ust have been preceded by ribonucleic acid (RNA). The main reason for believing this is that in today's world RNA molecules provide the catalytic machinery and the information for the formation from amino acids into proteins. One could argue, though, that no protein could arise without the help of protein enzymes. That means protein makes RNA, which then makes protein again, which makes RNA, and so on. This is comparable to the famous "chicken or egg" problem. Crick's "Central Dogma" stated that information flows only from nucleic acids to proteins, never the other direction. This means RNA came before proteins. "Gilbert defines that RNA world as a stage in the early development of life in which 'RNA molecules and cofactors [were] a sufficient set of enzymes to carry out all the chemical reactions necessary for the first cellular structures." What is RNA? RNA molecules are long chainlike assemblages made of a large number - up to many thousands - of units called nucleotides. Each nucleotide consists of three parts: phosphate, ribose, which is a 5-carbon sugar, and a base, of which there are three different kinds - adenine, guanine, cytosine, and uracil. It is not known how RNA got to be on earth. The first RNA molecules were probably random association of nucleotides. As we know, RNA translates information from the DNA and that is how proteins are made. With this scenario one or more RNA variants interacted with amino acids in such a way that the amino acid became linked to the ribose end of the RNA molecule. This is how the primordial gene started the long evolutionary journey that eventually gave rise to transfer RNAs. This is how it is today. They join with amino acids in what happens to be the first step of protein synthesis. What happened next? All the RNA strings probably formed hundreds of "minigenes". They had to do base pairing, replication, splicing, and translation. The more complex they became the more "work" needed to be ...
