1. Section 11.1 Summary – pages 281 - 287 Genetic information in the form of a code is held in the DNA molecules. Ultimately, this code determines an organism’s traits. DNA does this by holding the directions for making proteins. DNA

2. Section 11.1 Summary – pages 281 - 287 DNA is a large molecule made of repeating sub- units (smaller molecules) called nucleotides.

3. Section 11.1 Summary – pages 281 - 287 DNA Nucleotides have three parts: a simple sugar, Deoxyribose a phosphate group, and a nitrogenous base. Phosphate group Sugar Nitrogenous base

4. Section 11.1 Summary – pages 281 - 287 In DNA, there are four possible nitrogenous bases: Adenine (A)Guanine (G)Thymine (T)Cytosine (C) PURINES PYRIMIDINES

5. Section 11.1 Summary – pages 281 - 287 The phosphate groups and sugars form the backbone of the DNA molecule DNA STURCTURE “Legs of ladder” Phosphate & Sugar Backbone

6. Section 11.1 Summary – pages 281 - 287 The bonds in the backbone between the sugars and phosphates are covalent bonds (strong bonds) DNA STURCTURE

7. Section 11.1 Summary – pages 281 - 287 The nitrogenous bases make up the rungs of the DNA molecule. DNA STURCTURENitrogenous Base (A,T,G or C) “Rungs of ladder”

8. Section 11.1 Summary – pages 281 - 287 The bonds between the nitrogenous bases are weak hydrogen bonds (indicated by the dotted line) DNA STURCTURE

9. Section 11.1 Summary – pages 281 - 287 In DNA, adenine always pairs with thymine, A --- T and guanine always pairs with cytosine. G --- C DNA STRUCTURE These paired bases are called complementary base pairs.

10. BASE-PAIRINGS CG H-bonds T A When guanine and cytosine bond, they form triple hydrogen bonds When thymine and adenine bond, a double hydrogen bond is formed

11. DNA Double Helix P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C TA

12. Flip Flop Nature : sugar/phosphate backbone runs in opposite directions One strand runs 5’ to 3’, the other runs 3’ to 5’ One strand runs 5’ to 3’, the other runs 3’ to 5’ The nucleotides connect at the hydroxyl group of the pentose sugar (at the 3’ end)

13. Section 11.1 Summary – pages 281 - 287 Chromosome The sequence of nucleotides forms the unique genetic information of an organism. The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be. DNA STRUCTURE

14. Section 11.1 Summary – pages 281 - 287 Before a cell can divide, it must first make a copy of its DNA. WHY? The DNA is copied in a process called DNA replication. DNA REPLICATION This is when the cell PREPARES for division.

15. First, replication enzymes uncoil the DNA strand. Second, the weak Hydrogen bonds between the A,T,C, G’s break, unzipping the DNA strand. The enzyme Helicase is responsible for this action

16. DNA Replication Replication fork – where the DNA strand begins to unzip Helicase – Enzyme which unzips DNA during replication

17. Section 11.1 Summary – pages 281 - 287 Third, the enzyme DNA Polymerase attaches free nucleotides to the unwound portion.

18. An enzyme known as DNA polymerase adds nucleotides to the original strand’s free 3’ end, in a continuous fashion. The new DNA strand is therefore forming from a 5’ direction, and is referred to as the “leading strand”. Assembly is discontinuous because the exposed –OH group on the 3’ end is the only place where nucleotides can be joined together. As this replication fork opens up, it exposes the 3’ and the 5’ ends of the original DNA strand. In this diagram, that is represented by the blue molecule. On the open 5’ end, the new DNA is built in a discontinuous fashion, from the region nearest the fork opening. This new strand is known as the “lagging strand”. Okazaki fragments are formed, or little “chunks” of DNA.

19. Step 4: DNA ligase then helps to join the fragments and wind the strands back up into a double helix.

20. When DNA replication occurs, it begins somewhere in the middle of the molecule, and not at one end. The “replication fork” is within the bubble. As the new molecules are built, they progress in opposite directions from the point of origin.

22. When the process is complete, two DNA molecules have been formed identical to each other and to the parent molecule Errors during replication are rare, as each cell contains more than thirty enzymes to ensure the accurate replication of DNA DNA polymerase makes very few errors, and most of those that are made are quickly corrected by another enzyme which "proofreads" the nucleotides added into the new DNA strand. This is equal to a person copying 100 large dictionaries word for word, and symbol for symbol, with only one error for the whole process!