DNA as the Genetic Material PDF

Lecture 1 DNA as the Genetic Material?  Learning objectives Appreciation for the experiments that lead to the discovery of DNA Describe the experiments that lead to the discovery of DNA Explain the chemical composition and structure of DNA Who dis...

Lecture 1 DNA as the Genetic Material?  Learning objectives Appreciation for the experiments that lead to the discovery of DNA Describe the experiments that lead to the discovery of DNA Explain the chemical composition and structure of DNA Who discovered DNA? Timeline of events: 1890 Weismann - substance in the cell nuclei controls development. 1900 Chromosomes shown to contain hereditary information, later shown to be composed of protein & nucleic acids. 1928 Griffith’s Transformation Experiment 1944 Avery’s Transformation Experiment 1953 Hershey-Chase Bacteriophage Experiment 1953 Watson & Crick propose double-helix model of DNA 1956 First demonstration that RNA is viral genetic material. Search for the genetic material: 1. Stable source of information 2. Ability to replicate accurately 3. Capable of change (produce variation) 4. Transformation  1928 - Frederick Griffith’s Transformation Experiments Two strains of Streptococcus pneumoniae – S  Smooth Secrete a polysaccharide capsule Produce smooth colonies on solid media Virulent – R  Rough Unable to secrete a capsule Produce colonies with a rough appearance avirulent  Illustration of F. Griffith's discovery of transformation in S. pneumoniae using mice. http://www.visionlearning.com/en/library/Biology/2/DNA-I/149  Griffith’s Conclusions Something from the dead type S transformed type R into type S Called this process transformation The unknown substance was termed the transforming principle  WHAT IS LIFE? ERWIN SCHRODINGER First published 1944 ….suggested that genes must be molecules because like heritable traits they Can be very stable over time Do change from time to time At first protein thought to be genetic material: DNA structure too simple Proteins more complex-this is what is needed of the hereditary material Oswald T. Avery’s Transformation Experiment - 1944 Cell extract from type S cells treated with DNase, RNase, Protease. Only extract containing purified DNA transformed type R to type S Verify that DNA, not RNA or protein, is the genetic material Martha Chase (1930 - 2003) and Alfred Hershey (1908 - 1997) Cold Spring Harbor Laboratory, 1953 Along with Luria & Delbruck, Hershey was a Nobel Laureate in Chemistry (1968) for his work on phage genetics. Chase went on to receive a PhD. in bacterial genetics.  Hershey-Chase Bacteriophage Experiment - 1953 Confirm genetic material is DNA using phage T2 Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery. Bacteriophages are composed of DNA & protein Hershey-Chase Bacteriophage Experiment - 1953 T2 bacteriophage is composed of DNA and proteins: Set-up two experiments: Label DNA with 32 P Label Protein with 35 S Infected E. coli bacteria with two types of labeled T2 P is discovered within the bacteria 32 and progeny phages, whereas 35S is not found within the bacteria but released with phage ghosts. Click this link to watch an animation of the Hershey Chase experiment - https://www.youtube.com/watch?v=hubXqPrslR4 Conclusions about these early experiments: Griffith 1928 : Transformation principle Avery 1944: DNA is the transforming substance/genetic material Hershey-Chase 1953: Confirmed DNA (not RNA/ protein) is the genetic material. Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957: RNA (not protein) is genetic material of some viruses, but no known prokaryotes or eukaryotes use RNA as their genetic material. Structure of DNA  Structure of DNA strandthat make up DNA and RNA Nucleotide = monomers Three components 1. Pentose (5-carbon) sugar DNA = deoxyribose RNA = ribose (compare 2’ carbons) 2. Nitrogenous base Purines Adenine Guanine Pyrimidines Cytosine Thymine (DNA) Uracil (RNA) 3. Phosphate group attached to 5’ carbon Structure of DNA James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information: 1. Base composition studies of Erwin Chargaff indicated double-stranded DNA consists of ~50% purines (A,G) and ~50% pyrimidines (T, C) amount of A = amount of T and amount of G = amount of C (Chargraff’s rules) % AT and GC content varies from organism to organism Examples: %A %T %G %C %GC Homo sapiens 31.0 31.5 19.1 18.4 37.5 Zea mays 25.6 25.3 24.5 24.6 49.1 Drosophila 27.3 27.6 22.5 22.5 45.0 Aythya americana 25.8 25.8 24.2 24.2 48.4 Structure of DNA 2. X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins Conclusion-DNA is a helical structure with distinctive regularities, 0.34 nm & 3.4 nm. Adenine and Guanine Cytosine and Thymine Model building arrives at base pairing (NB Pairing in agreement with Chargaff’s rules) The double helix  Nobel Prize ceremony 1962 Wilkins Perutz Crick Steinbeck Watson Kendrew Rosalind Franklin 1920- 1958  The significance of all this in 2023 This debate raises many issues that are very much relevant to scientists today. How important is multidisciplinary research? How open should we be with our data? And does the Nobel Prize correctly reflect the work behind the scenes? This discovery exemplifies the large group of people with varying expertise that it takes to make such a significant discovery. It also highlights the many years it takes and the fact that everyone’s research is based on an earlier discovery by themselves or someone else. The discovery of the DNA structure was made due to knowledge acquired by chemists, biologists, physicists and others. Multidisciplinary research is just as important today as it was 70 years ago – we still have many questions to ask about human biology and disease which requires knowledge from many areas. The need for multi- disciplinary research and to ensure everyone is acknowledged for their contribution to a piece of research. DNA facts Memories may be passed down through generations in DNA, a process underlying cause of phobias We share a lot of DNA with other animals, plants and microorganisms Species How many genes do we share? Chimpanzee 98% Mouse 92% Zebrafish 76% Fruit Fly 51% Weed 26% Bacteria (E.coli) 18%  70 % https://www.youtube.com/watch?v=rESHv7d7O88 - Strawberry DNA cocktail “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Nature Publication of Watson and Crick 1953