DNA: The Secret of Life
Jan 8, 2008
Genetics and Society
Biol/ Phil 2510
DNA: The Secret of Life
•What is Life?
•History of the question
•DNA: the modern answer
Questions everyone asks
Who am I?
Where did I come from?
How are we different from each other?
Will I get cancer?
What will my children look like?
Who was in this room an hour ago?
Why do I love who I love?
Feb 28, 1953
Two guys walk into a bar and say they have the answer.
But let’s start at the beginning
QuestionWhat is Life?
Vitalism
Vitalism is the idea that living
organisms possess an inner force
or energy that gives them the
property of life.
Vitalists say that there is a
property that distinguishes life
from non-life.
Can you think of an example
of vitalist thought you have
heard of?
Materialism
Materialism says life has a physical
basis.
Living matter is made of the same
components as non-living matter.
Materialism in the 1800s
1828 Friedrich Wohler synthesizes urea from inorganic
materials
Substances normally produced by living
organisms are chemical compounds that can be
made by non-living sources
1897 Hans and Eduard Buchner show that fermentation
can be accomplished in a cell free system.
Fermentation, a reaction carried out by living
organisms, is a chemical, not a vital process
Heredity
Around 1900, the
debate begins to
focus on heredity.
Why are pandas not
people?
Why are pandas not people?
How would a vitalist answer this question?
How would a materialist?
Mendel’s laws show the rules
of heredity
(1866, rediscovered in 1900)
Inheritance occurs in packets
of information
Properties of a physical unit
•You can see it
•You can move it from one place to
another
•You can change it with other physical
processes
Developments in the 20th
century
1933: Thomas Hunt Morgan wins the Nobel prize
for demonstrating that chromosomes are the
fundamental unit of inheritance.
Demonstrated that here is an observable,
physical nature to cellular information
http://www.biol.unlp.edu.ar/historianobel-genetica.htm
http://www.cofc.edu/~bernardoj/Genetics%20Lab/212Lhome.html
The experiment
X
Breed mutant flies
Look at their
chromosomes
Observed
Traits can often be inherited together.
There are the same number of trait groups as
there are chromosomes.
In rare cases when chromosomes are not
inherited normally, the traits are also not inherited
normally.
Inheritance is on chromosomes- things
you can see in a microscope.
Properties of a physical unit
•You can see it
•You can move it from one place to
another
•You can change it with other physical
processes
Developments in the 20th
century
1944: Avery, McCarty and McLeod published a
paper that stated "Deoxyribonucleic acid (DNA)
plays a central role in determining specific
characteristics in the course of reproduction. "
There is a chemical substance, DNA, that holds genetic
information
http://genetics.gsk.com/history.htm
The experiment
+
Pneumonia
Bacteria (S)
The experiment
+
Pneumonia
Bacteria (R)
The experiment
+
Pneumonia
Bacteria (R)
Plus
DNA from
Pneumonia
Bacteria (S)
DNA alone is enough to
make R into S
Properties of a physical unit
•You can see it
•You can move it from one place to
another
•You can change it with other physical
processes
Developments in the 20th
century
1946: Hermann Müller wins a Nobel Prize for
his work on the genetic effects of radiation.
The information can be changed through
physical processes
http://nobelprize.org/nobel_prizes/medicine/laureates/1946/muller-bio.html
The experiment
X-rays
Broken
chromosomes
Mutant
chromosomes
Mutant offspring
Properties of a physical unit
•You can see it
•You can move it from one place to
another
•You can change it with other physical
processes
Heredity has a physical basis
Inheritance is on chromosomes.
You can see them.
DNA can carry new traits into an organism.
You can hold it in your hand.
Traits can be changed by physical processes.
You can change it by hitting it with a hammer.
The BIG Question
How does DNA make an
organism?
Surely there will be something
more to DNA than chemistry.
Vitalism??
What did we know about
DNA?
•It contains sugar, phosphate and nitrogencontaining bases
•There are 4 bases- A, C, G, T
Rosalind
Franklin
The Experiment
X-rays
DNA
crystal
Photographic
film
X-ray diffraction from DNA
crystal
Showed that DNA is a
double helix
Showed that the bases
are on the inside
More info on this at:
http://www.pbs.org/wgbh/nova/photo51/
Francis Crick and Jim Watson (1953)
The Experiment
Watson-Crick DNA Model (1953)
5'
3'
•Antiparallel double helix
•DNA bases in the
middle
•Sugar Phosphate
backbone running
along the outside
“Watson”
“Crick”
5'
3'
Watson-Crick DNA Model (1953)
•Antiparallel double helix
•DNA bases in the
middle
•Sugar Phosphate
backbone running
along the outside
•Bases are paired with
each other
What was the secret of life?
DNA bases can only pair one way
A T
G C
This is called complementarity
Complementarity
Complementarity allows the
hereditary information to be copied
digitally.
Question: Why is digital copying an
advantage?
DNA can be very accurately
copied
There have been hundreds of billions of cell divisions
since you were a single fertilized egg.
How many cell divisions have happened on Earth in
the last hour?
DNA can be very accurately
copied
This DNA sequence is still recognizable since the last
common ancestor of these animals.
Why is this a big deal?
When you know where the information
is stored, you can:
• Read it
Why is this a big deal?
When you know where the information
is stored, you can:
• Read it
•Change it
The chemical structure of
DNA is yet another blow to
vitalism
What Next?
Francis Crick
Francis Crick worked in his later years to understand the
neurobiological basis of consciousness.
“You’, your joys and your sorrows, your
memories and your ambitions, your sense of
personal identity and free will, are in fact no
more than the behavior of a vast assembly of
nerve cells…”
How does this relate to
vitalism?
1916-2004
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0020419
Postscript
Watson, Crick and
Wilkins
Nobel Prize 1962
Where was Rosalind Franklin?
Rosalind Franklin (1920 - 1958)
a feminist icon
a flawed perfectionist
Video:
Secret of Photo 51
http://www.pbs.org/wgbh/nova/photo51/
Call number: QH506.F72 S43 2003
Video:
DNA: The Secret of Life
http://www.pbs.org/wnet/dna/index.html
Call number: QP-624-D12-2004