Biology 9.3 Replication of DNA

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.

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

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.

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.

Replication of DNA Step 1: Before DNA replication can begin, the double helix unwinds in the first step separating the two strands. DNA helicases open 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.

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.

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.

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.

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.

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.

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

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.

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

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.

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.

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.