What is RNA? Cell biology and evolution

RNA: black background with a twisty white line on it

RNA molecule: A single strand of RNA, seen under an electron microscope

The RNA molecule was probably one of the earliest steps towards life on Earth. RNA is short for “ribonucleic acid”. That’s 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.

The beginning of life on earth might have been 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.

Five hundred million years later, still 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. All RNA molecules are made out of mainly these four molecules stuck together. RNA molecules are shaped like a spiral.

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.
any RNA molecule could make another RNA molecule just like itself – it could reproduce
any RNA molecule could make certain proteins.

Like one side of a spiral ladder with rungs sticking out

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 spiral 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 loose purine or pyrimidine molecules, they join up to the steps of the staircase. Each molecule matches 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. Finally the staircase splits apart like opening a zipper, making a new RNA molecule.

An RNA molecule makes a protein and lipids in the same way. It matches 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. Then 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.