Ch 5 and16 A Close Look at the Hereditary Molecules

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Ch 5 and16 A Close Look at the Hereditary
Molecules

• Protein sequence--programmed by genes
• Genes are made of DNA, a nucleic acid

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LE 5-25
Flow of genetic information
DNA
RNA
Protein
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The Roles of Nucleic Acids

• Two types
• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)
• DNA provides directions for its own replication.
• DNA directs synthesis of messenger RNA (mRNA)
• mRNA controls protein synthesis.
• Protein synthesis occurs on ribosomes.

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LE 5-26a
5 end
Nucleic acid building block
Nucleoside
Nitrogenous base
Phosphate group
Pentose sugar
Nucleotide
3 end
Polynucleotide, or nucleic acid
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Nucleic Acid Structure
Monomers nucleotide (3 parts) 1. nitrogenous
base 2. 5 C sugar 3. Phosphate
nucleoside
Polymer polynucleotide or nucleic acid
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LE 5-26b
Nitrogenous bases
Pyrimidines
Uracil (in RNA) U
Cytosine C
Thymine (in DNA) T
Purines
Adenine A
Guanine G
Pentose sugars
Deoxyribose (in DNA)
Ribose (in RNA)
Nucleoside components
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Important Nucleic Acid Distinctions
Two kinds of bases
Pyrimidines-one ring (T,U,C) Purines- two rings
(G,A)

• DNA
• the sugar deoxyribose
• NO 2 OH (hydroxyl)

• RNA
• the sugar ribose
• YES 2 OH

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Nucleotide Polymers

• Nucleotides (nt) connect through phosphodiester
  bond
• 5 Phosphate-- 3OH
• Creation of a sugar-phosphate backbone with bases
  as appendages.
• Sequence of bases along DNA or mRNA polymer
  unique for each gene.

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LE 16-7
5? end
Hydrogen bond
3? end
1 nm
3.4 nm
3? end
0.34 nm
5? end
Space-filling model
Partial chemical structure
Key features of DNA structure
Two DNA strands bind together through
complementary base-pairing.
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Structure of DNA double helix published in 1953
FrancisCrick
JamesWatson
Watson JD, Crick FHC. 1953. Molecular structure
of nucleic acids a structure for
deoxyribonucleic acids. Nature 171738.
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LE 16-6
Partly based on Franklins x-ray diffraction data
Franklins X-ray diffraction photograph of DNA
Rosalind Franklin
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LE 16-8
Chargaffs rules (1940s)
Amount of AT GC
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LE 16-UN298
Watson Crick built model of DNA and tested
possible combinations of bases
Did model support Chargaffs observations and
Franklins x-ray diffraction data?
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LE 16-7
Antiparallel DNA strands
5? end
Hydrogen bond
3? end
1 nm
3.4 nm
3? end
0.34 nm
5? end
Space-filling model
Partial chemical structure
Key features of DNA structure
Two DNA strands bind together through
complementary base-pairing.
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The DNA Double Helix

• Two polynucleotides (strands) base-paired
  together GC, AT (complementary base-pairing)
• Double helix
• Two sugar-phosphate backbones run in opposite 5
  to 3 directions - antiparallel
• One DNA molecule includes many genes

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Complementary base pairs
AT 2 H-bonds
GC 3 H-bonds
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Behavior of DNA
Draw a 10 base pair double-stranded DNA (dsDNA)
that is rich in AT.
Draw a 10 base pair double-stranded DNA (dsDNA)
that is rich in GC.
If these were placed in a tube of boiling water
what would happen?
DNA would become single stranded (ssDNA)
(denatured or melted).
Which DNA would denature first. Why?
AT rich fragment less stable 2 H-bonds/bp versus
3 H-bonds/bp
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DNA Used as Evolutionary Ruler

• Linear sequences of DNA in chromosomes
• passed from parents to offspring
• Two closely related species are more similar in
  DNA sequence than distantly related species
• Similarity of DNA sequence
• Determines evolutionary relatedness

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• Compare the human sequence to the frog and mouse.
  
• Which sequence is most similar to human?

human
5 GAACCTTCCAATTGATCT3
5 GAACCAACCAATTAAACT3
5 GAACCTTCGAATTGATCT3
mouse
frog
2. Write in the complementary strand for each.
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Earlier data suggested that DNA was hereditary
material
Model system Drosophila melanogaster Investigator
Thomas Hunt Morgan (early 1900s) Evidence
white eye phenotype associated with X-chromosome
Model system bacteria and viruses Investigators
Many Evidence various
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Evidence That DNA Can Transform Bacteria

• Evidence for genetic role of DNA (Frederick
  Griffith,1928)
• Heat-killed pathogenic S Streptococcus
  pneumoniae
• Rnon-pathogenic bacterial strain

Some living bacteria became pathogenic Transform
ation of R to S,
How could one determine pathogenicity
experimentally?
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LE 16-2
Mixture of heat-killed S cells and living R cells
Living S cells (control)
Living R cells (control)
Heat-killed S cells (control)
RESULTS
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells are found in blood sample
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What molecule was responsible for conferring a
new phenotype into an organism?

• Oswald Avery, Maclyn McCarty, and Colin MacLeod
  (1944)
• Published results
• Showed DNA from bacteria NOT protein-- caused
  transformation of R to S

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Independent confirmation

• Alfred Hershey and Martha Chase (1952)
• Used bacterial virus (bacteriophage) (T2) to ask
  whether DNA or protein was hereditary material

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LE 16-3
Phage head
Tail
Tail fiber
DNA
100 nm
Bacterial cell
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LE 16-4
Hershey Chase labeling experiment
Empty protein shell
Radioactive protein
Radioactivity (phage protein) in liquid
Phage
Bacterial cell
Batch 1 Sulfur (35S)
DNA
Phage DNA
Protein radiolabelled
Centrifuge
Pellet (bacterial cells and contents)
Radioactive DNA
Batch 2 Phosphorus (32P)
DNA radiolabelled
Centrifuge
Radioactivity (phage DNA) in pellet
Pellet
Phage produced in and released from
bacteria with radioactive DNA.
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Hershey Chase results -Suggest that DNA, not
protein, is transferred to bacteria by
phage. -DNA programs the reproduction of more
phage. Contains important
genetic instructions.
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