1. DNA Replication Section 9-3

2. DNA is Copied with the Help of Many Enzymes We know that the two DNA strands have a complementary relationship (A pairs with T and G pairs with C) This is how the DNA makes exact copies of itself One strand is used as a template, or pattern, on which the second strand is built.

3. DNA Replication – process of making a copy of DNA REMEMBER… This occurs during the S (synthesis) phase of the cell cycle. STEP 1: The double helix has to unwind. This is accomplished by an enzyme called DNA helicase. It opens the double helix by breaking the hydrogen binds that link the complementary nitrogen bases between the two strands. Once the two strands are separated, additional proteins attach to each strand to hold them apart. We now have what is called a replication fork (because of its Y shape).

4. STEP 2: At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands adding nucleotides to the exposed nitrogen bases. This is done according to the base pairing rules. As the DNA polymerase moves along, two new double helixes are formed. STEP 3: Once the DNA polymerase has begun adding nucleotides to a growing double helix, the enzyme remains attached until all of the DNA has been copied and it is signaled to detach. Each new double helix is composed of a new strand and an original strand of DNA. Both strands of DNA are identical to each other.

5. Checking for Errors In the course of DNA replication, errors sometimes occur and the wrong nucleotide is added to the new strand. DNA polymerases have a “proofreading” role – they can only add nucleotides to a growing strand of DNA if the previous nucleotide is correctly paired to its complementary base. If the previous nucleotide is incorrectly paired, DNA polymerase will backtrack and remove the incorrect nucleotide and replace it with the correct one. This prevents most errors in DNA replication.

6. Multiple Replication Forks Increase the Speed of Replication Circular DNA found in bacteria (prokaryotic cells) usually have two replication forks that begin at a single point. The replication forks move away from each other until the entire DNA molecule has been replicated – the two replication forks will meet. Eukaryotic Cells – each chromosome contains a single, long strand of DNA. This presents a challenge.

7. If a human chromosome has only one pair of replication forks spreading from a single point, it would take 33 days (792 hours) to complete replication. This problem is solved by multiple replication forks on each chromosome. Each chromosome is replicated in about 100 sections that are about 100,000 bases long. With multiple replication forks working in concert, an entire chromosome can be replicated in about 8 hours.

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