DNA REPLICATION We know we need to copy a cells DNA before a cell can divide, but how is DNA copied? There were 3 possible models for DNA copies to be made while obeying the Base Pairing or Chargaff’s Rules : – Conservative replication leaves the original DNA molecule intact and generates a completely new molecule Sort of like a photo copier – Dispersive replication produces two DNA molecules with sections of both old and new DNA interspersed along each strand. – Semi-conservative replication produces molecules with both old and new DNA, but each molecule would be composed of one old strand and one new one.

The Process of DNA Replication Simplified DNA Replication Activity 1.The double helix unwinds with the help of enzymes called DNA HELICASES. The enzymes break the hydrogen bonds between nitrogen bases Strands are held apart and prevented from twisting back into their double-helical shape by a SINGLE STRAND BINDING PROTEIN. The two areas on either end of the DNA where the DNA helix separates are called REPLICATION FORKS. 2. RNA PRIMASE- inserts as starter of RNA nucleotides at the initiation point. Animation

– Recall, one of the strands of DNA runs in the 3’ to 5’ direction and the other runs in the 5’-3’ direction – DNA Polymerase can only move/build in one direction- towards the 5’ end – Because DNA is antiparallel, DNA Polymerase molecules move in opposite directions along the two strands. – The two newly synthesized strands grow in opposite directions- one towards the fork, the other away from it. – The strand that is assembled continuously and towards the replication fork is called the LEADING STRAND, – The strand that is assembled discontinuously and away from the replication fork is called the LAGGING STRAND. These short fragments are called OKAZAKI FRAGMENTS. 3. DNA POLYMERASE III moves along each of the DNA strands reading the nucleotide on the template and joins the complementary nucleotide onto the end of the new strand by following the Base-Pairing Rules (as many as 1000 base pairs can be added each second)

4. DNA POLYMERASE I- strips away the RNA PRIMERS and inserts DNA nucleotides 5. LIGASE- seals the gaps in the sugar phosphate backbone where Okazaki fragments come together.

Checking for Errors Errors sometimes occur. An important feature of DNA replication is DNA polymerases ability to “proofread”. – This enzyme is capable of backtracking, removing incorrect nucleotides and replacing it with the correct one. – Still one error per one billion nucleotides typically occurs. – This type of error is called a MUTATION.

Multiple Forks Replication does not begin at one end of the DNA molecule and end at the other. Each chromosome contains a single long strand of DNA. The replication of a typical human chromosome with one pair of replication forks would take 33 days! For this reason, each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long (this takes only 8 hours).