1. DNA REPLICATION

2. DNA Replication Objectives: You should be able to:
Explain the need for replication
Describe and model the steps of replication
Identify the work of the enzymes of replication
Distinguish leading and lagging strands
Define “semiconservative”

3. Vocabulary Replication Semi-conservative Replication fork
Leading strand
Lagging strand
Okazaki fragments
Helicase
DNA polymerase
DNA ligase

4. Why DNA?

5. DNA REPLICATION
The purpose of replication is to create two identical strands of DNA (chromosomes) prior to cell division.
The exact copying of the DNA ensures that every cell in a person’s body will have all the chromosomes.

6. If either of the two new cells being produced lack a chromosome or have too many chromosomes or have an error in the DNA, a mutation will occur.
Mutations also occur if the nitrogen bases are copied or paired incorrectly (e.g. A with G).

7. DNA REPLICATION The replication of DNA is said to be semi-conservative
because one half of each newly created molecule is the original strand, and the other half is the newly synthesized strand.

8. Semi-conservative Replication

9. DNA REPLICATION DNA is unwound and “unzipped” by helicase.
Helicase breaks the hydrogen bonds between the nitrogen bases.
The point of separation (“unzipping”) of the DNA molecule is known as the replication fork.

10. Find the replication fork

11. DNA REPLICATION
DNA polymerase adds nucleotides in a 5’ to 3’ direction along both sides of the unzipped DNA strand.

12. THE LEADING STRAND
The unzipped side of the DNA molecule designated as 3’ is the template for the leading strand.
It is called leading because nucleotides can be added in a continuous motion toward the replication fork.

13. THE LAGGING STRAND
The unzipped side of the DNA molecule designated as 5’ is the template for the lagging strand.
Nucleotides cannot be added in a continuous motion because they are being added away from the replication fork.

14. The Lagging Strand (cont.)
The nucleotides cannot be added to the lagging strand until there is sufficient room at the replication fork for the DNA polymerase to attach a nucleotide.

15. THE LAGGING STRAND Nucleotides are added in small segments known as
Okazaki fragments.
Okazaki fragments are joined together by the enzyme called
DNA ligase.

17. DNA REPLICATION
Following replication, DNA polymerase “proofreads” the new strands for any mistakes.
If base pairs have been mismatched, DNA polymerase corrects the errors.

18. Replication in Action

19. Origin of Replication in Prokaryotes v. Eukaryotes

20. Replication in Prokaryotes
In prokaryotes, the circular DNA has only one origin of replication (where the replication process begins.)
Replication occurs in both directions; however, to speed the process.

21. Replication in Prokaryotes

22. Replication in Eukaryotes
Eukaryotic chromosomes can be quite long
To speed the replication process, there are multiple origins of replication along the chromosome that eventually run into each other to complete the process!

23. Replication in Eukaryotes