Transcript Origins of Life – Chapter 21

Origins of Life – Chapter 21
• “Other” theories
– comets may have delivered organic
compounds
• probably will never prove how life began
• have narrowed down possibilities based
on assumptions
• likely that life successfully only arose
once under unique conditions
Haldane and Operin (1930s)
• hypothesized that amino acids, sugars, and
nucleotide bases could form spontaneously from
molecules of ammonia, methane, and water
under early (4.3 b.y.o) earth conditions
• pre-biotic earth probably had high temps., high
UV levels, reducing atmosphere (no O2), and
frequent lightning storms.
• finally proposed that: amino acids →
polypeptides → proteins → enzymes (all
happened very slowly)
4. Early “cells”
c
• protobionts (IB) v liposomes
coacervates
– tiny spheres that form spontaneously when
certain macromolecules (proteins and carbos.)
are put in water at pH~4.
– surround themselves with a layer that is
selective in admitting certain molecules
– increase in size and then divide
– for the first time—there is an outside and an
inside where molecules can be concentrated
within a protected environment and rate of
chemical reactions can increase!
Miller and Urey (1950s)
• simulated primitive earth in the lab
• mixed water, methane, ammonia, water
vapor, hydrogen, and then used electrical
discharges to simulate lightning
• after a week the found aldehydes,
carboxylic acids and 15 amino acids!
Four processes necessary for spontaneous
origin of life on Earth (scientists still working
on)
1. Synthesis of simple organic molecules
• the following monomers (“building-block”
molecules) have been synthesized in the lab
under primitive earth conditions:
– all nucleotide bases of DNA and RNA
– sugars
– amino acids
– most vitamins
2. Polymerization
• dehydration linkages between monomers is
necessary to make polymers and only possible
when in high concentrations or when enzymes are
present
– monomers could have been concentrated inside
of coacervates
– clay particles would act as catalysts for chemical
reactions by allowing organic molecules stick to
them
– RNA
• could have acted as a catalyst; some reactions in
ribosomes are still catalyzed by RNA
• Sidney Fox (1950’s) demonstrated that
polymerization of amino acids can occur
– hot areas (volcanoes/pools) could
concentrate amino acids in order to make
polypeptides
– in the lab, he made polymers of 200+ amino
acids called thermal proteinoids
• when placed in water they can cluster
together in bodies called proteinoid
microspheres that automatically form twolayer membranes, grow, and even take up
molecules from the surrounding
environment
When lipid and amino acid molecules mix in water under the right conditions, some interesting things
happen. Tiny bubbles, called coacervates or proteinoids, about the size of bacteria form. But these are
not just simple bubbles. They are surrounded by a double membrane similar to cell membranes. The
bubbles can both get larger, by adding more material from the surrounding water, and bud off sections
and divide. They may even be able incorporate amino acids and carry on simple chemical reactions
similar to those found in cells.
3. Self-replicating systems
– a self-replicating system is not possible with
simple molecules
– therefore, maybe RNA or DNA arose first...
and maybe they became surrounded by a
coacervate or protenoid microsphere (this
would be similar to viruses of today)
– RNA can replicate (very slowly) through base
pairing without enzymes (DNA needs
enzymes)
– so what came first—nucleic acids or proteins?
Need nucleic acids to make enzymes yet need
enzymes to make nucleic acids?
The Earliest Cells— Prokaryotes?
• first genes were probably encoded in RNA, not DNA
– chemically simpler
– in the lab, can get spontaneous polymerization of
RNA nucleotides
– finally DNA could have evolved from RNA with the
help of reverse transcriptase
• membranes could have been formed from
phospholipids that automatically form bilayers and
then into cell-like spheres called micells.
Possible scenarios....
• early autotrophic cells (phototrophic cells)
started using water as the source of hydrogen
and producing O2 as a waste product.
• many O2-sensitive organisms probably became
extinct
• most of this early O2 was “used up” by
oxidizing (rusting) metals such as Fe & S
• finally (after about 2 billion years) O2 was left
over enough to make the oxygen-rich
atmosphere of today
• life evolves to utilize the abundant O2