. DNA Strand • Nucleotides bond together  polymer • Sugar & Phosphate attached by covalent bonds • Nitrogen bases point outward & attach to each other by hydrogen bonds (weaker)

8. Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G) DNA Double Helix

9. Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 Erwin Chargaff • Same amount of A&T and C&G • A pairs only w/T – (2 H-bonds) • C pairs only w/G – (3 H-bonds) • Complementary base pairing rule

11. Twisted Ladder • B/C Sugar & Phosphate groups do not line up exactly

12. Double Helix Discoveries • Rosalind Franklin – X-ray diffraction showed DNA helix was 2-3 strands • Watson & Crick – 1953 – exact structure double helix held together by nitrogen bases

14. Nucleosome Chromosome DNA double helix Coils Supercoils Histones/proteins

15. More DNA Facts • If DNA were the thickness of a clothesline, the length would be 8km (5mi) long. • In bacteria, nucleotides are replicated at a rate of about 500 per second; in mammals, about 50 per second. • During DNA replication, only about one error occurs for every one billion nucleotides made.

16. ENZYMES made of Proteins 100s to 1000s of chemical rxn in human body Enzymes speed up rxn by millions and even billions of times Single enzyme molec. acts on about 1000 substrate molecules/second.

17. IMPORTANCE OF ENZYMES • Enzymes are biological catalysts. • Lower the amount of energy needed for rxn • Not changed by rxn • Not used up by rxn

18. Reaction pathway without enzyme Activation energy without enzyme Activation energy with enzyme Reactants Reaction pathway with enzyme Products They lower the amount of energy needed for a reaction.

19. Responsible for metabolism: • Catabolism: breaking apart molecules (hydrolysis) • Anabolism: building molecules (dehydration synthesis/condensation rxn)

20. Lipase-lipids Protease-proteins Sucrase-sucrose Most enzymes are named after the substrate they work on (usually ending in “-ase”).

22. DNA Replication • Cells copy info for each new cell • Occurs in nucleus • 3 steps

23. 1. Unzip • DNA Helicase unzips (uncoils) part of DNA strand, breaking H-bonds btwn bases

24. 2. Complementary Base Pairing • DNA Polymerase pairs up free nucleotides with complementary bases (forming new H-bonds)

25. 3. Joining nucleotides • Ligase – joins nucleotides (S+P) w/covalent bonds  2 identical DNA strands, each w/1 new strand & 1 old strand • Semi-conservative replication

27. Checking for Errors • DNA polymerases are also “proofreaders” • Only add nucleotides to growing chain, if previous base is correctly paired • Backtracks to correct mistake • Only about 1 error for every one billion nucleotides made

28. DNA Replication Animation

29. Nucleosome Chromosome DNA double helix Coils Supercoils Histones/proteins