Biology 9.3 Replication of DNA

1. Biology 9.3 Replication of DNA Replication of DNA

2. Replication of DNA • When the double-helix structure of DNA was first discovered, scientists were excited about the complementary relationships between the sequences of nucleotides; the base-pairs.

3. Replication of DNA • They predicted that the structure was a basisthat allowed one DNA strand to serve as a template to make exact copies of the DNA when the cell divides.

4. Replication of DNA • Within five years of the discovery of DNA’s structure, scientists had firm proof that the complementary strands of DNA double-helix do indeed serve as the template to build new DNA.

5. Replication of DNA • The process of making an exact copy of DNA is called replication. • Recall from previous lessons that DNA replicates itself in the S phase of cell division; before the cell nucleus divides.

6. Replication of DNA • Step 1: • Before DNA replication can begin, the double helix unwinds in the first step separating the two strands. • DNA helicasesopen the double helix by breaking the weak hydrogen bonds that linked the nitrogen base-pairs (the rungs of the DNA ladder). Like opening a zipper, the two sides are unzipped.

7. Replication of DNA • Step 2: • Once the strands are separated, additional proteins attach to each strand, holding them apart and preventing them from returning to their previous double-helix shape. • These areas where the double-helix separates are called replication forks because of their Y shape.

8. Replication of DNA • Step 3: • At the replication fork, enzymes known as DNA polymerases add nucleotides to the exposed bases, according to the base-pairing rules • (adenine always links to thymine and guanine always links to cytosine). • As the DNA polymerases move along, two new full double-helixes form.

9. Replication of DNA • Step 4: • Once the DNA polymerases have begun adding nucleotides to a growing double-helix, the process continues until all of the DNA has been copied. • Than the polymerases receive a signal to detach and we are left with a two new strands of identical DNA. Replication is complete, each strand identical to the original strand.

10. Checking for Errors: • In the course of DNA replication, errors sometimes occur and the wrong nucleotide is added to the strand. • A “proofreading” function watches for this in the process and makes corrections before the replication goes farther.

11. Checking for Errors: • The process checks to make sure the correct nucleotide was added and does not proceed until this is verified. • If is senses a mistake, it backs up, replaces the incorrect nucleotide with the proper one, and only than continues on with replication.

12. Checking for Errors: • The proofreading reduces mistakes to about one error per 1 billion nucleotides.

13. The Rate of Replication • Replication does not begin at one end of the DNA molecule and end at the other. • The circular DNA molecules of prokaryotes usually have two replication forks that begin at a single point.

14. The Rate of Replication • The replication forks move away from each other until they meet on the opposite side of the DNA circle.

15. The Rate of Replication • In eukaryote cells, each chromosome contains a single long strand of DNA. The length presents a challenge. • To solve this and speed up replication, eukaryote cells replicate in about 100 sections.

16. The Rate of Replication • Each section has it’s own start and end point. With these 100 replication forks working in sync; the replication takes 8 hours instead of 33 days.

17. Computer Lab: • After completing worksheet 9.3; go to the computer lab and use the internet to research and answer the following three questions. • Explain the role that the two enzymes; helicases and polymerases, play in the role of DNA replication. • Explain the relationship between DNA polymerases and mutations. • State how multiple replication forks speed up replication in eukaryotes.