Fig. 16-8
Complementary Base Pairing
Showing hydrogen bonding
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Fig. 16-UN2
G
C
A
T
A
T
G
Sugar-phosphate
backbone
“sides of the ladder”
C
A
C
G
T
C
Nitrogenous
bases
“rungs of the
ladder”
G
Hydrogen bond
T
A
Fig. 16-7a
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
(a) Key features of DNA structure
Strands of DNA – run opposite
each other; this is called
antiparallel.
5 end
Fig. 16-5
Sugar–phosphate
backbone
5 end
Nitrogenous
bases
Carbon 1 – bonds to nitrogen base
Purines
•Adenine
•Guanine
Carbon 3 – bonds to next nucleotide
Carbon 5 – bonds to phosphate group
PurAsGold
Thymine (T)
Adenine (A)
Pyrimidines
•Cytosine
•Thymine
•Uracil
PyCUT
Cytosine (C)
DNA nucleotide
Phosphate
Sugar (deoxyribose)
3 end
Guanine (G)
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
• At first, Watson and Crick thought the bases
paired like with like (A with A, and so on), but
such pairings did not result in a uniform width
• Instead, pairing a purine with a pyrimidine
resulted in a uniform width consistent with the
X-ray
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Watson and Crick reasoned that the pairing
was more specific, dictated by the base
structures
• They determined that adenine (A) paired only
with thymine (T), and guanine (G) paired only
with cytosine (C)
• The Watson-Crick model explains Chargaff’s
rules: in any organism the amount of A = T,
and the amount of G = C
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 16.2: Many proteins work together in
DNA replication and repair
• The relationship between structure and
function is manifest in the double helix
• Watson and Crick noted that the specific base
pairing suggested a possible copying
mechanism for genetic material
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Semiconservative
The Basic Principle: Base Pairing to a Template
Strand
• Since the two strands of DNA are
complementary, each strand acts as a template
for building a new strand in replication
• In DNA replication, the parent molecule
unwinds, and two new daughter strands are
built based on base-pairing rules
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
end of codon)
end of codon)
First mRNA base (5
Third mRNA base (3
Fig. 17-5
Second mRNA base
Enzymes involved in DNA Replication
& Transcription
Enzyme
Function
Helicase
“molecular zipper” – unwinds double helix;
breaks hydrogen bonds that holds base
pairs together
Primase
Creates an RNA primer so DNA
polymerase will know where to start in
order to manufacture a new DNA strand.
DNA polymerase
Using a parent DNA strand, adds freefloating nucleotides (A, T, G, & C’s)
covalently to the new strand being
constructed.
ligase
“molecular glue” – joins fragments of the
New DNA strand together
RNA polymerase (used in transcription)
Uses one strand of DNA as a template to
construct mRNA – adds free-floating
nucleotide
Exonuclease
Removes RNA primers from the DNA
strand after replication has occurred, so
DNA polymerase can come in and fill in
the gaps with DNA nucleotides to finish
the process. Fixes mistakes on DNA
molecule.
Also can fix mutations that occur during
DNA replication.
Transcription & Translation
refer to diagram drawn on board
also watch animations:
DNAi.org
Khanacademy.org
Crash Course (youtube)
DNA – A Historical Perspective
1831-1836 – Charles Darwin, British naturalist - famous voyage on HMS Beagle. 1859 –
published famous book on the Origin of Species which reveals the idea of Evolution by
means of natural selection.
1858 – Alfred Wallace, – British biologist conducting field work in Malaysia. Sends a short
essay to Darwin with similar theory of evolution.
1865 – Gregor Mendel, Austrian monk – “Father of Heredity”
1869 – Johann Miescher (Swiss biochemist) – isolates DNA from WBC
1928 – Frederick Griffith – British Bacteriologist – discovers transformational factor
1944 – Oswald Avery et al. - Canadian-born American physician – shows that the
transformational factor was not a protein but DNA
1947 – Erwin Chargaff – Austrian biochemist – developed Chargaff’s ratios
1952 – Alfred Hershey & Martha Chase – provide conclusive evidence that DNA is the
transformational factor
1952 – Rosalind Franklin & Maurice Wilkins – use x-ray diffraction to analyze DNA
1953 – James Watson & Francis Crick construct double helix model of DNA
Johannes Friedrich Miescher 1844-1895
In 1869, first to isolate a substance
he called nuclein from the nuclei of
leucocytes or WBC
Collected these from pus he
obtained from bandages at nearby
hospitals.
He found that nuclein contained
phosphorus and nitrogen, but not
sulfur
Frederick Griffith 1871 - 1941
What is the transformational
factor??? Is it DNA or Protein???
Griffith’s research, working with
two strains of a bacterium, one
pathogenic and one harmless,
addresses this vital question
In 1941, Griffith was killed at work
in his London laboratory as a result
of an air raid in the London Blitz.
DNA – A Historical Perspective
Griffith and Transformation
1928 – British pathologist was researching
How certain types of bacteria produced pneumonia
He isolated 2 different strains: R which was harmless
and S - virulent
Live S-strain kills mouse
Injection of Rough Colonies ( R)
Results in Live Mice
Heat-killed Smooth colonies (S)
Result in Live Mice
Heat-Killed S + Live R =
Dead Mice
Fig. 16-2
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
Oswald Avery and DNA (1944)
Working along with Colin Macleod
& Maclyn McCarty
Repeated Griffith’s work with
modifications
Which molecule in the heat-killed
was the transformational factor?
The components of the Ground up
S were isolated, each mixed with R
and injected into mice
If the Heat-Killed S-strain’s DNA is destroyed with DNAase then R-strain can not be converted
to live S-strain. The gene to produce the capsule has been destroyed.
In 1952, Alfred Hershey and Martha
Chase performed experiments
showing that DNA is the genetic
material of a phage known as T2
To determine the source of genetic
material in the phage, they
designed an experiment showing
that only one of the two
components of T2 (DNA or
protein) enters an E. coli cell
during infection
They concluded that the injected DNA
of the phage provides the genetic
information
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-3
Phage
head
Tail
sheath
Tail fiber
Bacterial
cell
100 nm
DNA
Fig. 16-4-3
EXPERIMENT
Phage
Empty
protein
Radioactive
shell
protein
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
Fig. 16-6
(a) Rosalind Franklin
(b) Franklin’s X-ray diffraction
photograph of DNA
Erwin Chargaff (1905-2002)
and “Chargaff’s Rules”
The bases were not present in equal
quantities
They varied from organism to organism.
No matter where DNA came from —
yeast, people, or salmon — the number
of adenine bases always equaled the
number of thymine bases and the
number of guanine always equaled the
number of cytosine bases.
He published a review of his
experiments in 1950, calling the ratios
— which came to be known as
Chargaff’s Rules
Chargaff’s rules state that in any species there is
an equal number of A and T bases, and an
equal number of G and C bases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings