## What is DNA made of?

The basic structural unit of nucleic acids are nucleotides, which consist of a phosphoric acid residue, a specific sugar and nitrogenous bases (Figure 3.1). We distinguish between 2 types of nucleic acids, which differ at the nucleotide level. Deoxyribonucleic acid, called DNA, is usually a double-stranded molecule that contains the sugar deoxyribose, the nitrogenous bases adenine, thymine, cytosine, and guanine. RNA (ribonucleic acid) is usually a single-stranded molecule containing ribose. The arrangement of nitrogenous bases differs from DNA where thymine is replaced by uracil in RNA. Structurally, thymine and uracil are similar, but the production of uracil is less energy consuming. A relatively large number of RNA molecules are involved in various cellular processes, so it is advantageous to use less energy-demanding components. The presence of thymine in DNA ensures the necessary protection of the DNA molecules against damage that can occur in the cell nucleus, where most DNA molecules are located.

Individual nucleotides, placed one behind the other, are interconnected and thus form a DNA strand. The sequence (order) of nucleotides defines the genetic information, and is especially important. The connection of the two strands occurs through a hydrogen bond, which is formed between the nitrogenous bases of the nucleotides. The principle of base pairing is applied, according to which adenine pairs with thymine, while cytosine pairs with guanine (Figure 3.1). The complete genetic information of an individual is called the genome, which in humans consists of approximately 3.2 billion (3.2x109) base pairs. Only about a quarter of the base pairs contain information for the creation of products (RNA or proteins), and we refer to such sections as genes - genes encoding different types of RNA and genes encoding polypeptides from which proteins are formed. The human genome contains about 20,000 genes whose products are proteins, which represents about 1 to 2 % of the genome, and about 24 % of the genome is made up of genes for RNA.

All genetic information is organized into chromosomes. Human germ cells contain 22 autosomal chromosomes and the X or Y sex chromosome. The chromosomes differ from each other in size, shape and number of genes they contain. When a new individual is created by fertilisation, the germ cells of the parents fuse and a zygote is formed, which contains 23 pairs of chromosomes originating from both parents, with 22 pairs representing autosomes and one pair of sex chromosomes. The set of all chromosomes in the nucleus of a cell is called a karyotype (Figure 3.2). The total length of all the DNA located in the nucleus of one human cell exceeds one meter in length, although the largest of the chromosomes, which is chromosome 1, is only 10 µm long when viewed under a microscope. Thus, it is clear that the long DNA molecule undergoes several levels of packaging. The first level of packaging is represented by nucleosomes, in which DNA wraps around histone protein complexes. Subsequently, there is the formation of a chromatin fibre, which wraps around the protein scaffold of the chromosomes. In this way, DNA molecules shorten their length up to ten thousand times and form relatively small formations (spiralised chromosomes) that can be observed in the cell during division.