1. DNA Structure, Function and Replication

2. Learning Intentions B5 – Describe DNA replication:
Review DNA structure and function
Describe the purpose of DNA replication
Identify the site of DNA replication in a cell
Describe the three steps in DNA replication
Explain why DNA replication in called “semi- conservative”

5. Comic by Kate Beaton, http://harkavagrant.com/

6. Fig. 25.2

7. DNA structure Double helix
Backbone is sugar and phosphate joined by covalent bonds
Rungs are complementary base pairs joined by hydrogen bonds (purine-pyrimidine: A-T, G-C)
Note that the nucleotides in the two strands are laid out in opposite directions (antiparallel)

8. DNA Function DNA carries the genetic code
Replicates to pass on genetic information
Instructions for synthesis of proteins
Proteins are required to build an organism and catalyze all of its biochemical reactions (therefore controlling all of the functions of the cell and organism)

9. DNA Replication Questions for discussion:
What is meant by the term DNA replication?
Why is DNA replication necessary?
Where does it take place?
When does it take place?
What is the function of the genetic code?

10. (what do you think this means?)
DNA REPLICATION
DNA must be replicated in every cell before the cell divides
This is necessary so that each new cell has a complete copy of the genetic code
The method of DNA replication is known as “semi-conservative”
(what do you think this means?)

11. Fig. 25.3

12. Step 1 - Unzipping
The double-stranded DNA molecule “unzips” when the enzyme DNA helicase breaks the hydrogen bonds between complementary bases

13. Step 2 Each strand acts as a template to make new double helix
New complementary nucleotides attach to the exposed bases
Catalyzed by the enzyme DNA polymerase

14. Step 3
Adjacent phosphate and sugar groups are attached by another DNA polymerase
We now have two DNA molecules, each identical to the original

15. Semi-conservative
This is called semi-conservative replication because half of each DNA strand is “old” and the other half is “new”

16. Fig. 25.4