Download DNA structure

DNA contains the instructions (codes)
for making all the proteins in the body.
1)
2)
3)
4)
Hershey-Chase Experiment
Chargaff’s Rules
X-Ray Crystallography Evidence
Discovery of the Double Helix
1952 Alfred Hershey & Martha Chase (USA)
Phage viruses only have protein & DNA;
tested to see which is injected into host
Concludes DNA is the genetic material.
1950 Erwin Chargaff (USA)
Discovered that the
amounts of bases A & T
(adenine & thymine)
were roughly equal, as
were amounts of C & G
(cytosine & guanine).
Also found amounts of
bases varied in species.
1950’s Maurice Wilkins
& Rosalind Franklin (UK)
Used X-rays to get
images of molecular
structures in 3D
Photograph 51
(Franklin) evidence
that DNA has
helical structure
1953: James Watson (USA) & Francis Crick (UK)
Used models to propose structure of DNA.
Heavily influenced by fellow scientists.
Concluded that DNA must
be a double helix, bases
paired A-T & G-C, and the
strands must be antiparallel to keep shape.
DNA is a nucleic acid;
a very long polymer.
Monomer unit is a
nucleotide.
DNA shape is like a
twisted ladder/zipper.
A.K.A. Double Helix
DNA has four different bases:
•
Cytosine C
• Thymine T
• Adenine A
• Guanine G
O
O -P O
O
O
O -P O
O
One deoxyribose sugar together
with a phosphate group and a
nitrogenous base
O
O -P O
O
Phosphate
Nitrogenous
base
O
C
C
C
O Deoxyribose
• The backbone of a
single DNA strand is
alternating
phosphates and
5- carbon sugar
(deoxyribose )
• The rungs/teeth are
nitrogenous bases.
phosphate
nucleotide
deoxyribose
bases
DNA has two
strands that fit
together something
like a ladder/zipper.
The strands are
anti-parallel
The rungs/teeth are
nitrogenous bases
Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
N
C
N
C
cytosine
Adenine and guanine each have two rings
of carbon and nitrogen atoms.
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
N
C
N
N
C
C
N
C
C
C
C
N
N
N
C
O
Hydrogen bonds are weak
but there are millions and
millions of them in a single
molecule of DNA.
C
The bases attract each other
because of hydrogen bonds.
N
O
• Each cell has about 2 m of
DNA.
• The average human has 75
trillion cells.
• The earth is 150 billion m
from the sun
• The average human has
enough DNA to go from the
earth to the sun more than
400 times!