Genes
Genes are functional units of the genetic material in the chromosomes of each cell. This material is desoxyribonucleic acid, DNA. Its structure looks like an intertwined ladder, the double helix. It was detected by James Watson and Francis Crick in 1953, 50 years ago. The two backbones or strands of the ladder are long chains of phosphoric acid and desoxyribose, a kind of sugar. The rungs consist of four different chemical substances, the bases or genetic letters: adenine, guanine, thymine, and cytosine, abbreviated A, G, T, and C, two of which always face each other in one rung of the helix. For spatial reasons, the rungs can only contain the pairs A-T or G-C. If the sequence of these bases on one strand is e.g.
---AGGCTTAATCGT---
the sequence on the opposite strand must be
---TCCGAATTAGCA---
i.e., the sequences are complementary to each other.
Each of the about 100 trillion (100 x 1012) body cells in a human being contains in its nucleus 46 chromosomes with a total of more than 6 billion genetic letters, grouped in about 25,000 to 35,000 genes. Almost all details of the sequence of these letters are now known. It is the genetic information, which is passed on from generation to generation with very little changes or mutations. These mutations, which were necessary for the evolution of all living beings, can also have negative consequences as, e.g., hereditary diseases.
Most of the genes carry the information for the construction of one or more proteins, which consist of amino acids. The sequence of the amino acids, of which there are 20 different kinds, is important for the function of the proteins such as enzymes, the catalysts for biochemical reactions in the body, as regulators for other genes, or as structural material.
In the cell nucleus, where the chromosomes reside, the genetic information of the genes is copied or transcribed to another genetic substance of a similar structure, the pre-messenger ribonucleic acid, pre-mRNA. The genes of multicellular organisms consist of active sections, exons, and inactive ones, introns. After the transcription, the introns, which are often much longer than the exons, are removed from the pre-mRNA, and the transcribed exons spliced together to the messenger RNA, mRNA, which is then exported to the ribosomes, the protein synthesizing structures in the cytoplasma of the cell. In the ribosomes, catalytic acting RNAs, ribozymes, use the genetic information of the mRNA to construct specific proteins out of amino acids which are delivered to the ribosomes by another kind of RNA, the transfer RNAs or tRNAs.
The RNAs use the base U, uracil, instead of the very similar base T of the DNA. In the mRNA, three consecutive genetic letters always signify one of the 20 different amino acids according to a genetic dictionary, the genetic code, which is the same for all life on earth. Thus, the genetic script uses only four letters, and its words, the codons, are always three letters long, triplets. There are no spaces between the words, and three different stop codons exist, UAA, UAG, and UGA, where the protein synthesis is terminated.