The genetic code
It is now important how this genetic information is translated into amino acids from which the proteins are built, which are the essential elements in every cell and every organism. When this question was investigated, it was already known that an RNA was first copied from the DNA strand with the genetic information for a protein. During this transcription, a C on the DNA becomes a G on the RNA, a T becomes A and the G becomes C. Only the A is not simply transformed into a T, but into the nucleotide uracil (U). This RNA strand is then translated into a chain of different amino acids – and this chain is the basic structure of the resulting protein. What was not known was the code that translated the four nucleotides into the 22 amino acids known today. According to abbreviationfinder.org, RNA stands for Ribonucleic acid.
The German biochemist Heinrich Matthaei (born 1929) and his US colleague Marshall Nirenberg (1927–2010) solved this puzzle. On May 21, 1961, they found out that exactly three nucleotides of an RNA strand stand for a certain amino acid: If three U’s follow one another, the cell translates this into the amino acid phenylalanine. If, on the other hand, a C replaces the middle nucleotide, the group of three UCU stands for the amino acid serine. The translators of the genetic code into amino acids are the so-called ribosomes, the protein factories in the cells.
They also attach the amino acids to one another, so that the protein is largely ready.
In the following years, many researchers tried to find out what DNA and RNA look like and how they manage to pass on genetic information. Erwin Chargaff (1905–2002) from Columbia University in New York took science a big step further: From studies by other researchers, the scientist knew that DNA is composed of several basic building blocks. So-called adenine (A), cytosine (C), guanine (G) and thymine bases There are (T) in it as well as sugar and phosphate groups. How these molecules are connected, however, nobody knew until Erwin Chargaff noticed that the DNA of each species contains the same amounts of adenine and thymine and that the same amount of cytosine and guanine is always present. The researcher suspected that the bases would appear in pairs.
The British Rosalind Franklin (1920–1958) and the New Zealander Maurice Wilkins (1916–2004) used this knowledge to try to produce DNA crystals. It was recognized that not only salts can form crystals, but also nucleic acids, for example, if they are isolated in a very pure form. Such crystals can be used to produce X-ray images from which the arrangement of individual atoms and groups of molecules in a substance can be determined. In 1952, Rosalind Franklin obtained photographs on which she could recognize the basic structure of DNA: two, three or four DNA threads wound spirally around each other; the sugar and phosphate content of the molecules hung on the outside, while the bases A, C, G and T were on the inside.
Two theorists, the Briton Francis Crick (1916–2004) and his American doctoral student James Watson (born 1928), were then able to interpret the X-ray images more precisely: Exactly two strands of DNA form elongated spirals. The bases that connect and hold together the two strands on the inside are decisive. If an A protrudes inwards from one strand, it always connects to a T of the other strand. If a C protrudes inwards, on the other hand, it connects with a G. The bases are structured in such a way that the connections can only arise at certain points, as a result of which the two DNA strands wind around each other like a spiral staircase. The researchers speak of a double helix here.
The chemical fine structure looks as follows: A pentagonal sugar molecule (the so-called deoxyribose) forms the basic building block. At one corner there is a phosphate group, at another corner there is either a purine base (adenine or guanine) or a pyrimidine base (cytosine or thymine), the so-called nucleotides. Since the phosphate group of one nucleotide can connect to a free corner of the sugar pentagon of another nucleotide, whose phosphate group in turn reaches for the next sugar pentagon, long nucleotide chains are created.
Organisms use two types of sugar pentagons for nucleic acids. One of these rings contains one less oxygen atom than the other. Biochemists call the chains of sugars with one more oxygen atom ribonucleic acid (RNA), those sugar chains with one oxygen atom less are called deoxyribonucleic acid (DNA).
Two so-called hydrogen bonds serve as a connection between an adenine base (A) and a thymine base (T) of the DNA; There are even three of these hydrogen bonds between a guanine base (G) and a cytosine base (C). (Chemists refer to a relatively weak bond created by a hydrogen atom as hydrogen bonds.) Without such hydrogen bonds, nucleic acids would be a kind of ball of thread, but with their help the typical spiral staircase structure of a deoxyribonucleic acid is created.