Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
DNA sequencing wikipedia , lookup
Zinc finger nuclease wikipedia , lookup
DNA repair protein XRCC4 wikipedia , lookup
Homologous recombination wikipedia , lookup
DNA replication wikipedia , lookup
DNA profiling wikipedia , lookup
DNA polymerase wikipedia , lookup
DNA nanotechnology wikipedia , lookup
Microsatellite wikipedia , lookup
MOLECULAR BIOLOGY OF INHERITANCE: PART I: HISTORICAL PERSPECTIVES Chapter 16 Learning Objectives 1. Summarize the experiments and conclusions conducted by Griffiths; Avery, McCarty & MacLeod; Hershey & Chase. 2. Summarize the experimental evidence of Franklin, Chargaff that were used by Watson & Crick to propose the structural model of DNA. 3. Describe the chemical and structural features of DNA. Concept 16.1: DNA is the genetic material. Historical Context. • DNA was first isolated from cells in 1863. • Discovery of chromosomes: • The movement of chromosomes during cell division was discovered in 1883. • the presence of chromosomes in reproductive cells was discovered in 1902. • chromosomes were the physical basis of inheritance was discovered in 1903. • The experimental evidence that genes are located was published in 1903. Hereditary Molecule: DNA or Protein? • The monomers of DNA were discovered in 1910 by Phoebus Levine.* • He thought that DNA was structural component of chromosomes and the genes were thought likely to be made of the protein component of chromosomes. (* Phoebus Levene. (2015, May 20). In Wikipedia, The Free Encyclopedia. Retrieved 19:17, October Phoebus Levene 15, 2015, from https://en.wikipedia.org/w/index.php?title=Phoebu Image: Wikimedia Commons s_Levene&oldid=663259196) Experiments by Frederick Griffith • The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928. • Griffith was a bacteriologist who provided the first experimental evidence of genetic transformation, changing the form and function of a bacterium. Frederick Griffith. Image Wikipedia Commons Griffith’s Transformation Experiment • He observed that mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form. • He called this phenomenon transformation, now defined as a change in genotype and phenotype. • Griffith’s referred to a “transforming principle” as causing the change, but didn’t know what it was. Figure 16.2 Experiments by Avery, MacLeod & McCarty • In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod repeated Griffiths’ experiments and announced that the transforming substance was DNA. • Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria. • Many biologists remained skeptical, mainly because little was known about DNA. Experiments by Hershey & Chase • More evidence for DNA as the genetic material came from studies of viruses that infect bacteria, called bacteriophages (or phages. • In 1952, Alfred Hershey and Martha Chase research with T2 phages proved that DNA was Griffith’s transforming principle. Alfred Hershey Image: Wikipedia Commons Martha Chase Image: Wikipedia Commons T2 Phage: DNA & Protein The T2 phage, consisting almost entirely of DNA and protein, attacks E. coli transforming the cells into a T2-producing factory that releases phages when the cell ruptures. Figure 16.3 DNA is transforming principle. • DNA contains phosphorus, whereas protein does not. • DNA was labeled with a radioactive phosphorus; protein was labeled with a radioactive sulfur. • The fraction containing the radioactive phosphorus—DNA—was the only component that could transform E. coli into phage producing factories. • Their results confirmed Avery, MacLeod& McCarty’s results and confirmed that the DNA of the phage was Griffith’s transforming principle. Summary of Hershey & Chase Experiment Figure 16.4 Erwin Chargaff’s Rules • By 1947, Erwin Chargaff had developed a series of rules based on a survey of DNA composition in organisms. • Chargaff found a peculiar regularity in the ratios of nucleotide bases, known as Chargaff’s rules, in all organisms he studied. • The number of A is approximately equal to the number of T and the number of G is approximately equal to the number of C; but A + T did not equal C + G. Erwin Chargaff Image original source: unknown Determining the structure of DNA • After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity. • Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure. Maurice Wilkins. Image: Cold Spring Harbor Archives Figure 16.6 Franklin’s X-ray Diffraction of DNA • X-rays are diffracted as they pass through crystalized DNA. • The diffraction pattern can be used to deduce the threedimensional shape of molecules. • An unpublished annual report summarizing Franklin’s work, she had concluded that the sugar-phosphate backbones were on the outside of the double helix. Figure 16.6 DNA Structural Features • Watson & Crick built models based on X-rays and chemistry of DNA. • Only a pyrimidine-purine pair produces the 2-nm diameter indicated by the X-ray data. • Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions). • The ladder forms a full turn of the helix every ten base pairs, or every 3.4 nm. DNA Structural Features, cont. • Watson & Crick determined that chemical side groups of the nitrogenous bases form hydrogen bonds, connecting the two strands. • Based on details of their structure, adenine forms two hydrogen bonds only with thymine, and guanine forms three hydrogen bonds only with cytosine. • This finding explained Chargaff’s rules. Figure 16.1. James Watson and Francis Crick with their DNA model at the Cavendish Laboratories in 1953. Who won Nobel Prizes? • Griffith—no. • Avery—nominated, but never won; MacLeod--no, & McCarty—no. • Alfred Hershey—yes, but for previous work. • Martha Chase—no. • Erwin Chargaff-no, and he was not happy about it. • Watson—yes. • Crick-yes. • Wilkins—yes—but who remembers? • Franklin—no (died before award given).