DNA: The Genetic Material Biology Ch. 9 Ms. Haut
Identifying the Genetic Material Mendel’s experiments—inherit chromosomes that contain genes The Question now: What are genes made of? Scientists searching for the answer: Griffith and Avery Hershey and Chase
Introduction Basics of molecular biology began with the study of bacteriophages (viruses that infect bacteria) DNA—deoxyribonucleic acid and RNA—ribonucleic acids were identified through various experiments Polymers of nucleotides Actual structure of these molecules not discovered until 1953
Griffith-Avery Experiment: Transformation of Bacteria 2 strains of Streptococcus pneumonia --S strain—pathogenic (disease-causing) --R strain—non-pathogenic -Griffith found that if he mixed heat-killed S-strain bacteria with the harmless strain, some of the living cells were converted to the pathogenic form (TRANSFORMATION—assimilation of external DNA) -Avery purified various chemicals from the heat-killed pathogenic bacteria, only DNA worked to transform the cells -Discovery met with criticism—most still believe protein is the hereditary material Controls
Hershey-Chase Experiment: DNA, the Hereditary Material in Viruses Bacteriophages—viruses that infect bacteria --attach to host cell and inject their DNA into the cell --experiment demonstrated that it was DNA, not protein, that functioned as the phage’s genetic material Radioactively-labeled (sulfur) viral proteins remained outside the host cell during infection Radioactively-labeled (phosphorus) viral DNA traveled inside the host cell -the injected DNA molecules cause the cells to to produce new viral DNA and proteins
Nucleotides of DNA Nucleotides are the monomeric units that make up DNA 3 main parts: 5 carbon sugar—deoxyribose Phosphate group Nitrogenous base
Nitrogenous bases Pyrimidines: single-ring structures Thymine (T) Cytosine (C) Purines: larger, double-ring structures Adenine (A) Guanine (G)
Discovery of the Double Helix 1953—James Watson and Francis Crick determined the structure of the DNA molecule to be a double helix 2 strands of nucleotides twisted around each other
Rosalind Franklin contributed to this discovery by producing an X-ray crystallographic picture of DNA Determined helix was a uniform diameter and composed of 2 strands of stacked nucleotides
Sugar-Phosphate backbones of DNA oriented in opposite direction Base-Pairing Rule: a purine forms a hydrogen bond with a pyrimidine adenine=thymine guaninecytosine
Structure of DNA is related to 2 primary functions: 1. Copy itself exactly for new cells that are created 2. Store and use information to direct cell activities
Complementary Strands of DNA If one strand is known, the other strand can be determined 3’ = T 5’ A C G T G C C = A = T = A G G 3’ 5’
DNA Replication Replication occurs simultaneously at many sites (replication bubbles) on a double helix Allows DNA replication to occur in a shorter period of time
DNA Replication DNA helicases unwind the double helix DNA polymerases can only attach nucleotides to the 3’ end of a growing daughter strand Thus, replication always proceeds in the 5’ to 3’ direction
DNA Replication Within the replication bubbles, one daughter strand is made continuously while the other daughter strand must be made in short pieces which are then joined together by DNA ligase These short pieces of DNA are called Okazaki fragments Overall Direction of Replication-5’ to 3’
DNA Replication This replication process assures that daughter cells will carry the same genetic information as each other and as the parent cell. Each daughter DNA has one old strand of DNA and one new strand of DNA
DNA Replication
Checking for Errors 1/1,000,000,000 chance of an error in DNA replication Can lead to mutations DNA polymerases have a “proofreading” role Can only add nucleotide to a growing strand if the previous nucleotide is correctly paired to its complementary base If mistake happens, DNA polymerase backtracks, removes the incorrect nucleotide, and replaces it with the correct base