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Download ch. 16 Molecular Basis of Inheritance
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Transcript
Molecular Basis of Inheritance •DNA Structure •DNA Replication Evidence that DNA is the Hereditary Material of Life? • Griffith-Avery Experiment—DNA can transform bacteria • Hershey-Chase Experiment—Viral DNA can program cells • Chargaff—Analysis of DNA composition Griffith-Avery Experiment: Transformation of Bacteria Controls Hershey-Chase Experiment: DNA, the Hereditary Material in Viruses Chargaff:The Composition of DNA • Base composition of DNA varies between species • Regularity in ratios of nucleotide bases Adenine = 30.9% Thymine = 29.4% Guanine = 19.9% Cytosine = 19.8% • Chargaff’s Rules: A = T and G = C Scientists in the Race for the Double Helix • Linus Pauling • James Watson • Francis Crick • Rosalind Franklin Structure of DNA Purines Adenine Pyrimidines Thymine Guanine Cytosine Structure of DNA • DNA is a double helix with a uniform width • Purine and pyrimidine bases stacked • Purine + Pyrimidine: width consistent with X-ray data, • base ratios consistent with Chargaff’s rules: A = T and G C Structure of DNA is related to 2 primary functions: 1. Copy itself exactly for new cells that are created 2. Store and use information to direct cell activities Complementary (Anti-Parallel) Strands of DNA • If one strand is known, the other strand can be determined 3’ A = T 5’ C G G T A T C C 5’ G C C =A =T =A G G 3’ DNA Replication: Semi-Conservative Model • 2 strands of the parental DNA separate, and each functions as a template for the synthesis of a new complementary strand DNA Replication • This replication process assures that daughter cells will carry the same genetic information as each other and as the parent cell. Each daughter DNA has one old strand of DNA and one new strand of DNA DNA Replication • Replication occurs simultaneously at many sites (replication bubbles) on a double helix Allows DNA replication to occur in a shorter period of time Elongating a New Strand • DNA polymerases can only attach nucleotides to the 3’ end of a growing daughter strand • Thus, replication always proceeds in the 5’ to 3’ direction Priming DNA Synthesis • Primase adds RNA primer to strand • DNA polymerase adds DNA nucleotides to strand • Another DNA polymerase replaces RNA with DNA DNA Replication • Within the replication bubbles, one daughter strand is made continuously (leading strand) while the other daughter strand must be made in short pieces (lagging strand) which are then joined together by DNA ligase These short pieces of DNA are called Okazaki fragments Overall Direction of Replication-5’ to 3’ DNA Replication Checking for Errors • 1/1,000,000,000 chance of an error in DNA replication – Can lead to mutations • DNA polymerases have a “proofreading” role – Can only add nucleotide to a growing strand if the previous nucleotide is correctly paired to its complementary base • If mistake happens, DNA polymerase backtracks, removes the incorrect nucleotide, and replaces it with the correct base Mismatch Repair • DNA polymerase proofreads each nucleotide and catches mistakes • The polymerase removes mistake and replaces with correct nucleotide Excision Repair • Nuclease • Used by skin cells when repairing genetic damage caused by UV rays of sunlight End-Replication Problems End-Replication Problems Telomerase— enzyme lengthens telomeres (ends of nucleotide sequences)