Presentation on theme: "The Search for the Genetic Material of Life"— Presentation transcript:
1 The Search for the Genetic Material of Life What is a gene?Stable source of informationAbility to replicate accuratelyCapable of change
2 The Search for the Molecular Basis of Heredity Search for genetic material---nucleic acid or protein/DNA or RNA?Griffith’s Transformation ExperimentAvery’s Transformation ExperimentHershey-Chase Bacteriophage ExperimentTobacco Mosaic Virus (TMV) ExperimentNucleotides - composition and structureDouble-helix model of DNA - Watson & CrickOriginal Source for portions of slide content: by Kevin McCracken University of Alaska Fairbanks.
3 Timeline of events1890 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 Experiment1944 Avery’s Transformation Experiment1953 Hershey-Chase Bacteriophage Experiment1953 Watson & Crick propose double-helix model of DNA1956 Gierer & Schramm/Fraenkel-Conrat & Singer Demonstrate RNA is viral genetic material.
4 Frederick Griffith’s Transformation Experiment - 1928 “transforming principle” demonstrated with Streptococcus pneumoniaeGriffith hypothesized that the transforming agent was a “IIIS” protein.
6 Hershey-Chase Bacteriophage Experiment - 1953 Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery.Structure of T2 phageDNA & protein
8 Hershey-Chase Bacteriophage Experiment - 1953 T2 bacteriophage is composed of DNA and proteins:Set-up two replicates:Label DNA with 32PLabel Protein with 35S3. Infected E. coli bacteria with two types of labeled T24. 32P is discovered within the bacteria and progeny phages, whereas 35S is not found within the bacteria but released with phage ghosts.1969: Alfred Hershey
9 Gierer & Schramm Tobacco Mosaic Virus (TMV) Experiment – 1956 & Fraenkel-Conrat & Singer - 1957 Used 2 viral strains to demonstrate RNA is the genetic material of TMV
10 Conclusions about these early experiments: Griffith 1928 & Avery 1944:DNA (not RNA) is transforming agent.Hershey-Chase 1953:DNA (not protein) is the genetic material.Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957:RNA (not protein) is genetic material of some viruses.
11 Nucleotide = monomers that make up DNA and RNA (Figs. 2.9-10) Three components1. Pentose (5-carbon) sugarDNA = deoxyriboseRNA = ribose(compare 2’ carbons)2. Nitrogenous basePurinesAdenineGuaninePyrimidinesCytosineThymine (DNA)Uracil (RNA)3. Phosphate group attached to 5’ carbon
12 Nucleotides are linked by phosphodiester bonds to form polynucleotides. Covalent bond between the phosphate group (attached to 5’ carbon) of one nucleotide and the 3’ carbon of the sugar of another nucleotide.This bond is very strong, and for this reason DNA is remarkably stable. DNA can be boiled and even autoclaved without degrading!5’ and 3’The ends of the DNA or RNA chain are not the same. One end of the chain has a 5’ carbon and the other end has a 3’ carbon.
14 James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNATwo sources of information:Base composition studies of Erwin Chargaffindicated 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)%GC content varies from organism to organismExamples: %A %T %G %C %GCHomo sapiensZea maysDrosophilaAythya americana
15 James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNA Two sources of information:X-ray diffraction studies - Rosalind Franklin & Maurice WilkinsConclusion-DNA is a helical structure withdistinctive regularities, 0.34 nm & 3.4 nm.
16 Double Helix Model of DNA: Six main features Two polynucleotide chains wound in a right-handed (clockwise) double-helix.Nucleotide chains are anti-parallel: 5’ 3’3’ 5’Sugar-phosphate backbones are on the outside of the double helix, and the bases are oriented towards the central axis.Complementary base pairs from opposite strands are bound together by weak hydrogen bonds.A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds).e.g., 5’-TATTCCGA-3’3’-ATAAGGCT-3’Base pairs are 0.34 nm apart. One complete turn of the helix requires 3.4 nm (10 bases/turn).Sugar-phosphate backbones are not equally-spaced, resulting in major and minor grooves.
19 James D. Watson Francis H. Crick Maurice H. F. Wilkins What about? 1962: Nobel Prize in Physiology and MedicineJames D.WatsonFrancis H.CrickMaurice H. F.WilkinsWhat about?Rosalind Franklin
20 RNA (A pairs with U and C pairs with G) Examples: mRNA messenger RNA tRNA transfer RNArRNA ribosomal RNAsnRNA small nuclear RNARNA secondary structure:single-strandedFunction in transcription(RNA processing) and translationYeast Alanine tRNA
21 Organization of DNA/RNA in chromosomes Genome = chromosome or set of chromosomes that contains all theDNA an organism (or organelle) possessesProkaryotic chromosomes1. most contain one double-stranded circularDNA molecule2. typically arranged in arranged in a denseclump in a region called the nucleoidEukaryotic chromosomes1. Eukaryotic chromosome structureChromatin - complex of DNA and chomosomal proteins ~ twice as much or more protein as DNA.2. Eukaryotic chromosomes or chromatin found in the nucleus of the cell.3. Cells from different species contain varying numbers of chromosome of different sizes and morphologies -the karyotype (e.g., pea, 2N = ; human, 2N = 46, fruit fly, 2N= 8).