CHAPTER 16 THE MOLECULAR BASIS OF INHERITANCE

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CHAPTER 16THE MOLECULAR BASIS OF INHERITANCE
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• 1940S THE SEARCH FOR DNA LEAD MOST SCIENTISTS
  TO BELIEVE THAT PROTEIN MAY BE THE GENETIC
  MATERIAL, DUE TO THE FACT, THAT THERE WAS SOME
  EVIDENCE THAT DNA CAN TRANSFER BACTERIA

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• 1928 FREDERICK GRIFFITH
• HE ATTEMPTED TO FIND A VACCINE FOR STREPTOCOCCUS
  PNEUMONIAE
• HE KNEW THAT THERE WERE 2 STRAINS OF THE BACTERIA
• SMOOTH COLONIES ONES THAT WERE ENCAPSULATED
  WITH A POLYSACCHARIDE COAT
• ROUGH COLONIES COLONIES NOT COVERED

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• GRIFFITHS EXPERIMENT
• DEMONSTRATED TRANSFORMATION
• ASSIMILATION OF EXTERNAL GENETIC MATERIAL BY A
  CELL, HOWEVER, GRIFFITH WAS UNABLE TO DETERMINE
  WHAT THE TRANSFORMING AGENT WAS
• HIS EXPERIMENT HINTED THAT PROTEIN WAS NOT THE
  GENETIC MATERIAL
• HEAT DENATURES PROTEIN

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• AVERY, McCARTY _at_ MacLOED
• THROUGH EXPERIMENTATION ANNOUNCE THAT THE
  TRANSFORMING AGENT HAD TO BE DNA

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• ALFRED HERSHEY _at_ MARTHA CHASE
• 1952
• DISCOVERED THAT DNA IS THE GENETIC MATERIAL OF A
  PHAGE KNOWN AS T-2
• BACTERIOPHAGE (PHAGE) IS A VIRUS THAT INFECTS
  BACTERIA

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• HERSHEY _at_ CHASE KNEW
• ONE OF THE MANY PHAGES TO INFECT THE ENTERIC
  BACTERIA E.COLI
• LITTLE MORE THAN DNA IN A PROTEIN COAT (MOST
  VIRUS)
• WERE ABLE TO REPROGRAM AN (E.COLI) CELL TO
  PRODUCE T-2 PHAGES AND RELEASE THE VIRUS WHEN
  THE CELL LYSIS
• THEY DID NOT KNOW IF IT WAS DNA OR PROTEIN THAT
  WAS RESPONSIBLE FOR REPROGRAMMING THE BACTERIAL
  CELL

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• HERSHEY, CHASE ONLY GAVE SOME EVIDENCE THAT DNA
  WAS THE HERIDITARY MATERIAL IN A VIRUS, IT WAS
  ONLY CIRCUMSTANTIAL IN EUKARYOTIC CELLS

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• 1947 ERWIN CHARGRAFF
• OFFERED EXPERIMENTAL EVIDENCE THAT DNA IS THE
  HEREDITARY MATERIAL IN EUKARYOTIC CELLS
• HE USED PAPER CHROMATOGRAPHY
• HE REPORTED THAT DNA COMPOSITION IS SPECIES
  SPECIFIC, AMOUNTS AND RATIOS VARY FROM ONE
  SPECIES TO ANOTHER (NITROGENOUS BASES)
• THERE WAS A REGULARITY IN BASE RATIOS
• THIS BECAME KNOWN AS CHARGRAPHS RULE

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• JAMES WATSON _at_ FRANCIS CRICK
• USED INFORMATION FROM
• MAURICE WILKINS
• ROSALIND FRANKLIN
• LINUS PAULING

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• FRANKLIN HAD PRODUCED AN X-RAY PHOTOGRAPH OF DNA,
  FROM THIS INFORMATION WATSON _at_ CRICK DEDUCED THE
  FOLLOWING
• DNA IS A HELIX WITH A UNIFORM WIDTH OF 2nm, THEY
  SUGGESTED IT HAD 2 STRANDS
• PURINE _at_ PYRIMADINE BASES ARE STACKED .34 nm
  APART
• THE HELIX MAKES ONE FULL TURN EVERY 3.4nm ALONG
  ITS LENGTH
• THERE ARE 10 LAYERS OF BASE PAIRS IN EACH TURN OF
  THE HELIX
• WIDTH SUGGESTED IT WAS MADE UP OF 2 STRANDS,
  UNLIKE 3 SUGGESTED BY PAULING

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• WATSON _at_ CRICK SOLVED THE PROBLEM
• THEY BUILT A SCALE MODEL THAT CONFORMED TO THE
  X-RAY DATA AND KNOWN CHEMISTRY OF DNA
• PURINE MUST PAIR WITH PYRAMIDINE
• BASE STRUCTURE WILL DICTATE WHICH PAIRS CAN
  HYDROGEN BOND

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• BASE PAIRING RULE
• CHARGRAFFS RULE
• SUGGESTED THE GENERAL MECHANISM FOR DNA
  REPLICATION
• ONE STRAND COMPLEMENTS THE OTHER
• THE BASE SEQUENCE CAN BE HIGHLY VARIABLE, MAKES
  IT SUITABLE FOR CODING INFORMATION
• HYDROGEN BONDS AND VAN DER WAALS FORCES HELP
  STABILIZE DNA

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• 1953 WATSON _at_ CRICK (MODEL)
• SUGGESTED A TEMPLATE MECHANISM FOR DNA
  REPLICATION
• PROPOSED THAT GENES ON THE ORIGINAL DNA STRAND
  ARE COPIED BY A SPECIFIC PAIRING OF COMPLENATARY
  BASES WHICH WILL CREATE A COMPEMENTARY DNA STRAND
• THE COMPLEMENTARY STRAND CAN THEN FUNCTION AS A
  TEMPLATE TO PRODUCE A COPY OF THE ORIGINAL STRAND

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• WATSON _at_ CRICKS
• PRODUCED A SECOND PAPER, THAT FOLLOWED THE
  ANNOUNCMENT OF THE DOUBLE HELIX
• SUGGESTED THE MODEL FOR DNA REPLICATION
• BASED ON CHARGRAFFS RULE

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• MATTHEW MESELSON _at_ FRANKLIN STAHL
• PROVIDED EXPERIMENTAL EVIDENCE TO SUPPORT WATSON
  _at_ CRICKS SEMICONSERVATIVE MODEL OF DNA
  REPLICATION

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• ENZYMES AND PROTEINS _at_ DNA REPLICATION
• THE CONCEPT IS SIMPLE
• ACTUAL MECHANISM IS QUITE COMPLEX
• A HELICAL MOLECULE MUST UNTWIST WHILE IT COPIES
  ITS 2 ANTIPARRALLEL STRANDS SIMULTANEOUSLY
• OVER A DOZEN ENZYMES AND PROTEINS ARE NEEDED
• PROKARYOTES NUCLEOTIDES ADDED 500/SECOND
• EUKARYOTES 50/SECOND
• ACCURACY ONLY ABOUT 1 IN A BILLION NUCLEOTIDES
  ARE INCORRECT

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• ORIGIN OF REPLICATION
• WHERE DOES DNA REPLICATION ACTUALLY BEGIN
• SPECIFIC PROTEINS ARE NEEDED TO INITIATE THIS
  PROCESS
• FORMATION OF REPLICATION FORKS
• A Y SHAPED REGION OF REPLICATING DNA WHERE THE
  NEW STRANDS WILL GROW
• REPLICATION BUBBLE POSITION WHERE THE HELIX
  OPENS, CREATED BY THE FORMATION OF THE
  REPLICATION FORK

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• EUKARYOTIC CHROMOSOMES HAVE HUNDREDS OR EVEN
  THOUSANDS OF REPLICATING ORIGINS.
• VIRAL OR BACTERIAL DNA HAVE ONLY ONE REPLICATION
  ORIGIN

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• ELONGATION OF A NEW DNA STRAND
• FIRST THE STRAND WILL SEPARATE, 2 PROTEINS ARE
  INVOLVED
• HELICASES WHICH ARE ENZYMES THAT WILL CATALYZE
  THE UNWINDING OF PARENTAL DOUBLE HELIX TO EXPOSE
  THE TEMPLATE
• SINGLE STRAND BINDING PROTEIN, WHICH KEEP THE
  STRANDS SEPARATE AND STABALIZE THE UNWOUND DNA,
  UNTIL NEW COMPLIMENTARY STRANDS ARE FORMED

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• ELONGATION OF A NEW STRAND OF DNA
• DNA POLYMERASE THE FUNCTIONAL ENZYME
• DNA POLYMERASE LINKS THE NUCLEOTIDES TO THE
  GROWING STAND
• THE NEW NUCLEOTIDES ARE ADDED TO THE 3 END OF
  THE GROWING STRAND
• NUCLEOSIDE TRIPHOSPHATE DRIVES THIS REACTION
  (ENERGY SOURCE SIMILAR TO ATP)

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• ANTIPARRALLEL
• SUGAR PHOSPHATE BONDS IN DNA RUN IN OPPOSITE
  DIRECTIONS
• DNA CAN ONLY ELONGATE STRANDS IN THE 5 TO 3
  DIRECTION
• PROBLEM OF ANTIPARRALLEL DNA STRANDS IS SOLVED BY
  THE CONTINUOUS SYNTHESIS OF THE
• LEADING STRAND THIS IS A STRAND THAT IS A
  SINGLE POLYMER IN THE 5 TO 3 DIRECTION, WHICH
  IS TOWARDS THE REPLICATION FORK

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• OKAZAKI FRAGMENTS
• PRODUCE THE LAGGING STRAND
• A SERIES OF SHORT SEGMENTS
• SYNTHESIZED IN THE 5 TO 3 DIRECTION
• DNA LIGASE IS THE ENZYME THAT LINKS OKAZAKI
  FRAGMENTS INTO A NEW DNA STRAND
• OKAZAKI FRAGMENTS SOLVE THE PROBLEM OF THE
  LAGGING STRAND

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• PRIMING DNA SYNTHESIS
• PRIMER A SHORT RNA SEGMENT THAT IS
  COMPLIMENTARY TO THE DNA SEGMENT NEEDED TO BEGIN
  DNA REPLICATION
• PRIMASE THE ENZYME THAT JOINS RNA NUCLEOTIDES
  TO FORM THE PRIMER
• ABOUT 10 NUCLEOTIDES LONG (EUKARYOTES)
• ONLY 1 PRIMER IS NEEDED FOR THE LEADING STRAND
• AN RNA PRIMER IS NEEDED FOR EACH OKAZAKI FRAGMENT
  IN THE FORMATION OF THE LAGGING STRAND

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• DAMAGE REPAIR
• PROOF READING OF DNA
• DNA REPLICATION IS HIGHLY ACCURATE
• PAIRING ERRORS ARE ONLY ABOUT 1-10,000
• A COMPLETE DNA MOLECULE ERROR IS ONLY ABOUT 1 IN
  1 BILLION

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• MISMATCH REPAIR
• MISTAKES ARE CORRECTED AS THE DNA IS BEING COPIED
• POLYMERASE PROOF READS EACH NUCLEOTIDE AND
  REMOVES INCORRECTLY PAIRED NUCLEOTIDES AND ALLOWS
  SYNTHESIS TO CONTINUE

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• EXCISION REPAIR
• ACCIDENTAL DNA CHANGES DUE TO EXPOSURE TO
  CHEMICALS, RADIATION, UV LIGHT ETC.
• NUCLEASE IS THE REPAIR ENZYME
• THE DAMAGED SEGMENT IS EXCISED AND THE GAP IS
  FILLED BY DNA POLYMERASE AND SEALED BY LIGASE

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• TELOMERES
• DO NOT CONTAIN GENES
• MULTIPLE REPITITIONS OF A SHORT NUCLEOTIDE
  SEQUENCE
• TTAGGG IN EUKARYOTES
• VARIES BETWEEN 100-1000
• PROTECTS THE EROSION OF DNA
• TELOMERASE (ENZYME) CATALYZE THE LENGTHENING OF
  THE TELOMERE

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