Download Intro to DNA - April 16

Genetics in ~1920:
1. Cells have chromosomes
Sketch of Drosophila chromosomes (Bridges, C. 1913)
Genetics in ~1920:
1. Cells have chromosomes
Sketch of Drosophila chromosomes (Bridges, C. 1913)
2. Specific locations on chromosomes control different phenotypes
Mutant phenotypes
Short
aristae
0
Black
body
48.5
Cinnabar
eyes
57.5
Vestigial
wings
67.0
Brown
eyes
104.5
Genetics in ~1920:
Chromosomes are made of:
DNA
Protein
Which one is the genetic material?
DNA: consists of nucleotides
5’
4’
3’
1’
2’
The structure of a nucleotide
DNA: consists of nucleotides (4 types)
5’
4’
3’
1’
2’
Pyrimidines
The structure of a nucleotide
Purines
DNA is a polymer
Sugar–phosphate
backbone
5 end
-Nucleotides are joined together by
linking 5’ and 3’ carbons of sugar
groups to phosphate groups
Nitrogenous
bases
Thymine (T)
- A chain of nucleotides has a 5’ end
and a 3’ end
Adenine (A)
Cytosine (C)
DNA nucleotide
Phosphate
Sugar (deoxyribose)
3 end
Guanine (G)
Protein: Consists of amino acids
Amino
group
Carboxyl
group
Protein: Consists of amino acids (20 different types)
Nonpolar
Glycine
(Gly or G)
Valine
(Val or V)
Alanine
(Ala or A)
Methionine
(Met or M)
Leucine
(Leu or L)
Trypotphan
(Trp or W)
Phenylalanine
(Phe or F)
Isoleucine
(Ile or I)
Proline
(Pro or P)
Polar
Amino
group
Carboxyl
group
Serine
(Ser or S)
Threonine
(Thr or T)
Cysteine
(Cys or C)
Tyrosine
(Tyr or Y)
Asparagine
(Asn or N)
Glutamine
(Gln or Q)
Electrically
charged
Acidic
Aspartic acid
(Asp or D)
Glutamic acid
(Glu or E)
Basic
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H)
A polypeptide
(a)
Side chains
Peptide
bond
Backbone
(b)
Amino end
(N-terminus)
Carboxyl end
(C-terminus)
Griffith (1928) Transformation of bacteria
Mixture of
heat-killed
Living S cells Living R cells Heat-killed
S cells and
(control)
(control)
S cells (control) living R cells
EXPERIMENT
RESULTS
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cells
Griffith (1928) Transformation of bacteria
Mixture of
heat-killed
Living S cells Living R cells Heat-killed
S cells and
(control)
(control)
S cells (control) living R cells
EXPERIMENT
RESULTS
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cells
Avery (1944) DNA is the transforming material
How could he have shown this?
Hershey + Chase (1952) T4 infection of E. coli
Phage
head
Tail
sheath
Tail fiber
Bacterial
cell
100 nm
DNA
Hershey + Chase (1952) T4 infection of E. coli
EXPERIMENT
Phage
Radioactive
protein
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Radioactive DNA
Batch 2:
radioactive
phosphorus (32P)
Hershey + Chase (1952) T4 infection of E. coli
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Radioactive DNA
Batch 2:
radioactive
phosphorus (32P)
Hershey + Chase (1952) T4 infection of E. coli
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Radioactivity
(phage
protein)
in liquid
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive DNA
Batch 2:
radioactive
phosphorus (32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Hershey + Chase (1952) T4 infection of E. coli
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Radioactivity
(phage
protein)
in liquid
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive DNA
Batch 2:
radioactive
phosphorus (32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Chargaff (1949)
02_UnTable01.jpg
1953: The double helix
Watson and Crick
Rosalind Franklin
Watson and Crick- 1953
Key aspects of the Watson-Crick model
- The 2 strands are in shape of a double helix
-10.5 base pairs per turn of the helix
5 end
Hydrogen bond
1 nm
3
end
3.4 nm
0.34 nm
(a) Key features of DNA
structure
3
end
(b) Partial chemical
structure
5
end
Data used to deduce double helix:
Sugar–phosphate
backbone
5 end
Nitrogenous
bases
1) Chemical structure of DNA polymer
2) Chargaff’s rules
Thymine (T)
3) Franklin’s X-ray diffraction data
Adenine (A)
Cytosine (C)
DNA nucleotide
Phosphate
Sugar (deoxyribose)
3 end
Guanine (G)
(b) Franklin’s X-ray diffraction
photograph of DNA
This told them:
-2 anti-parallel DNA strands
-Helical shape
-Width, period of helix
Key aspects of the Watson-Crick model
-2 anti-parallel strands of DNA
-Sugar-phosphate backbone on outside, bases on inside
-Bases form pairs through hydrogen bonding
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
0.34 nm
(a) Key features of DNA
structure
3
end
(b) Partial chemical
structure
5
end
Purine
Pyrimidine
Base pairing
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Why A and C can’t base pair:
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width consistent
with X-ray data
Watson and Crick- 1953
“It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material.”
Fig. 16-9-2
A
T
A
T
C
G
C
G
T
A
T
A
A
T
A
T
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
Fig. 16-9-3
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
T
A
T
A
T
A
T
A
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
(c) “Daughter” DNA molecules, each
consisting of one parental strand
and one new strand
Fig. 16-10
Parent cell
(a) Conservative
model
(b) Semiconserva- tive
model
(c) Dispersive model
First
replication
Second
replication
Fig. 16-11a
EXPERIMENT
1 Bacteria cultured in
medium containing
15N
2 Bacteria transferred
to medium
containing 14N
RESULTS
3 DNA sample
centrifuged after 20
min (after first
application)
4 DNA sample
centrifuged after 20
min (after second
replication)
Less
dense
More
dense
Fig. 16-11b
CONCLUSION
First replication
Conservative model
Semiconservative model
Dispersive model
Second replication