Transcript How Did Life Begin? And What is Life?

How Did Life Begin?
And What is Life?
Barry Selinsky
Chemistry Department
Villanova University
How was life established on
earth?
• God did it.
God School
God School – Final Written
Examination
As a community committed to the Augustinian
ideals of truth, unity and love, God School prides
itself on maintaining the highest standards of
academic integrity and does not tolerate any
forms of academic dishonesty or misconduct.
Accordingly, each student who takes an
examination is expected to sign the following
statement:
I, (sign your name)
have not had any unsanctioned prior access to this
examination and will conduct myself in an honest
manner in regard to all aspects of this
examination. Unless authorized, I will not discuss
the contents of this examination with other
students.
Final Thesis Project
• Create life.
• You may use any atomic materials up to
and including the element iron.
Periodic Table
Final Written Examination Continued
• You may also use
small molecules
consisting of no more
than 5 total atoms
and no more than 3
different elements.
That is, water and
HCN are ok; acetate
(CH3COOH) is not.
O
H
H
H
H
H
N
O
C
H
C
C
O
H
Final Written Examination Continued
• You may also assume
any environmental
conditions needed to
complete the
assignment.
Good news and bad news
• The bad news – there is no partial credit.
Either you make a living organism or you
don’t.
• The good news – you have 800 million
years to complete the exam!
Assumptions in our attempts to
determine how life began
• All of today’s organisms arose from a
single ancestor organism.
• Support – all organisms today share many
common features, proteins, and metabolic
pathways.
Evolution in pictures
Corollary to first assumption
• If multiple types of organisms developed
simultaneously (or nearly so), one type
dominated and eliminated the others.
What is the simplest life as we
know it?
• Bacteria – single cell organisms with the ability to
independently reproduce and generate new bacteria.
Bacteria are comprised of
numerous small molecules and four
classes of macromolecules:
• Nucleic acids (DNA and RNA)
• Proteins
• Polysaccharides (polymers of sugars)
• Lipids
King of Prussia Mall: A Multicellular
Organism
Costco – A Modern Single Celled
Organism
The General store – A primitive
single cell organism
Definitions of Life:
• Any population of entities which has the
properties of multiplication, heredity, and
variation. – John Maynard Smith
• Life is an expected, collectively self-organized
property of catalytic polymers. – Stuart
Kauffman
• Life possesses the properties of replication,
catalysis, and mutability. – Norman Horowitz
Definitions of life
• Life is a self-sustained chemical system
capable of undergoing Darwinian
evolution. – Gerald Joyce
• From this definition, what minimal
elements must life contain?
1. Life must be chemical
As a result, computer generated artificial
intelligence is excluded.
2. Life sustains itself by gathering atoms
and energy form its environment.
If it doesn’t eat, it’s not alive. Life requires
metabolism.
3. Living organisms must display variation.
No two people, bunnies, or bacteria are
necessarily the same. Life requires
reproduction and genetic variability.
Bacteria are comprised of
numerous small molecules and four
classes of macromolecules:
• Nucleic acids (DNA and RNA; polymers of
nucleotides)
• Proteins (polymers of amino acids)
• Polysaccharides (polymers of sugars)
• Lipids
How can we make these?
Miller’s experiments: A production
of amino acids under possible
primitive earth conditions.
Miller’s amino acid synthesis
H
H
H
C
H
H
H
+
C
+
N
H
H
N
H
H2O
H
H
H
O
C
H
O
H
methane
2:
ammonia
2:
hydrogen
1
glycine
(an amino acid
Oro’s purine synthesis
NH2
N
H
C
N

N
hydrogen cyanide
(poisonous gas)
N
H
N
adenine (forms adenosine
triphosphate (ATP), DNA, and RNA
Amino acids, sugars, purines, and
pyrimidines can be made from
simple precursors.
Next, is it possible to generate
polymers of these materials to
make biomacromolecules important
to life?
Polymerization on crystalline
templates
The chirality issue
Chiral molecules
Order out of chaos: the entropy
problem
• Second law of
thermodynamics:
In any spontaneous
process there is
always an increase in
the entropy of the
universe.
The theory of “emergence”
• While systems
become more
disordered with time,
local order and
complexity arise due
to the input of
energy.
The Iron-Sulfur World
• Hypothesis – life
began at
hydrothermal vents at
the bottom of the
ocean, fed by a
constant stream of
hot volcanic gas.
Energy from volcanic gas
Feo + H2S
FeS + H2 + energy
FeS + H2S
FeS2 + H2 + energy
The Reverse Citric Acid Cycle
CO2
Pyruvate
Acetyl-CoA
CoA
CO2
Citrate
Oxaloacetate
H2O
H2
Aconitate
Malate
H2 O
H2 O
Isocitrate
Fumerate
H2
H2
Succinate
CO2
Succinyl-CoA
CoA
2-Oxoglutarate
CO2
In support of the reverse citric acid
cycle:
• At hydrothermal vents, there is a constant
•
•
stream of hydrogen gas and energy generated
from the reaction of hydrogen sulfide with iron
sulfide.
The reactions can be catalyzed by iron sulfide,
pyrite, or other iron-sulfur complex.
In organisms, the reactions are catalyzed by
enzymes. Most enzymes require iron-sulfur
clusters to work.
Problems with the Iron-Sulfur world
• The iron sulfur world does not explain the
generation of genetic material, and does
not include reproduction or genetic
variability.
The RNA World
• Hypothesis: Life began as a collection of a
small number of self-reproducing catalytic
RNA molecules localized within a small
area.
What is RNA?
Evidence in support of the RNA
World
• Some RNA molecules can catalyze
chemical reactions (mostly done by
proteins).
• Some RNA molecules can replicate
themselves if ribonucleotides are present.
• This explains both metabolism and genetic
variability.
Problems with the RNA world
• Self-replicating RNA molecules have only
been made up to 14 nucleotides long.
Catalytic RNAs are estimated to require at
least 50 nucleotides.
• There is no known physical
encouragement for multiple self-replicating
RNA molecules to collect in the same local
area.
Thanks for living!