Transcript DNA Structure and history10

DNA
The Genetic Material
2006-2007
Chromosomes related to phenotype
• T.H. Morgan
– working with Drosophila
– associated phenotype with specific
chromosome
• white-eyed male had specific X
chromosome
Genes are on chromosomes
• Morgan’s conclusions
– genes are on chromosomes
– but is it the protein or the DNA ?
The “Transforming Principle”
• Frederick Griffith
– Streptococcus pneumonia bacteria
– Harmless bacteria (“rough”) + heat-killed disease
bacteria (“smooth”) = fatal disease in mice
– Something passed from dead bacteria to live bacteria
to change their phenotype
• “Transforming Principle”
The “Transforming Principle”
live pathogenic
strain of bacteria
A.
mice die
live non-pathogenic
strain of bacteria
B.
mice live
heat-killed
pathogenic bacteria
C.
mice live
mix heat-killed
pathogenic &
non-pathogenic
bacteria
D.
mice die
Transformation = change in phenotype
something in heat-killed bacteria could still transmit
disease-causing properties
DNA is the “Transforming Principle”
• Avery, McCarty & MacLeod
– purified DNA & proteins from Streptococcus bacteria
– injected protein into bacteria
• no effect
– injected DNA into bacteria
• transformed harmless bacteria into
virulent bacteria
mice die
Confirmation of DNA
• Hershey & Chase
– “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
Protein coat labeled
with 35S
DNA labeled with 32P
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
Hershey
& Chase
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
Chargaff
• DNA composition: “Chargaff’s rules”
– varies from species to species
– all 4 bases not in equal quantity
– bases present in characteristic ratio
• humans:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
Structure of DNA
• Watson & Crick
– developed double helix model of DNA
• other leading scientists working on question:
– Rosalind Franklin
– Maurice Wilkins
– Linus Pauling
Franklin
Wilkins
Pauling
1953 article in Nature
Watson and Crick
Watson
Crick
Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic material.”
Watson & Crick
Directionality of DNA
• You need to
number the
carbons!
nucleotide
PO4
N base
– it matters!
5 CH2
O
4
1
ribose
3
OH
2
5
The DNA backbone
PO4
• Putting the DNA backbone together
base
5 CH2
O
– refer to the 3 and 5 ends of the
DNA
• the last trailing carbon
4
1
C
3
O
–O P O
O
5 CH2
2
base
O
4
1
2
3
OH
3
Anti-parallel strands
• DNA backbone is bonded from phosphate
to sugar between 3 & 5 carbons
5
3
3
5
– DNA molecule has “direction”
– complementary strand runs in
opposite direction
Bonding in DNA
hydrogen
bonds
5
3
covalent
phosphodiester
bonds
3
5
Base pairing in DNA
• Purines
– adenine (A)
– guanine (G)
• Pyrimidines
– thymine (T)
– cytosine (C)
• Pairing
– A:T
• 2 bonds
– C:G
• 3 bonds
But how is DNA copied?
• Replication of DNA
– base pairing allows each side to
serve as a template for a new
strand
Models of DNA Replication
• Alternative models
– become experimental predictions
conservative
P
1
2
semiconservative
dispersive
Semiconservative replication
• Meselson & Stahl
– label “parent” nucleotides in DNA strands with
heavy nitrogen = 15N
– label new nucleotides with lighter isotope = 14N
parent
15N/15N
15N
parent
strands
replication
Predictions

14N/14N
1st round of
replication
15N/15N
conservative
2nd round of
replication
15N/15N
1
15N/15N
2 15N parent
strands
semiconservative
15N/14N
dispersive
 
14N/14N
P
15N/14N
conservative
14N/14N
15N/14N
semiconservative
15N/14N
dispersive