In another thread Oholiab wrote:
This is the most recent "gap" creationists try to put God into. God-of-the-gaps theology. The quote from ICR misstates the original Urey-Miller experiments. It ignores that there are lots of ways to get amino acids by different chemical reactions either on a primitive earth or on comets who can then dump the amino acids on earth.
12. MP Bernstein, SA Sandford, LF Allamandola, Life's far-flung raw materials. Scientific American 281: 42-49, July 1999. Astrochemistsshow that complex carbon compound, including amino acids, are present ininterstellar clouds.
15. J. P. Amend, E. L. Shock , Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems, Science 281: 1659 - 1662 ,11 Sep 1998.
The first cell can be formed with just amino acids. When amino acids are heated the chemical reaction forms proteins. If the proteins are in the presence of water, they spontaneously form cells.
http://www.siu.edu/~protocell/
http://www.theharbinger.org/articles/rel_sci/fox.html
Now, the interesting thing about these cells (called protocells to distinguish them from modern cells) is that they metabolize (including a primitive photosynthesis), respond to stimuli (they have membrane action potentials just like nerve cells), grow, and reproduce.
What is more, the proteins formed are also catalysts and catalyze the synthesis of sugars, lipids, new proteins, and RNA/DNA.
The "tricky part" is chirality. You can understand this by taking 4 toothpicks of different colors and putting them equally spaced in an orange or apple. Then take a second piece of fruit and put the toothpicks in it as the mirror image of the first. Same chemical structure. But you will find that you can't line up the two fruits so that the colors of all the toothpicks match. One version is called D and the other L for molecules.
Now, in modern life MOST amino acids are L and MOST sugars are D. Not all. You don't need to have all L amino acids to make a functional protein and you don't need all D sugars to make a RNA or DNA helix.
So, how did life get this way over time? There are several processes, all of which were probably operating and contributing.
As noted, preferential destruction of D amino acids and L sugars on comets and meteorites would produce an initial overabundance of the L amino acids and D sugars:
R Irion, Did twisty starlight set stage for life? Science 281:626-627, Jul. 31, 1998. Polarized UV light from stars can selectively destroy either D amino acids, setting the stage for chirality. Primary article is J Bailey, A chrysostomous, JH Hough et al., Circular polarization in star-formation regions: implicactions for biomolecular homochirality. Science 281: 672-674, Jul. 31, 1998.
Supplementing this is that homochiral proteins and RNA/DNA formed by chemistry are more stable:
Z Naturforsch [C] 1997 Jan;52(1-2):89-96
Plural origins of molecular homochirality in our biota Part II. The relative
stabilities of homochiral and mixed oligoribotides and peptides. Having an L-ribose would terminate an growing D-ribose DNA chain. So only the D-ribose chain could grow, thus selecting for homochirality.
And yes, proteins that replicate do make homochiral offspring:
Nature 2001 Feb 15;409(6822):797-801 A chiroselective peptide replicator. Saghatelian A, Yokobayashi Y, Soltani K, Ghadiri MR.
"The origin of homochirality in living systems is often attributed to the generation of enantiomeric differences in a pool of chiral prebiotic molecules, but none of the possible physiochemical processes considered can produce the significant imbalance required if homochiral biopolymers are to result from simple coupling of suitable precursor molecules. This implies a central role either for additional processes that can selectively amplify an initially minute enantiomeric difference in the starting material, or for a nonenzymatic process by which biopolymers undergo chiroselective molecular replication. Given that molecular self-replication and the capacity for selection are necessary conditions for the emergence of life, chiroselective replication of biopolymers seems a particularly attractive process for explaining homochirality in nature. Here we report that a 32-residue peptide replicator, designed according to our earlier principles, is capable of efficiently amplifying homochiral products from a racemic mixture of peptide fragments through a chiroselective autocatalytic cycle. The chiroselective amplification process discriminates between structures possessing even single stereochemical mutations within otherwise homochiral sequences. Moreover, the system exhibits a dynamic stereochemical 'editing' function; in contrast to the previously observed error correction, it makes use of heterochiral sequences that arise through uncatalysed background reactions to catalyse the production of the homochiral product."
Not myths, but observations. The "experience" that Malthus said formed the basis of knowledge.
This is the most recent "gap" creationists try to put God into. God-of-the-gaps theology. The quote from ICR misstates the original Urey-Miller experiments. It ignores that there are lots of ways to get amino acids by different chemical reactions either on a primitive earth or on comets who can then dump the amino acids on earth.
12. MP Bernstein, SA Sandford, LF Allamandola, Life's far-flung raw materials. Scientific American 281: 42-49, July 1999. Astrochemistsshow that complex carbon compound, including amino acids, are present ininterstellar clouds.
15. J. P. Amend, E. L. Shock , Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems, Science 281: 1659 - 1662 ,11 Sep 1998.
The first cell can be formed with just amino acids. When amino acids are heated the chemical reaction forms proteins. If the proteins are in the presence of water, they spontaneously form cells.
http://www.siu.edu/~protocell/
http://www.theharbinger.org/articles/rel_sci/fox.html
Now, the interesting thing about these cells (called protocells to distinguish them from modern cells) is that they metabolize (including a primitive photosynthesis), respond to stimuli (they have membrane action potentials just like nerve cells), grow, and reproduce.
What is more, the proteins formed are also catalysts and catalyze the synthesis of sugars, lipids, new proteins, and RNA/DNA.
The "tricky part" is chirality. You can understand this by taking 4 toothpicks of different colors and putting them equally spaced in an orange or apple. Then take a second piece of fruit and put the toothpicks in it as the mirror image of the first. Same chemical structure. But you will find that you can't line up the two fruits so that the colors of all the toothpicks match. One version is called D and the other L for molecules.
Now, in modern life MOST amino acids are L and MOST sugars are D. Not all. You don't need to have all L amino acids to make a functional protein and you don't need all D sugars to make a RNA or DNA helix.
So, how did life get this way over time? There are several processes, all of which were probably operating and contributing.
As noted, preferential destruction of D amino acids and L sugars on comets and meteorites would produce an initial overabundance of the L amino acids and D sugars:
R Irion, Did twisty starlight set stage for life? Science 281:626-627, Jul. 31, 1998. Polarized UV light from stars can selectively destroy either D amino acids, setting the stage for chirality. Primary article is J Bailey, A chrysostomous, JH Hough et al., Circular polarization in star-formation regions: implicactions for biomolecular homochirality. Science 281: 672-674, Jul. 31, 1998.
Supplementing this is that homochiral proteins and RNA/DNA formed by chemistry are more stable:
Z Naturforsch [C] 1997 Jan;52(1-2):89-96
Plural origins of molecular homochirality in our biota Part II. The relative
stabilities of homochiral and mixed oligoribotides and peptides. Having an L-ribose would terminate an growing D-ribose DNA chain. So only the D-ribose chain could grow, thus selecting for homochirality.
And yes, proteins that replicate do make homochiral offspring:
Nature 2001 Feb 15;409(6822):797-801 A chiroselective peptide replicator. Saghatelian A, Yokobayashi Y, Soltani K, Ghadiri MR.
"The origin of homochirality in living systems is often attributed to the generation of enantiomeric differences in a pool of chiral prebiotic molecules, but none of the possible physiochemical processes considered can produce the significant imbalance required if homochiral biopolymers are to result from simple coupling of suitable precursor molecules. This implies a central role either for additional processes that can selectively amplify an initially minute enantiomeric difference in the starting material, or for a nonenzymatic process by which biopolymers undergo chiroselective molecular replication. Given that molecular self-replication and the capacity for selection are necessary conditions for the emergence of life, chiroselective replication of biopolymers seems a particularly attractive process for explaining homochirality in nature. Here we report that a 32-residue peptide replicator, designed according to our earlier principles, is capable of efficiently amplifying homochiral products from a racemic mixture of peptide fragments through a chiroselective autocatalytic cycle. The chiroselective amplification process discriminates between structures possessing even single stereochemical mutations within otherwise homochiral sequences. Moreover, the system exhibits a dynamic stereochemical 'editing' function; in contrast to the previously observed error correction, it makes use of heterochiral sequences that arise through uncatalysed background reactions to catalyse the production of the homochiral product."
Not myths, but observations. The "experience" that Malthus said formed the basis of knowledge.
