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2004-20051
Half Hollow Hills High School Ms. FogliaAP Biology
2004-2005AP Biology
Chapter 16.
DNAThe Genetic Material
Replication
2004-2005AP Biology
Scientific History The march to 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 Drosophila (fruit flies) genes are on chromosomes but is it the protein or the DNA of the
chromosomes that are the genes? through 1940 proteins were thought to be
genetic material… Why?
1908|1933
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The “Transforming Factor” Frederick Griffith
Streptococcus pneumonia bacteria was working to find cure for pneumonia
harmless live bacteria mixed with heat-killed infectious bacteria causes diseasein mice
substance passed from dead bacteria tolive bacteria = “Transforming Factor”
1928
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The “Transforming Factor”
Transformation: change in genotype & phenotypedue to assimilation of external DNA by a cell
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DNA is the “Transforming Factor” Avery, McCarty & MacLeod
purified both DNA & proteins fromStreptococcus pneumonia bacteria which will transform non-pathogenic bacteria?
injected protein into bacteria no effect
injected DNA into bacteria transformed harmless bacteria into virulent
bacteria
What’s the conclusion?
1944
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Avery, McCarty & MacLeod
Oswald Avery
Maclyn McCarty
Colin MacLeod
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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: 35S in their proteins 32P in their DNA
infected bacteria withlabeled phages
1952|1969
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Hershey & Chase
Alfred HersheyMartha Chase
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Blender experiment Radioactive phage & bacteria in blender
35S phage radioactive proteins stayed in supernatant therefore protein did NOT enter bacteria
32P phage radioactive DNA stayed in pellet therefore DNA did enter bacteria
DNA is confirmed as “transformingfactor”
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Chargaff DNA composition: “Chargaff’s rules”
varies from species to species amounts of 4 bases not equal bases present in characteristic ratio
humans: A=30.9% T=29.4% G=19.9% C=19.8%
1947
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Structure of DNA Watson & Crick
developed double helix model of DNA other scientists working on question:
Linus PaulingMaurice WilkinsRosalind Franklin
1953|1962
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Rosalind Franklin (1920-1958)
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Watson and Crick 1953 article in Nature
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Double helix structure of DNA
the structure of DNA suggested a mechanism forhow DNA is copied by the cell
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Anti-parallel strands Phosphate to sugar bond
involves carbons in 3’ & 5’positions DNA molecule has
“direction” complementary strand
runs in opposite direction
“It has not escaped our notice thatthe specific pairing we havepostulated immediately suggests apossible copying mechanism for thegenetic material.”
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Bonding in DNAhydrogen
bonds
….strong or weak bond?How do the bonds fit the mechanism for copying DNA?
3’
5’ 3’
5’
phosphodiesterbonds
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Base pairing in DNA Purines
adenine guanine
Pyrimidines thymine cytosine
Pairing A : T C : G
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Copying DNA Replication of DNA
base pairing allowseach strand to serveas a pattern for anew strand
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Models of DNA Replication Alternative models
needed to be verified through experimentation
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Semi-conservative replication Meselson & Stahl
labeled nucleotides of “parent” DNA strandswith heavy nitrogen = 15N
labeled new nucleotides with lighter isotope =14N
replicated strands were found to be half 15N &half 14N
“The Most Beautiful Experiment in Biology”
1958
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DNA Replication Large team of enzymes coordinates replication
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Helicase Opens DNA helix enabling replication
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DNA Polymerase III Adds nucleotides only to 3’ end
nucleoside-P-P-P links to sugar-P backbone losing 2-P provides energy for bonding
Does this sound familiar??
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A common thread in biology…
5’
3’
3’
5’
You don’t justget energy fromATP → ADP
Why 3Ps?
TTP → TMPGTP→ GMPCTP → CMP
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Leading & Lagging strands
Leading strand continuous synthesis
Lagging strand Okazaki fragments joined by ligase
“spot welder”
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Okazaki fragments
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Priming DNA synthesisDNA polymerasecan only extend anexisting DNAmolecule. Cannotstart a new one. short RNA primer
is built first onparent DNA strandby primase
RNA primer laterremoved by DNApolymerase I
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Replication fork
3’
5’3’
5’
5’
3’
leading strand
Okazaki fragments
lagging strand
3’ 5’
DNA polymerase
ligase
helicase
direction of replication
primase
DNA polymerase
SSB
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Replication bubbleAdds 1000 bases/second!
Which direction does DNA build?List the enzymes & their role
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Replication enzymes
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DNA polymerases DNA polymerase I
20 bases/second editing, repair & primer removal
DNA polymerase III 1000 bases/second main DNA building enzyme
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Editing & proofreading DNA At 1000
bases/second,lots of typos!
DNA polymerase Iexcisesmismatched bases reduces error rate
from 1 in 10,000 to1 in 100 millionbases
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Fast & accurate! It takes E. coli <1 hour to copy
5 million base pairs in its singlechromosome & divide to form 2identical daughter cells
Human cell copies its 6 billion bases ÷ into daughter cells in only fewhours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle
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Problems in the end Ends of
chromosomesare erodedwith eachreplication
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Telomeres expendable,
non-codingsequences atends of DNA short sequence
of basesrepeated 1000stimes
TTAGGG inhumans
Telomeraseenzyme in certaincells
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