BMS100_PHL1.17_W23_Post-learning 2 - DNA synthesis_TO.pptx

Physiology Concept 1.17 Cell cycle – Post-learning 2: DNA synthesis 2 Dr. Hurnik BMS 100 Week 9 Video Link https://ccnm.ca.panopto.com/Panopto/ Pages/Viewer.aspx?id=dc0e0edd-042e46d3-a430-afbb000a174b Today’s Overview In-class Phases: G1, S, G2, M Checkpoints: Start transition, G2/M, metaphase-...

Physiology Concept 1.17 Cell cycle – Post-learning 2: DNA synthesis 2 Dr. Hurnik BMS 100 Week 9 Video Link https://ccnm.ca.panopto.com/Panopto/ Pages/Viewer.aspx?id=dc0e0edd-042e46d3-a430-afbb000a174b Today’s Overview In-class Phases: G1, S, G2, M Checkpoints: Start transition, G2/M, metaphase-to-anaphase Checkpoint regulation Cell cycle regulation in the presence of growth factors Cell cycle regulation in the presence of unfavourable conditions CKIs, RB, p53 Cell survival Post-learning 1. DNA synthesis 1 2. DNA synthesis 2 3. Mitosis DNA proof-reading mechanisms • Only about 1 mistake occurs for every 1010 nucleotides during DNA replication  Reminder: this is much lower than the mistake rate for transcription (RNA polymerase) DNA polymerase proofreading 1 • DNA polymerase has several proofreading mechanisms:  1. DNA polymerase activity • Takes place just prior to a new nucleotide being covalently added to the growing daughter chain  Correct nucleotide has higher affinity for the DNA polymerase than an incorrect nucleotide • So, energetically, incorrectly paired nucleotides are less favourable and therefore more likely to diffuse away before the DNA polymerase can add them by mistake DNA polymerase proofreading 2 • DNA polymerase has several proofreading mechanisms:  2. Exonucleolytic proofreading • Occurs immediately after an incorrect nucleotide has been covalently added to the growing daughter chain  An incorrectly added nucleotide will not provide an effective 3’-OH end for DNA polymerase to add on the next nucleotide  Separate catalytic site on DNA polymerase will initiate DNA polymerase to move in the 3’  5’ direction, cliping off any unpaired or mispaired residues • Catalytic site: 3’-to-5’ proofreading exonuclease DNA polymerase proofreading 3 • DNA polymerase has several proofreading mechanisms:  3. Strand-directed mismatch repair system • This mechanism is FYI Molecular Biology of the Cell (Alberts et al) 6 th ed. Figure 519. Page 251 Telomeres • Since DNA replication occurs discontinuously on the lagging strand we end up with a shorter DNA fragment on the daughter strand once the RNA primer has been removed  Without a mechanism to deal with this problem, DNA would be lost from the end of all chromosomes each time it divides Molecular Biology of the Cell (Alberts et al) 6 th ed. Figure 534. Page 263 Telomeres - GGGTTA • Eukaryotes have specialized nucleotide sequences at the end of their chromosomes called telomeres  Many tandem repeats of GGGTTA (in humans) • FYI – 1 telomere = ~1000 GGGTTA repeats Telomere & telomerase • Telomere DNA sequences are recognized by telomerase  Can replenish these sequences each time a cell divides  The activity of telomerase varies based on the cell type • Some cells (eg. Stem cells) have full telomerase activity • Most cells have low/ minimal telomerase activity  telomeres gradually shorten until a descendant cell inherits chromosome that lack telomere function • Initiates a response causing them to withdraw permanently from the cell cycle and cease dividing • P53 and P16 are involved in withdrawing cell from cell cycle • Called replicative cell senescence Telomerase activity • Telomerase will recognize the tip of an existing telomere DNA repeat on the parent strand and elongate it in the 5’ to 3’ direction  It uses a intrinsic RNA template as a template Molecular Biology of the Cell (Alberts et al) 6 th ed. Figure 534. Page 263 Telomerase activity • Then replication of the daughter strand can be complete by the conventional DNA polymerase  The extended telomere will be used as a template for synthesis of the daughter strand Molecular Biology of the Cell (Alberts et al) 6 th ed. Figure 534. Page 263 References • Alberts et al. Molecular Biology of the Cell. Garland Science. • Betts et al. Anatomy and Physiology (2ed). OpenStax • Pathologic Basis of Disease(Robbins and Cotran) 10th ed.