DNA: The Genetic Material Teacher: Maher Zeidan Section 1: DNA: The Genetic Material The discovery that DNA is the genetic code involved many experiments. Essential Questions • • • Which experiments led to the discovery of DNA as the genetic material? What is the basic structure of DNA? What is the basic structure of eukaryotic chromosomes? Copyright © McGraw-Hill Education DNA: The Genetic Material Vocabulary Review • nucleic acid Copyright © McGraw-Hill Education New • • double helix nucleosome DNA: The Genetic Material Discovery of the Genetic Material • After the rediscovery of Mendel’s work, scientist began to look for the molecule involved in inheritance. • For many years, scientists struggled to determine if DNA or protein was the source of genetic information. Copyright © McGraw-Hill Education DNA: The Genetic Material Discovery of the Genetic Material Griffith • First major experiment searching for the genetic material • Involved transformation between two forms of S. pneumoniae • Set the stage for the search to identify the transforming substance Copyright © McGraw-Hill Education DNA: The Genetic Material Griffith's experiment Discovery of the Genetic Material Avery • Identified the molecule that transformed the R strain of S. pneumoniae into the S strain • Concluded that when the S cells were killed, DNA was released • R bacteria incorporated this DNA into their cells and changed into S cells Copyright © McGraw-Hill Education DNA: The Genetic Material Discovery of the Genetic Material Hershey and Chase • Used radioactive labeling to trace bacteriophage DNA and protein. • Concluded that the bacteriophage DNA was injected into the cell and provided the genetic information needed to produce new viruses. Copyright © McGraw-Hill Education DNA: The Genetic Material Summary of Hershey-Chase Results Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure Nucleotides • Nucleotides are the subunits of nucleic acids, and consist of • 5-carbon sugar • Phosphate group • Nitrogenous base Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure Chargaff • Analyzed the amount of A, G, T, and C in the DNA of various species • Chargaff’s rule: C = G and T = A Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure The structure question • Search for the structure of DNA was lead by four scientists: • Rosalind Franklin, British chemist • Maurice Wilkins, British physicist • Francis Crick, British physicist • James Watson, American biologist Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure X-ray diffraction • X-ray diffraction techniques indicated that DNA was a double helix, or twisted ladder shape. • The double helix shape was formed by two strands of nucleotides twisted around each other. Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure Watson and Crick • Using Franklin and Chargaff’s data, Watson and Crick measured the width of the helix and the spacing of the bases. • They built a model that conformed the Franklin and Chargaff’s data. Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure DNA structure • DNA often is compared to a twisted ladder. • Rails of the ladder are represented by the alternating deoxyribose and phosphate. • The pairs of bases (cytosineguanine or thymine-adenine) form the steps. • Purine base always binds to a pyrimidine base Copyright © McGraw-Hill Education DNA: The Genetic Material DNA Structure Orientation • DNA molecules have specific orientations of the two strands • On the top rail, the strand is said to be oriented 5’ to 3’. • The strand on the bottom runs in the opposite direction and is oriented 3’ to 5’. Copyright © McGraw-Hill Education DNA: The Genetic Material Structure of DNA Copyright © McGraw-Hill Education DNA: The Genetic Material Chromosome Structure • In prokaryotes, DNA molecules are contained in the cytoplasm, and consist mainly of a ring of DNA and associated proteins. • In eukaryotes, DNA is organized into individual chromosomes. Copyright © McGraw-Hill Education DNA: The Genetic Material DNA in Prokaryotes & Eukaryotes Chromosome Structure • To fit into a cell, DNA coils around a group of beadlike proteins called histones. • DNA + histones form a nucleosome, which group together into chromatin fibers, which supercoil to form a chromosome. Copyright © McGraw-Hill Education DNA: The Genetic Material Review Essential Questions • • • Which experiments led to the discovery of DNA as the genetic material? What is the basic structure of DNA? What is the basic structure of eukaryotic chromosomes? Vocabulary • • double helix nucleosome Copyright © McGraw-Hill Education DNA: The Genetic Material
