The genetic information of every living organism is contained in its chromosomes. These are microscopically small filamentous components of every cell, whose structure and location vary and constitute one of the fundamental differences between viruses (not counted as living organisms), bacteria and archaea (prokaryotes, single-celled organisms without a cell nucleus), and plants, animals and fungi (eukaryotes, living organisms with a cell nucleus in the cell).
The chromosomes of viruses can consist of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). In prokaryotic unicellular organisms, only one ring-shaped chromosome made of DNA is found and this lies freely in the cell. In contrast, animals, plants and fungi have several chromosomes, often even a species-specific number of them. These consist mainly of DNA, but can also contain RNA and are located in the cell nucleus. This means they are enclosed by the membrane of the cell nucleus.
The chromosomes are nothing more than the tightly packed version of the long DNA molecule they are made of. If, for example, the 46 human chromosomes were to be unpacked, their genetic material in the form of DNA strands would come to a total length of about two metres. Each DNA molecule, in turn, looks like a twisted rope ladder. It consists of two parallel, interconnected polynucleotide chains. The backbone chains (the “ladder’s” outer sides) consist of sugars and phosphates; the “rungs” consist of two interconnected nucleotide bases each, with exactly four different bases occurring in each strand, each of which combines with only one other to form a base pair – adenine (A) with thymine (T) and guanine (G) with cytosine (C). Because its two nucleotide strands wind around each other like a helix, the DNA molecule is also called a double helix.
A gene is a specific section of the DNA molecule that stores specific genetic information. Different species vary in the number of genes in their DNA. The human genome consists of an estimated 30,000 genes, while scientists have identified precisely 5416 genes in the bacterium Escherichia coli O157:H7.
If in the cell the information of a gene is needed for a certain process, the DNA double-strand splits at the relevant section. Free complementary RNA nucleotides from the cell now attach themselves to the exposed nucleotide bases of the DNA molecule: adenine to thymine, guanine to cytosine and vice versa. The only difference is that in RNA uracil replaces thymine as the complementary base to adenine. In this manner, the RNA nucleotides copy the DNA information and then, as messenger RNA (mRNA), migrate to the part of the cell where the information is needed.
There, in a process called translation, the mRNA is translated into a sequence of amino acids from which proteins are then produced and cellular processes can be set in motion. For this reason, mRNA is also called bioactive.
fig. 7.2 > A human’s genetic information is stored in the cell nucleus, or more precisely in the 46 chromosomes whose individual DNA strands, strung together, would come to a length of two metres.