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Chapter 16 – The Molecular Basis of Inheritance - Coggle Diagram
Chapter 16 – The Molecular Basis of Inheritance
DNA as the Genetic Material
Griffith’s Transformation Experiment (1928)
Harmless bacteria transformed into pathogenic
Conclusion: "transforming principle" passed heritable material
Avery, McCarty & MacLeod (1944)
Identified DNA as the transforming substance
Hershey-Chase Experiment (1952)
Used radioactive sulfur (protein) and phosphorus (DNA) in phages
DNA entered bacterial cells → proved DNA carries genetic info
Structure of DNA
Chargaff’s Rules
DNA composition varies by species
In any species: A = T and G = C
Watson, Crick, Franklin, Wilkins
Rosalind Franklin used X-ray crystallography to image DNA
Watson & Crick built double helix model (1953)
Double Helix Model
Sugar-phosphate backbone (antiparallel strands)
Nitrogenous bases: A–T, C–G (complementary pairing)
Held together by hydrogen bonds
One full turn every 10 base pairs
DNA Replication: Basic Concept
Semiconservative Model
Each daughter molecule has 1 old and 1 new strand
Supported by Meselson-Stahl Experiment
DNA Replication: Detailed Process
Origin of Replication
Eukaryotes: multiple origins
Prokaryotes: single origin
DNA opens at replication bubble
Enzymes Involved
Helicase: unwinds DNA
Single-strand binding proteins: stabilize open strands
Topoisomerase: relieves tension ahead of fork
Primase: synthesizes RNA primer
DNA polymerase III: adds DNA nucleotides
DNA polymerase I: replaces RNA primer with DNA
DNA ligase: joins Okazaki fragments
Leading vs Lagging Strand
Leading strand: synthesized continuously toward fork
Lagging strand: synthesized in fragments (Okazaki fragments) away from fork
Needs multiple primers
Accuracy and Repair
Proofreading
DNA polymerases check and correct errors
Mismatch Repair
Enzymes fix incorrectly paired bases post-replication
Nucleotide Excision Repair
Nuclease cuts out damaged DNA
DNA polymerase fills in
Ligase seals
Defects
Mutations in repair genes → cancer risk (e.g., XP: xeroderma pigmentosum)
DNA Packing in Chromosomes
Levels of Organization
DNA double helix
Nucleosome – DNA wrapped around histone proteins
30-nm fiber – nucleosomes coiled
Looped domains
Chromatin – further folded
Chromosome – most compact form during mitosis
Types of Chromatin
Euchromatin: loosely packed, actively transcribed
Heterochromatin: tightly packed, transcriptionally inactive
Telomeres and Replication Limits
DNA polymerase can’t complete 5′ ends → shortening with each replication
Telomeres: repetitive sequences at chromosome ends
Protect genes, act as buffers
Telomerase: enzyme that extends telomeres in germ cells
Active in cancer cells too