CHAPTER 7 DNA Structure and Replication Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 7 1 DNA Is the Hereditary Molecule of Life Hereditary molecule any molecular substance that carries and conveys genetic information Before DNA was known to be the hereditary molecule five essential characteristics were identified 1 Localized to the nucleus component of chromosomes Present in stable form in cells 2 3 Sufficiently complex to contain information needed for structure development and reproduction 4 Able to accurately replicate itself so that daughter cells contain the same information as the parent cell 5 Mutable undergoing a low mutations rate that introduces genetic variation and serves as a foundation for evolutionary change Copyright 2012 Pearson Education Inc 2 Genetics Analysis An Integrated Approach Chromosomes Contain DNA DNA was first noticed in 1869 when Friedrich Meischer isolated it from nuclei of white blood cells It was weakly acidic and phosphorous rich He called it nuclein In the 1870s microscopic studies identified fusion of male and female nuclei during reproduction and chromosomes were observed soon after Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 3 Early Suggestion That DNA Was the Hereditary Material In 1895 Edmund Wilson first suggested that DNA might be hereditary material He observed that sperm and eggs contribute the same number of chromosomes during reproduction He made a connection between the substance observed by Meischer and chromatin of chromosomes In 1900 Mendel s hereditary principles were rediscovered In 1903 Sutton and Boveri independently described the parallels between chromosome portioning Copyright 2012 Pearson Education Inc 4 Genetics Analysis An Integrated Approach Focus on the Nucleus and Chromosomes By 1920 DNA was identified as the principle component of nuclein The basic chemistry of DNA was deciphered It is a polynucleotide consisting of four repeating subunits A T C G bound by covalent bonds In 1923 DNA was localized to chromosomes and made a candidate for the hereditary material However both proteins and RNA are also found in chromosomes candidates Genetics Analysis An Integrated Approach Lipids and carbohydrates were also considered to be Copyright 2012 Pearson Education Inc 5 The Transformation Factor Frederick Griffith identified two strains of Pneumococcus S which caused fatal pneumonia in mice and R which did not A single nucleotide change can convert the R rough strain into the S smooth strain These strains occur in four antigenic types I II III and IV that cannot be altered by mutation alone smooth rough Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 6 Griffith s Experimental Results Mice infected with strain SIII developed pneumonia and Mice infected with the strain RII or with heat killed strain died SIII survived Mice infected with heat killed strain SIII and live strain RII developed pneumonia and died live type SIII bacteria were recovered from the mice Griffiths had described the process of transformation Biochemical tests of the heat killed SIII extract showed that it contained mainly DNA with small amounts of RNA proteins lipids and polysaccharides Further tests required to required to identify transforming material Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 7 Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 8 Experimental Results Avery MacLeod and McCarty used heat killed SIII bacteria and live RII bacteria and infected mice The extract of heat killed SIII bacteria was divided into aliquots and treated to destroy either DNA RNA proteins or lipids and polysaccharides All aliquots killed the mice except the one with the DNA destroyed Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 9 Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 10 DNA Is the Hereditary Molecule Hershey Chase 1952 showed that DNA is responsible for bacteriophage infection of bacteria Bacteriophages are viruses that infect bacteria Phages such as T2 have a protein shell with a tail that attaches to the host cell and a head that contains DNA Phages must infect bacterial hosts to reproduce Infection begins when the phage injects DNA into the bacterial cell and leaves its protein shell on the surface The phage DNA replicates in the bacterium and produces proteins that are assembled into progeny phage these are released by lysis of the host cell Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 11 Phage Infection of Bacteria Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 12 Hershey and Chase Experiments Proteins contain large amounts of sulfur but almost no phosphorus DNA contains large amounts of phosphorus but no sulfur Hershey and Chase separately labeled either phage proteins with 35S or DNA with 32P and then traced each radioactive label in the course of infection After infection agitation by a blender separated the empty phage particles from the infected bacteria In the protein labeling experiment the radioactivity was detected in the empty phage particles ghosts In the DNA labeling experiment the radioactivity was detected inside the infected bacteria Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 13 Phage DNA Not Protein Is Responsible for Infecting the Bacteria Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc 14 7 2 The DNA Double Helix Consists of Two Complementary and Antiparallel Strands Watson and Crick s model of the secondary structure of DNA shows that it is fairly simple in structure It is composed of 4 nucleotides joined by covalent phosphodiester bonds with two polynucleotide chains that come together to form a double helix Despite the structural simplicity DNA is a complex informational molecule 3 conformational types A DNA right handed 1 2 3 B DNA right handed Z DNA left handed Genetics Analysis An Integrated Approach Copyright 2012 Pearson Education Inc DNA Nucleotides A DNA nucleotide is composed of a sugar one of four nitrogenous bases and up to three phosphate groups Deoxyribose is the sugar of DNA nucleotides it has 5 carbons identified as 1 2 3 4 and 5 A nucleotide base is attached to the 1 carbon an OH hydroxyl group is attached to the 3 carbon and one to three phosphates are attached to the 5