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DNA: The Genetic Material
Chapter 14
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Outline
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Genetic Material Experiments
Chemical Nature of Nucleic Acids
Three-Dimensional Structure of DNA
– Watson and Crick
Replication
– Semi Conservative
– Replication Process
Eukaryotic DNA Replication
One-Gene/One-Polypeptide Hypothesis
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Hammerling Experiment
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Hammerling Experiment
– Cells of green alga (Acetabularia) were cut
into pieces and observed to see which
were able to express hereditary
information.
 Discovered hereditary information is
stored in the cell’s nucleus.
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Transplantation Experiments
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Briggs and King (1952), and Steward (1958)
conducted several experiments that
conclusively determined each nucleus in a
eukaryotic cell contains a full set of genetic
instructions.
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Transplantation Experiments
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Several experiments were required to
conclusively determine which substance made
up genes.
– Griffith experiment
 documented movement of genes from
one organism to another (transformation)
 movement of material can alter the
genetic makeup of the recipient cell
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Avery and Hershey-Chase Experiments
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Avery experiment
– removed almost all protein from bacteria,
and found no reduction in transforming
activity
Hershey-Chase
– labeled DNA and protein with radioactive
isotope tracer
 determined hereditary information was
DNA, not protein
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Chemical Nature of Nucleic Acids
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DNA made up of nucleic acids
– Each nucleotide is composed of a five
carbon sugar, a phosphate group, and an
organic base.
 nucleotides distinguished by the bases
 reaction between phosphate group of
one nucleotide and hydroxyl group of
another is dehydration synthesis
 phosphodiester bond
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Chemical Nature of Nucleic Acids
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Purines - large bases
– adenine and guanine
Pyrimidines - small bases
– cytosine and thymine
 Chargaff’s rule
 A = T and G = C
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Nucleotides
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Three-Dimensional Structure of DNA
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X-ray diffraction suggested DNA had helical
shape with a 2 nanometer diameter.
– Watson and Crick deduced DNA is an intertwined double helix.
 complementary base-pairing
 purines pairing with pyrimidines
 constant 2 nanometer diameter
 antiparallel configuration
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DNA Double Helix
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Semi-Conservative Replication
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Each chain in the helix is a complimentary
mirror image of the other.
– double helix unzips and undergoes semiconservative replication
 each strand original duplex becomes
one strand of another duplex
 confirmed by Meselson-Stahl
experiment
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Replication Process
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Replication of DNA begins at one or more
sites (replication origin).
– DNA polymerase III and other enzymes
add nucleotides to the growing
complementary DNA strands.
 require a primer
 can only synthesize in one direction
 endonucleases
 exonucleases
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DNA Replication
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Replication Process
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DNA polymerase cannot link the first
nucleotides in a newly synthesized strand.
– RNA polymerase (primase) constructs an
RNA primer.
DNA polymerase adds nucleotides to 3’ end.
– Leading strand replicates toward replication
fork.
– Lagging strand elongates from replication
fork.
 Okazaki fragments
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DNA Synthesis
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Replication Process
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DNA ligase attaches fragment to lagging
strand.
– Because synthesis of the leading strand is
continuous and the lagging strand is
discontinuous, the overall replication of DNA
is referred to as semi-discontinuous.
DNA gyrase removes torsional strain
introduced by opening double helix.
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Replication Process
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Opening DNA double helix
– initiating replication
– unwinding duplex
– stabilizing single strands
– relieving torque
Building a primer
Assembling complementary strands
Removing the primer
Joining Okazaki fragments
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DNA Replication Fork
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Replisome
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Replisome is a macromolecular protein
machine (replication organelle).
– fast, accurate replication of DNA during
cell division
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Stages of Replication
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Initiation
– always occurs at the same site
Elongation
– majority of replication spent in elongation
Termination
– exact details unclear
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Eukaryotic DNA Replication
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Eukaryotes usually have multiple, large
chromosomes.
– multiple origins of replication
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One-Gene/One-Polypeptide Hypothesis
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Genes produce their effects by specifying
the structure of enzymes.
– Each gene encodes the structure of one
enzyme (Beadle and Tatum).
 Many enzymes contain multiple
polypeptide subunits, each encoded by
a separate gene.
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One-Gene / One-Polypeptide
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Summary
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Genetic Material Experiments
Chemical Nature of Nucleic Acids
Three-Dimensional Structure of DNA
– Watson and Crick
Replication
– Semi Conservative
– Replication Process
Eukaryotic DNA Replication
One-Gene/One-Polypeptide Hypothesis
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