Chapter 12 DNA the Carrier of Genetic Information Virulence ability to cause disease death Avirulence inability Transformation one strain of dead cells conferred different strain of living cells griffith Hershey and Chase demonstrated that DNA is taken up by cells 1 2 3 4 5 showed the significance of DNA in viral reproduction by bacteriophage labled DNA and Protein allowed the viruses to infect E Coli viral heads were sheered off by blender agitation centrifuged the cells found that infected cells contained DNA but not protein Structure Watson and Crick DNA building block nucleotide deoxyribose pentose sugar 1 phosphate 2 1 of 4 nitrogenous bases 3 Bases Purines Adenine and Guanine Pyrimidines Thymine and Cytosine the nucleotides are linked by covalent bonds to form an alternating sugar phosphate boackbone the 3 carbon of one sugar is bonded to the 5 phosphate of the adjacent sugar to form a result is a polymer of indefinite length with the nucleotides linked in any order 3 5 phosphodiester linkage no matter how long the chain is 5 end is attached to phosphate 3 end is attached to hydroxyl group DNA 2 polynucleotide chains intertwined to form a double helix X Ray Diffraction to fit the two strands must run in opposite directions of each other antiparallel 2 coiled chains are an outside of sugar phosphate backbones and base method for elucidating 3 D structure of the molecule pairs along axis hydrogen bonds form between A T and G C width 2 0 nm 3 AGCTAC 5 5 TCGATG 3 DNA acts as a template for it s own duplication DNA Replication way in which DNA is precisely copied Semiconservative Replication 4 bonds between the two strands to break 2 chains separate pair up with complimentary nucleotides to replace other strand forms 2 new double helixes Conservative Replication Dispersive Replication both parent strands stay together 2 newly synthesized form second helix parent and new randomly mixed during replication Proteins Involved in DNA Replication Helicase opens the double helix by disrupting the hydrogen bonds that hold the two strands together creates replication fork Single Strand Binding SSB Protein binds to single strands of DNA and prevent the helix from re forming before it can be used as a template for replication prevents hydrolysis of single strand by other enzymes Topoisomerase breaks one of both DNA strands preventing excessive coiling during replication and then rejoins them in a more relaxed configuration DNA Polymerase links nucleotide subunits to form a new DNA strand from a DNA template add nucleotides only to 3 end of existing strand exergonic reactions always grows 5 to 3 RNA Primer short piece of RNA synthesized where replication begins RNA ribonucleic acid is a polymer consisting of nucleotide subunits that associate by complementary base pairing DNA Primase synthesizes short RNA primers on the lagging strand Begins replication of the leading strand displaces primase and adds subunits to 3 end of the RNA primer Leading Strand add nucleotides going towards replication fork synthesized smoothly and continuously Lagging Strand adds away from replication fork only can do short pieces okazaki fragments at a time seperate RNA primer initiates each okazaki fragment DNA Ligase links Okazaki fragments by joining the 3 end of the new DNA fragment to the 5 end of the adjoining DNA forms phosphodiester linkage also helps with repair Telomerase lengthens telomeric DNA by adding repetitive nucleotide sequences to the ends of eukaryotic chromosomes bacterial DNA is circular but eukaryotic chromosomes have free ends DNA poly merases don t complete replication of the strand neatly at the end of the SNA a small portion is left unreplicated and a small single strand segment is lost with each cell cy cle the important info is retained because chromosomes have protective end caps or telomeres that do not contain protein coding genes Contain many copies of the same sequence so even if some is lost the information is still accounted for eventually cells lose telomerase therefore normal cells can only replicate a number of times cancerous cells retain telomerase to replenish their telomeres im mortal