DNA Replication Section 14.1-3 Learning Goals To determine how prokaryotic and eukaryotic cells copy their DNA messages To discuss the types of DNA errors and damage that can occur Major Challenges of DNA Replication Major Challenges Explained Replication fork or bubble Location of DNA synthesis Replication bubble forms in the chromosome As the replication fork slides and copies DNA, the DNA builds up torsional strain Strain is released by topoisomerases What is a “replisome”? The cellular machinery that is responsible for synthesizing new DNA Comprised of several enzymes and proteins that unwind the double helix, synthesize DNA, and proofread the newly synthesized strand DNA Synthesis Runs in opposite direction (5'  3' direction) As DNA unwinds to the replication fork Leading strand – copied continuously as double helix unwinds Lagging strand - synthesized in short segments (Okazaki fragments) in the reverse direction then joined together Major Challenges of DNA Replication Another Major Challenge = Time Proofreading Method of checking newly synthesized strand for errors and fixing those errors DNA Replication in Prokaryotes vs Eukaryotes Hint: How do the Replisomes compare? Nucleosomes, Histones, and Eukaryotic Replication Nucleosomes – discoidal octamers of histone proteins wrapped by DNA Complexes must be removed to replicate DNA Nascent DNA helices must be reorganized into new nucleosomes DNA is organized in chromosomes and the enzymes that remodel chromatin by modifying histones Telomeres Ends of the linear chromosomes found in eukaryotes DNA sequence consists of numerous repeats of a single sequence (TTAGGG) Allow DNA replication to occur without loss of DNA at the telomere protect the chromosome from the cell’s own DNA repair machinery Reproduced and maintained by telomerases Action of Telomerase DNA Repair Sections 14.4-5 Learning Goals Why are DNA repair mechanisms necessary? Class activity! Mismatch Repair Two theories: mismatch repair is recognized by Methylation (hemimethylated guanine) Nicks (marker of nascent strand) Base Excision Repair DNA glycosylase – excises base from DNA helix AP endonuclease – recognizes abasic site and nicks DNA at the AP site Nucleotide Excision Repair Mechanism of Photolyase Uses the energy of a photon of light to hydrolyze thymine dimers generated through exposure to UV light Direct repair Nucleotides involved remain in place Non-homologous end joining Takes two ends of chromosomes and joins the phosphodiester backbones together Homologous Recombination Trading or swapping of different pieces of chromosomes Generates two daughter chromosomes that have components of both parent chromosomes but are not complete copies of either Increases genetic diversity Prokaryotic Recombination Proceeds through a double-strand break repair process Process has two parts: Branch migration Resolution Reciprocal breaks form Holliday junctions Eukaryotic Recombination Several models proposed Two best characterized are: double-strand break repair (DSBR) mechanism synthesis-dependent strand-annealing (SDSA) path