CELL

Every living system consists of one or more cells. Vertebrates, including humans, are complex organisms consisting of several tissues formed by individual cells. Tissues include, for example, muscles, skin, and the nervous system.

A cell has many different parts, each with its own special function in maintaining the activities of the cell. The heart of the cell is its nucleus, which contains DNA (deoxyribonucleic acid), storing all the information needed to direct the activities of the cell.

DNA

A DNA strand is built of small molecules called nucleotides. There are four different types of DNA nucleotides that are called by their one letter symbols: A, T, G, and C. Thus, a piece of DNA could look something like this: AGATTCAAAAGGGCC. Human beings differ from each other only by small changes in the nucleotide structure. In the previous example, another person could have G instead of A as the first nucleotide. DNA is organised in chromosomes, of which human beings have 46 in total (23 chromosomes from both parents). The number of chromosomes and genes varies greatly between species – for example roundworm has 13000 genes and dogs have 78 chromosomes. A human genome of 23 chromosomes contains over 3.2 billion nucleotides (A, T, G, or C) and approximately 21000 genes.

GENE

A gene is a segment of DNA that can serve as a template for a functional protein. Humans have approximately 21000 genes, and all of them are included in every cell of the body (except for red blood cells). Only approximately 2% of the whole DNA is genes. The DNA strand between the genes is used by the machinery and different proteins that are involved in activating or silencing genes.

FROM GENE TO PROTEINS

Genes vary by their length and the number of introns (non-coding segments) and exons (protein coding segments). When a specific protein is needed, a respective gene is transcribed into RNA (ribonucleic acid), which is further processed so that introns are cut away. Thus the main structure of a protein is dependent on the nucleotide structure of exons. One gene may result in several different proteins depending on what exons are retained in the final RNA molecule.