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Molecular Basis of Inheritance

Explore the molecular basis of DNA replication in cells, focusing on key concepts like double helix structure, replication origins, enzyme catalysis, and error correction mechanisms. Investigate telomere maintenance and its role in cell longevity. Witness the fascinating world of genetic information duplication.

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Molecular Basis of Inheritance

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  1. Molecular Basis of Inheritance AP Biology Crosby High School

  2. Frederick Griffith (1928) • Streptococcus pneumoniae • Lethal smooth strain vs. Harmless rough

  3. Hershey – Chase Experiment • T2 reprogrammed E. coli • Alfred Hershey and Martha Chase (1952)

  4. Additional Support for DNA • Eu. Cells copy DNA exactly before Mitosis • Diploid cells exactly twice as much DNA as Haploid gametes • Chargaff’s rule • %A = %T • %G = %C

  5. The Players

  6. Watson and Crick • James Watson visited Maurice Wilkins at Cambridge • Noticed Rosalind Franklin’s X-ray image of DNA • Took it to Francis Crick who recognized a helix structure

  7. Double – Helix Calculations • Width of helix suggested two strands • One complete turn every 3.4 nm • Base pairs .34 nm apart • Purines must be paired specifically with Pyrimidines • Supported Chargaff’s rule

  8. The Structure

  9. DNA Replication • Proposed types • Conservative: Original DNA remains intact • Semi-conservative: Half original and half daughter • Dispersive: All four strands have old and new • 6 billion base pairs copied • Takes only a few hours • 1 mistake out of 1 billion nucleotides

  10. Proposed Replications

  11. Origin of Replication • Recognize specific portion to open molecule • Begins copying in both directions • Number of Origins • Prokaryotic: 1 origin of replication • Eukaryotic: May have hundreds or thousands • Replication fork • Y-shaped region of origin of replication

  12. Elongation of New DNA Strands • Catalyzed by DNA Polymerases • Bacteria: 500 nucleotides / sec • Humans: 50 nucleotides / sec • Energy supplied by nucleoside triphosphates • ATP, GTP, CTP, TTP • Nucleotide and phosphate join strand • Pyrophosphate releases energy through hydrolysis

  13. Elongation

  14. Antiparallel DNA Strands

  15. Leading and Lagging Strand • Only elongates in 5’  3’ Direction • Lagging strand contains Okazaki fragments (100-200) • DNA Ligase joins fragments

  16. Primase Begins Replication • Primase: joins RNA together to form a primer • DNA Polymerase adds to the primer • Another polymerase replaces the RNA primer with DNA • Leading strand needs only one primer • Lagging strand needs one primer for each Okazaki fragment

  17. DNA Proofreading • DNA polymerase checks for errors and corrects them as it elongates • Mismatch Repair: fixes incorrectly paired nucleotides • Nucleotide Excision Repair • Thymine – Thymine Dimers

  18. Excision Repair

  19. Telomeres • Exist at the ends of DNA • Multiple repetitions of TTAGGG • Between 100 – 1,000 repeats • Telomerase replaces telomeres in germ cells • If the repeats run out then the cells die

  20. Telomerase

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