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”

• protobionts (IB) 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!

cv

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 two-layer 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