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AP Biology 2008-2009 DNA The Genetic Material Biology---Yippee!
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Scientific History Understanding that DNA is the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958)
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Genes are on chromosomes T.H. Morgan working with fruit flies Determined: genes are on chromosomes The question then was: is it the protein or the DNA of the chromosomes that are the genes? through 1940 proteins were thought to be genetic material because their structure is more complex 1908 | 1933
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Frederick Griffith 1928 was working to find cure for pneumonia (Streptococcus pneumonia bacteria) harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice substance passed from dead bacteria to live bacteria = “Transforming Factor”
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The “Transforming Factor” Transformation something in heat-killed bacteria could still transmit disease-causing properties to the harmless bacteria live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.
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Avery, McCarty & MacLeod purified both DNA & proteins from Streptococcus pneumonia bacteria to determine which will transform non- pathogenic bacteria? injected protein into bacteria no effect injected DNA into bacteria transformed harmless bacteria into virulent bacteria Concluded DNA is the transforming factor 1944 What ’ s the conclusion?
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Confirmation of DNA Hershey & Chase classic “blender” experiment worked with bacteriophage viruses that infect bacteria grew phage viruses in 2 media, radioactively labeled with either 35 S in their proteins 32 P in their DNA infected bacteria with labeled phages 1952 | 1969 Hershey
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Blender experiment Radioactive phage & bacteria in blender 35 S phage radioactive proteins did NOT enter bacteria 32 P phage radioactive DNA did enter bacteria Confirmed DNA is “transforming factor” Taaa-Daaa!
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Summary Used radioactive labels to identify genetic material Labeled DNA with radioactive phosphorus ( 32 P) Labeled protein coats with radioactive sulfur ( 35 S) Blender Examined newly infected host cells (pellet) and found only radioactive phosphorus label (not sulfur) ***DNA is the genetic material
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Hershey & Chase Alfred HersheyMartha Chase 1952 | 1969 Hershey
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Nucleic Acids (review) section 10.2 Examples: RNA (ribonucleic acid) single helix DNA (deoxyribonucleic acid) double helix Structure: monomers = nucleotides RNADNA
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Nucleotides 3 parts 1. nitrogen base Adenine, guanine, cytosine & thymine 2. pentose sugar (5 Carbon) ribose in RNA deoxyribose in DNA 3. phosphate (PO 4 ) group Nitrogen base I’m the A,T,C,G or U part!
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Erwin Chargaff DNA composition: “Chargaff’s rules” varies from species to species Amount of thymine = amount of adenine Amount of cytosine = amount of guanine 1947
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Structure of DNA Watson & Crick developed double helix model of DNA other scientists working on question: Rosalind Franklin Maurice Wilkins Linus Pauling 1953 | 1962 Franklin WilkinsPauling
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Rosalind Franklin (1920-1958)
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Rosalind Franklin & Maurice Wilkins (1950) 1. used X-ray crystallography to study the structure of DNA. In this technique, X-rays are diffracted as they passed purified DNA. The diffraction pattern can be used to deduce the three-dimensional shape of molecules.
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Watson and Crick 1953 article in Nature CrickWatson
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James Watson and Francis Crick 1. Concluded that the structure of DNA is a double helix. (After Watson viewed Franklin’s x-ray diffraction photo) 2. Backbone consists of alternating sugar & phosphate units. 3. Attached to the backbone are four kinds of bases. I. AdenineIII. Cytosine II. GuanineIV. Thymine
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Twist
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 16.5
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Copying DNA Replication of DNA base pairing allows each strand to serve as a template for a new strand
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Franklin Stahl Matthew Meselson Meselson & Stahl experiment used radioactive nitrogen to determine that DNA replication is semi-conservative (each of the 2 new DNA molecules is half original or parent DNA and half newly made) Semi-conservative replication 1958
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AP Biology 2008-2009 DNA Replication
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DNA used to make DNA Making an exact copy of the DNA before the cell divides original strand serves as a template for the new strand Each resulting double-stranded DNA molecule is made of one original and one new strand ( ½ parent template and ½ new DNA) semi-conservative replication
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Anti-parallel strands DNA molecule has “direction” complementary strand runs in opposite direction Replication only occurs in the 5’ to 3’ direction 3 5 5 3
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Bonding in DNA weak bonds 3 5 3 5 covalent phosphodiester bonds hydrogen bonds strong bonds
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Base pairing in DNA Pairing A : T 2 bonds C : G 3 bonds
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Replication: 1st step Unwind DNA Helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins single-stranded binding proteins replication fork helicase
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DNA Polymerase III Replication: 2nd step Build daughter DNA strand add new complementary bases to 3’’ end of growing DNA strand Enzyme DNA polymerase III strand only grows 5 3
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Loss of DNA With each replication, small segments at the end of our chromosomes (called telomeres) are lost This may be part of the aging process When enough DNA is lost the cell can no longer divide An enzyme called telomerase is able to add on to the end of chromosomes – but it is inactivated in most of our cells Cancer cells keep the enzyme active and can divide forever
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