Genes and gene products

In the middle of the DNA ladder are located the base pairs, whose sequence depicts the genetic information. The sequence of the bases can be read like a text that provides instructions for making gene products. Thus, a gene is a part of the DNA that is read off and translated into a molecule, mainly a protein. Humans have between 25,000 and 30,000 genes, whose size vary between a few hundred DNA bases up to more than 2 million bases.

In the first step of making gene products, the DNA double helix is opened between the base pairs. Then the sequence of the bases is transcribed into a molecule called RNA (ribonucleic acid). Like DNA, RNA is build up chain-like and consists of a sugar-phosphate-backbone with attached bases. But RNA is only single stranded and distinguishes from DNA in the sugar, which is ribose instead of desoxyribose. Additionally, the base thymine is replaced by the similar base uracil.

RNA that contains the instruction for making a protein (mRNA), carries the information from the DNA out of the nucleus into the cytoplasm (the fluid that surrounds the nucleus), where the sequence is translated into the protein. Here the mRNA interacts with a specialized complex called ribosome, which reads the sequence of the bases. Each sequence of three bases, called a codon, usually codes for one particular of 20 amino acids. There are also codons that determine the beginning and the end of the translation.

At the ribosome, the amino acids are stringed together in collaboration with the tRNA, that carries the amino acids to the ribosome. The resulting chain can consist from a few to more than 1000 amino acids and folds into a unique three dimensional structure, the protein. Proteins can either act alone or as a (sub)unit of other molecules and are required for the structure, function and regulation of the body's tissues and organs.

A process called alternative splicing, that can occur in the nucleus of eukaryotes after transcription of the gene, leads to different proteins from a single gene. When the mRNA has been transcribed from the DNA, it includes sections which can be excised (introns). The remaining sections (exons) are assembled and translated into protein. Due to the possibility to alternatively excise sections of the mRNA, a few genes can encode a high number of proteins.

There exist also direct gene products which don't need the translation from the mRNA into proteins. These direct gene products are e.g. the rRNA, which is part of the ribosome, and the tRNA.