In detail

The DNA (deoxyribonucleic acid)

Structure of deoxyribonucleic acid

The DNA (deoxyribonucleic acid, sometimes referred to as DNA in DE) is a nucleic acid molecule. Already in 1943, Oswald Avery made DNA as the site of genetic material in experiments with bacteria. However, it was unclear what the structure of the DNA would look like. In a scientific race for DNA molecular structures, James Watson and Francis Crick found that deoxyribonucleic acid was a double helix. For this discovery, they were awarded the 1962 Nobel Prize.
Essentially, there are three components that make up the individual nucleotides:
a Phosphate acid residue
a Monosaccharide with 5 C atoms (Pentose)
one organic base: The purines adenine and guanine, as well as the pyrimidines cytosine and thymine
The five carbon atoms are numbered from 1 'to 5'. At the first (1 ') carbon atoms, the respective base is bound. The last (5 ') carbon atom links the phosphoric acid residue to the pentose. In a Polynukleotidstrang, so a strand of many individual nucleotides, connects the pentose with the sugar of the fifth carbon atom (5 ') of the next pentose on the third carbon atom (3').
In this way, one ends with a polynucleotide strand: one 5'end and one 3'end. The difference between 5 'and 3' end is essential in replication, because only at the 3 'end new nucleotides can be deposited.
The DNA double helix consists of two entwining DNA single strands, with opposite directions of the 3 'and 5' ends. Stability is achieved by the stacking interactions of successive bases and not, as one would expect, by the hydrogen bonds.
Adenine always associates with thymine and cytosine with guanine (see picture). From this two rules can be derived, which always apply:
Adenine and thymine are present in a 1 to 1 ratio
Cytosine and guanine are present in a 1 to 1 ratio
It seems logical, because the respective base can only connect with its complementary tag. On the other hand, no statements can be made about the ratio of e.g. Adenine and guanine.
The individual sequence of base pairs determines the expression of all our phenotypic traits (body size, eye color, hair, etc.) using only the four bases adenine, thymine, cytosine and guanine. How this works in detail, you can read in gene expression (or protein biosynthesis).