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AP Biology 2008-2009
DNAThe Genetic Material
Biology---Yippee!
Scientific History 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)
Genes are on chromosomes T.H. Morgan
working with fruit flies Determined: genes are on
chromosomes The question then was: is it
the protein or the DNA of the chromosomes that are the genes? through 1940 proteins
were thought to be genetic material because their structure is more complex
1908 | 1933
Frederick Griffith 1928
was working to find cure for pneumonia (Streptococcus pneumonia bacteria)
harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice
substance passed from dead bacteria to live bacteria = “Transforming Factor”
The “Transforming Factor”
Transformationsomething in heat-killed bacteria could still transmit disease-causing properties to the harmless bacteria
live pathogenicstrain of bacteria
live non-pathogenicstrain of bacteria
mice die mice live
heat-killed pathogenic bacteria
mix heat-killed pathogenic & non-pathogenicbacteria
mice live mice die
A. B. C. D.
Avery, McCarty & MacLeod purified both DNA & proteins from Streptococcus pneumonia bacteria to determine which will transform non-pathogenic bacteria?
injected protein into bacteria no effect
injected DNA into bacteria transformed harmless bacteria
into virulent bacteria
Concluded DNA is the
transforming factor
1944
What’s theconclusion?
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 with labeled phages
1952 | 1969Hershey
Blender experiment Radioactive phage & bacteria in blender
35S phage radioactive proteins did NOT enter bacteria
32P phage radioactive DNA did enter bacteria
Confirmed DNA is “transforming factor”
Taaa-Daaa!
Summary Used radioactive labels to identify genetic
material Labeled DNA with radioactive phosphorus (32P) Labeled protein coats with radioactive sulfur
(35S) Blender Examined newly infected host cells (pellet) and
found only radioactive phosphorus label (not sulfur)
***DNA is the genetic material
Hershey & Chase
Alfred HersheyMartha Chase
1952 | 1969Hershey
Nucleic Acids (review) section 10.2
Examples: RNA (ribonucleic acid)
single helix DNA (deoxyribonucleic acid)
double helix
Structure: monomers = nucleotides
RNADNA
Nucleotides3 parts
1. nitrogen base Adenine, guanine, cytosine & thymine
2. pentose sugar (5 Carbon)ribose in RNA
deoxyribose in DNA
3. phosphate (PO4) group
Nitrogen baseI’m the
A,T,C,G or Upart!
Erwin Chargaff DNA composition: “Chargaff’s rules”
varies from species to species Amount of thymine = amount of adenine Amount of cytosine = amount of guanine
1947
Structure of DNA Watson & Crick
developed double helix model of DNA other scientists working on question:
Rosalind FranklinMaurice WilkinsLinus Pauling
1953 | 1962
Franklin Wilkins Pauling
Rosalind Franklin (1920-1958)
Rosalind Franklin & Maurice Wilkins (1950)1. used X-ray crystallography to study the structure
of DNA. In this technique, X-rays are diffracted as they
passed purified DNA. The diffraction pattern can be used to deduce the
three-dimensional shape of molecules.
Watson and Crick1953 article in Nature
CrickWatson
James Watson and Francis Crick
1. Concluded that the structure of DNA is a double helix. (After Watson viewed Franklin’s x-ray diffraction photo)
2. Backbone consists of alternating sugar & phosphate units.
3. Attached to the backbone are four kinds of bases.
I. Adenine III. Cytosine
II. Guanine IV. Thymine
Twist
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 16.5
Copying DNA Replication of DNA
base pairing allows each strand to serve as a template for a new strand
Franklin Stahl
Matthew Meselson
Meselson & Stahl experiment used radioactive nitrogen to determine that DNA replication is semi-conservative (each of the 2 new DNA molecules is half original or parent DNA and half newly made)
Semi-conservative replication 1958
AP Biology 2008-2009
DNA Replication
DNA Replication DNA used to make DNA Making an exact copy of the DNA before
the cell divides original strand serves as a template for
the new strand Each resulting double-stranded DNA
molecule is made of one original and one new strand ( ½ parent template and ½ new DNA) semi-conservative replication
Anti-parallel strands DNA molecule has
“direction” complementary strand runs
in opposite direction Replication only occurs in
the 5’ to 3’ direction
3
5
5
3
Bonding in DNA weak bonds
3
5 3
5
covalentphosphodiester
bonds
hydrogenbonds
strong bonds
Base pairing in DNA Pairing
A : T 2 bonds
C : G 3 bonds
Replication: 1st step Unwind DNA
Helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins
single-stranded binding proteins replication fork
helicase
DNAPolymerase III
Replication: 2nd step Build daughter DNA
strand add new complementary
bases to 3’’ end of growing DNA strand
Enzyme DNA polymerase III
strand only grows 53
Loss of DNA With each replication, small segments at the
end of our chromosomes (called telomeres) are lost This may be part of the aging process When enough DNA is lost the cell can no longer
divide
An enzyme called telomerase is able to add on to the end of chromosomes – but it is inactivated in most of our cells Cancer cells keep the enzyme active and can
divide forever
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