Chapter 16.2 DNA Replication and Repair

Recap Nitrogen base pairings A – T C – G Adenine and Guanine are purines -2 rings Cytosine and Thymine are pyrimidines -1 ring

DNA Replication According to Watson and Crick this is the basic principle of replication -only a few hours to complete - one error per 10 billion nucleotides - more than a dozen enzymes and proteins aid in process

Origins of Replication Replication begins at special sites in DNA stretch that have specific sequence of nucleotides. Proteins recognize and attach creating a bubble. DNA is replicated in two directions until entire sequence is copied

Antiparallel Elongation Two strands of DNA are oriented in opposite directions or antiparallel Replication follows same pattern Nucleotides are added to 3’ end and proceed in 5’ to 3’ direction Leading strand copied toward replication fork continuously Lagging stand copied away from the fork in series of segments

Proteins and Enzymes for Replication Helicase is an enzyme that untwists the double helix to initiate copying Single-strand binding protein stabilizes unpaired DNA stands until synthesis of new strands occurs Topoisomerase relieves strain created from unwinding of parent strand

Priming synthesis Primase is an enzyme required for synthesis of both strands that connects nucleotides and makes primer complementary to parent strand Primer is usually 5 to 10 nucleotides long

DNA Polymerase Once DNA is uncoiled a bubble forms DNA polymerase is the enzyme that will catalyze the synthesis of a new stand * Eukaryotes have 11 different polymerases acting in replication With primer is in place and base pairs are added to by DNA polymerase

Leading Strand Leading stand uses DNA polymerase to add nucleotides toward replication fork continuously *5’ to 3’ direction Another DNA polymerase will replace primer nucleotides with complementary sequence DNA Ligase will attach all sequenced nucleotides to complete copied strand

Lagging Strand Lagging strand replicates away in segments. *Segments are called Okazaki fragments Primase adds primer sequence DNA polymerase add nucleotides to segments Another polymerase replaces primer segments DNA Ligase joins fragments together to complete complementary copy

Synthesis of Leading and Lagging Strands

Proofreading and Repair DNA polymerases are responsible for proofreading and correcting errors in the sequences Mismatched pair occurs if nucleotides are damaged or evade proofreading Cells will use special enzymes to repair nucleotides 130 enzymes have been discovered in humans that repair DNA

Repair of DNA Nucleotide Excision repair Enzyme nuclease cuts out damaged segment of DNA DNA polymerase replaces missing nucleotides DNA ligase joins segments together

Telomeres Telomere – nucleotide sequences at the ends of eukaryotic DNA that do not contain genes - typical repetition of 6 base sequence TTAGGG - does not prevent the shortening of DNA molecules from replication - will protect genes from erosion through successive rounds

Telomerase Enzyme that catalyzes the lengthening of telomeres and compensates for shortening that occurs with replication Mainly visible in germ cells May be link to explain why cancer cells are able to stabilize and persist.