Chapter 16 Molecular Basis of Inheritance

Deciphering DNA

The Search for Genetic Material Known Genes on chromosomes Chromosomes made of DNA and protein Unknown Which chromosomal component was the genetic material Protein Heterogeneous class of macromolecules with specific functions Case stronger initially Nucleic acids Physical and chemical properties too uniform for amount of variation Experimentation gradually changed perceptions DNA’s role clarified by studying bacteria and their viruses

Frederick Griffith o Streptococcus pneumoniae model o S encapsulated and virulent; R non- encapsulated and non- virulent o Heat killed S cells mixed with R cells created S cells o Concluded that S cells have a chemical component that can transform other cells

Oswald Avery Identified the transforming substance from Griffith’s work as DNA Focused on DNA, RNA, and protein Extract components from pathogenic bacteria Each individually inactivated and tested for transformation ability Degradation of DNA only substance to prevent Not uniformly supported Proteins better candidates Doubted bacterial DNA similar to that of complex organisms Little still known about DNA

Alfred Hershey and Martha Chase o Tracked protein and DNA of E. coli phage T2 o Bacteriophage is a virus that infects bacteria o Radioactive isotopes to label cells o Determined that DNA entered bacteria and directed virus reproduction not protein

Existing Knowledge of DNA Polymer of nucleotides with 3 components Pentose sugar (deoxyribose) and a phosphate group Purines = two rings Adenine (A) Guanine (G) Pyrimidines = one ring Thymine (T) Cytosine (C)

Erwin Chargaff The amount of A, T, G, and C in the DNA vary from species to species Evidence of molecular diversity to increase DNA credibility Chargaff’s rules In each species, the amount of A = T while the amount of C = G Importance unknown until discovery of double helix OrganismATCG Human30.3% 19.9%19.5% Chicken28.8%29.2%20.5%21.5% Grasshopper29.3% 20.5%20.7% Sea Urchin32.8%32.1%17.7%17.3% Wheat27.3%27.1%22.7%22.8% Yeast31.3%32.9%18.7%17.1% E. coli24.7%23.6%26.0%25.7%

Rosalind Franklin X-ray diffraction image of DNA DNA is helical in structure Uniform in width and spacing between bases Suggested that there were 2 strands = double helix Concluded that sugar- phosphate backbones were on the outside Evidence was groundwork for Watson and Crick

James Watson and Francis Crick Double helix with anti-parallel strands Sugar-phosphate backbone on outside Paired nitrogenous bases on inside Complimentary hydrogen binding of a purine and a pyrimidine A with T form 2 bonds, G with C form 3 bonds Consistent with Chargoff and Franklin Awarded the Nobel Prize

DNA Double-Helix Structure

DNA Replication

Each strand of original DNA serves as a template Nucleotides match to template according to base pairing rules 1 ‘parent’ DNA strand produces 2 new ‘daughter’ strands

A) Two parent strands eventually come back together B) Watson and Crick: each daughter strand with 1 old parent strand C) All four strands have a mixture of new and old DNA Matthew Meselson and Franklin Stahl’s work confirmed the semi- conservative model DNA Replication Models

Replication Efficiency E. coli with 4.6 million nucleotide pairs replicates in less than an hour Humans with 6 billion pairs a few hours, with only about 1 error every 10 billion nucleotides Enzymes and proteins are responsible Better understood in prokaryotes than eukaryotes Process is fundamentally similar

Origins of Replication Short specific nucleotides sequences Prokaryotes with 1, eukaryotes with multiple Proteins recognize and attach Separates strands and opens them up to form a replication bubble Proceeds in both directions until fully copied

Overall DNA Replication COMPONENTS (Table 16.1) Helicase Single-strand binding protein Topoisomerase Primer and primase DNA pol III and I Leading and lagging strands Okazaki fragments DNA ligase KEY POINTS: DNA pol binds to 3’ end Strands grow 5’ 3’ only

Proofreading and Repairing DNA DNA polymerases also proofread each nucleotide Incorrect pairs are removed Mismatch pairs result from those that evaded the polymerases Alternate enzymes remove and replace Nucleases cut out damaged DNA Polymerases and ligases fill gap with nucleotides Skin cell repair from UV light damage