DNA Replication. The Organization of DNA. Recall:

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1 Recall: The Organization of DNA DNA Replication Chromosomal form appears only during mitosis, and is used in karyotypes. folded back upon itself (chromosomes) coiled around itself (chromatin) wrapped around histone proteins double helix Each double helix of DNA is really long and unruly. They need to be organized in a compact manner within the nucleus. Eukaryotic cells have much more DNA than bacteria and viruses, and so the organization is more sophisticated.

2 Semi Conservative Replication Original DNA 1 st Replication 2 nd Replication Semi Conservative Replication Recall: During interphase of the cell cycle, cells make an extra copy of their DNA to prepare for mitosis. Each strand of the original double helix (parent strand) serves as a template to match the sequence of base pairs in a complimentary strand.

3 Phases of Replication There are three phases of replication which happen simultaneously on the DNA molecule. Phase 1: Initiation Phase 2: Elongation Phase 3: Termination

4 Phase 1: Initiation Replication Origins: These sites are characterized by a specific sequence of base pairs. A group of enzymes recognize the sequence and bind to the site, unraveling the strands and separating them to form an opening called a replication bubble. Replication fork As the replication bubble expands, the junction where the two strands are still attached is called the replication fork.

5 Initiation DNA Polymerase Helicase An enzyme called DNA helicase unwinds the double helix just ahea of the replication fork. Direction of Replication RNA Primer Start Point of Bubble Within the replication bubble, DNA polymerase then binds to the parent strands within the bubble, using them as a template to add complementary nucleotides one at a time. Once the strands have separated, a small piece of Start Point of Bubble RNA called a primer attaches to the separated parent strands to begin replication.

6 Phase 2: Elongation Special Circumstance: DNA polymerase lengthens the new strands by adding nucleotides to the free 3 hydroxyl end of a pre existing chain. Therefore replication can only take place in the 5 to 3 direction of the new strand. DNA polymerase

7 But the two strands run antiparallel...what about the strand running in the other direction? The leading The lagging strand must be built in small sections. As the fork continues to open, a new fragment is begun with a primer and elongates to meet the fragment behind it. DNA Ligase links the Okazagi fragments together to form the daughter strand. strand is able to replicate continuously. It adds nucleotides to the new strand in the same direction as the fork opens. Only one primer is required.

8 LEADING Remember, in replication there are two forks, and hence two leading and lagging strands. LEADING LAGGING LAGGING Original point of origin

9 Phase 3: Termination Lagging Strand Issue: Once the RNA primers are removed at the end, or telomere of the strand,there is a small gap that cannot be filled in. As a result, each daughter strand is slightly shorter than the parent strand! Fortunately, telomeres are composed of highly repetitive non coding nucleotide sequences that act as a buffer.

10 Mistakes Despite the specific H bond arrangement between complementary base pairs, occasionally an incorrect nucleotide is inserted into the replicated strand. If the nucleotide error is missed, a single point mutation occurs. DNA polymerase can recognize when H bonding does not occur, and rejects the nucleotide that was inserted.

11 DNA Replication clip 1 DNA Replication clip 2 DNA Replication clip 3 DNA Replication clip 4 - Real time animation