1. Notes 4-3 continued… DNA

2. Scientists 1952- Rosalind Franklin used X-ray method to take photographs of DNA 1953- Watson and Crick use the photographs and discovered the structure of DNA is a double helix (twisted ladder)

3. DNA is found inside the nucleus of cells

4. Structure of DNA The DNA molecule, supported by proteins, is shaped like a twisted ladder.

5. Chromosomes DNA coils around proteins, then coils up some more, and condenses into chromosomes

6. DNA Deoxyribonucleic Acid Belongs to the organic molecule group known as “nucleic acids” that we discussed in chap 3 Sides of the ladder are made up of molecules of sugar called deoxyribose and phosphate molecules The rungs of the ladder are made up of 2 nitrogen bases

7. Nitrogenous bases DNA has 4 possible nitrogen bases: Adenine, thymine, guanine, and cytosine They are represented by the capital letters A, T, G, C In DNA, A always pairs with T and G always pairs with C Look at the DNA molecule on the next slide to see what I mean

8. Draw this in your notes Notice the base pairs in the rungs, A-T and C-G Notice the alternating sugars and phosphates on the sides

9. Replication of DNA Happens inside the nucleus during interphase An enzyme (a protein) comes and “unzips” the DNA ladder between its base pairs New bases are then added to the old strand As a result, you end up with 2 identical strands of DNA This is an important part of the cell cycle, as each new daughter cell needs a copy of DNA, since DNA holds the instructions for making proteins

10. Replication of DNA Because of the way in which the nitrogen bases pair with one another, the order of the bases in each new DNA molecule exactly matches the order in the original DNA molecule.

11. Problem: If the left side of the ladder has the following sequence of base pairs: A-C-T-G-G-T-G-C-G-A-C-T, what would the sequence on the right side of the ladder be? Write down your answer…. T-G-A-C-C-A-C-G-C-T-G-A