A strand of RNA (seen through an electron microscope)
Many scientists think that RNA was one of the earliest steps towards life on Earth. RNA is short for "ribonucleic acid", which is a very long, complicated molecule made out of amino acids linked together. The amino acids are made out of atoms of carbon, hydrogen, and a few other elements.
If this theory is right, then the beginning of life on earth would look something like this. When the planet Earth first formed, about four and a half billion years ago, amino acids from space were among the molecules that formed the Earth. More than four billion years ago, some of these amino acids were joining up to make larger molecules called purines and pyrimidines. Four of these big molecules were adenine, guanine, cytosine, and uracil. These four molecules combined to make very large molecules of ribonucleic acid, or RNA for short. RNA molecules are shaped like a spiral. All RNA molecules are made out of mainly these four molecules stuck together.
RNA was different from amino acid or purine or pyrimidine molecules in three very important ways: there were many different kinds of RNA molecules, because the molecule could build the purine molecules together in any order, and any RNA molecule could make another RNA molecule just like itself - it could reproduce, and in addition any RNA molecule could make certain proteins.
Model of an RNA molecule
The way RNA reproduces is that each molecule of RNA is shaped like a spiral staircase with one side missing. The vertical pillar of the staircase is made mainly of carbon and oxygen atoms (also some phosphate atoms). Sticking out from this vertical part are lots of horizontal steps. Each step is made of one of the four purines and pyrimidines: adenine, guanine, cytosine, or uracil. When the RNA runs into other purine or pyrimidine molecules, they join up to the steps of the staircase, each one matching up to the ones that are already there. Then they connect themselves together with carbon and oxygen atoms to make the other side of the staircase, and finally the staircase splits apart, making a new RNA molecule.
An RNA molecule makes a protein and lipids in the same way - by matching some of its purines or pyrimidines with other loose molecules that happen to be floating around. Originally, the RNA probably did this on its own, in the oceans. About three and a half billion years ago, RNA began to build proteins inside prokaryote cells. About one and a half billion years ago, when eukaryote cells evolved with specialized parts, the RNA began to do its work of building proteins and lipids mainly inside the endoplasmic reticulum.
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