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
NUCLEUS
DNA
CYTOPLASM
mRNA
mRNA
Ribosome
Aminoacids
Synthesis ofmRNA in the nucleus
Movement ofmRNA into cytoplasmvia nuclear pore
Synthesis of protein
Polypeptide
DNA
RNA
Protein
Flow of genetic information
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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 5-26a5 end
3 end
Nucleoside
Nitrogenousbase
Phosphategroup
Nucleotide
Polynucleotide, ornucleic acid
Pentosesugar
Nucleic acid building block
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Nucleic Acid Structure
Monomers
nucleotide (3 parts)
1. nitrogenous base
2. 5 C sugar
3. Phosphate
Polymerpolynucleotide or nucleic acid
nucleoside
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LE 5-26b
Nitrogenous bases
Pyrimidines
Purines
Pentose sugars
CytosineC
Thymine (in DNA)T
Uracil (in RNA)U
AdenineA
GuanineG
Deoxyribose (in DNA)
Nucleoside components
Ribose (in RNA)
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Important Nucleic Acid Distinctions
Pyrimidines-one ring (T,U,C)
Purines- two rings (G,A)
DNA the sugar = deoxyribose
NO 2’ OH (hydroxyl)
Two kinds of bases
RNA the sugar= ribose
YES 2’ OH
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Nucleotide Polymers
• Nucleotides (nt) connect through phosphodiester bond
5’ Phosphate--> 3’OH
• 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
3 end
5 end
3 end
Space-filling modelPartial chemical structure
Hydrogen bond
Key features of DNA structure
0.34 nm
3.4 nm
1 nm
Two DNA strands bind togetherthrough complementary base-pairing.
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FrancisCrick
JamesWatson
Structure of DNA double helix: published in 1953
Watson JD, Crick FHC. 1953. Molecular structure of nucleic acids: a structure for deoxyribonucleic acids. Nature 171:738.
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LE 16-6
Franklin’s X-ray diffractionphotograph of DNA
Rosalind Franklin
Partly based on Franklin’s x-ray diffraction data
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LE 16-8
Chargaff’s rules (1940s):
Amount of A=T
G=C
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LE 16-UN298
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: widthconsistent with X-ray data
Watson & Crick: built model of DNA and tested possible combinations of bases
Did model support Chargaff’s observations and Franklin’s x-ray diffraction data?
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LE 16-7
5 end
3 end
5 end
3 end
Space-filling modelPartial chemical structure
Hydrogen bond
Key features of DNA structure
0.34 nm
3.4 nm
1 nm
Antiparallel DNA strands
Two DNA strands bind togetherthrough 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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Adenine (A) Thymine (T)
Guanine (G) Cytosine (C)
Sugar
Sugar
Sugar
Sugar
Complementary base pairs
G=C3 H-bonds
A=T
2 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 stable2 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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5’ GAACCTTCCAATTGATCT3’
5’ GAACCAACCAATTAAACT3’ 5’ GAACCTTCGAATTGATCT3’
1. Compare the human sequence to the frog and mouse. Which sequence is most similar to human?
human
mousefrog
2. Write in the complementary strand for each.
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Earlier data suggested that DNA was hereditary material
Model system: Drosophila melanogasterInvestigator: Thomas Hunt Morgan (early 1900’s)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+
“R”non-pathogenic bacterial strain
Some living bacteria became pathogenicTransformation of “R’ to ‘S”,
How could one determine pathogenicity experimentally?
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LE 16-2
Living S cells(control)
Living R cells(control)
Heat-killedS cells (control)
Mixture of heat-killedS cells and livingR cells
Mouse dies
Living S cellsare found in blood sample
Mouse healthy Mouse healthy Mouse dies
RESULTS
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• Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944)• Published results
– Showed DNA from bacteria NOT protein--> caused transformation of “R” to “S”
What molecule was responsible for conferring a new phenotypeinto an organism?
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• Alfred Hershey and Martha Chase (1952)
– Used bacterial virus (bacteriophage) (T2) to ask whether DNA or protein was hereditary material
Independent confirmation
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LE 16-3
Bacterialcell
Phagehead
Tail
Tail fiber
DNA
100
nm
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LE 16-4
Bacterial cell
Phage
DNA
Radioactiveprotein
Emptyprotein shell
PhageDNA
Radioactivity(phage protein)in liquid
Batch 1:Sulfur (35S)
RadioactiveDNA
Centrifuge
Pellet (bacterialcells and contents)
PelletRadioactivity(phage DNA)in pellet
Centrifuge
Batch 2:Phosphorus (32P)
Hershey & Chase labeling experiment
Protein radiolabelled
DNA radiolabelled
Phage produced in and released from bacteriawith 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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I’m a pretty cool molecule butI’ll still answer yourquestions.