The mixture was kept circulating by continuously boiling and then condensing the water. The gases passed through a chamber containing two electrodes with a spark passing between them. At the end of a week, Miller used paper chromatography to show that the flask now contained several amino acids as well as some other organic molecules. In the years since Miller's work, many variants of his procedure have been tried. In solution, hydrolysis of a growing polymer would soon limit the size it could reach.
Abiotic synthesis produces a mixture of L and D enantiomers.
Each inhibits the polymerization of the other. Link to a discussion of enantiomers. All metabolism depends on enzymes and, until recently, every enzyme has turned out to be a protein. The synthesis of DNA and RNA requires proteins. So proteins cannot be made without nucleic acids and nucleic acids cannot be made without proteins.