How is DNA made in a cell

Science in dialogue

How did DNA come about?

The DNA - or in German DNA (deoxyribonucleic acid) - consists of lined up building blocks, the nucleotides. Each nucleotide is composed of phosphoric acid, the sugar deoxyribose and an organic base.

Since there are four different organic bases in DNA, there are also four different DNA nucleotides. The genetic code is based on the special sequence of the four nucleotides in DNA. It contains the building instructions for the various proteins that make up a living being and that are responsible for its properties.

The DNA probably originated more than four billion years ago. At that time there was already liquid water on earth and it contained all the necessary chemical components: phosphoric acid, formaldehyde and hydrogen cyanide. Sugar was formed from the formaldehyde, with mineral surfaces possibly serving as catalysts. The reaction of several hydrocyanic acid molecules led to organic bases. The individual building blocks combined to form the nucleotides and these reacted with each other in a random sequence to form DNA strands of different lengths. The energy required for the chemical reactions was provided by electrical discharges (e.g. lightning) or the UV radiation from the sun, which without the protective ozone layer on the early earth was much more intense than it is today.

The bases in DNA easily combine into base pairs. This can be done either between two individual strands or between sections within the same strand. As a result, DNA molecules do not break apart so easily due to thermal molecular movement and are therefore very stable. As early as 1953, the American scientist Stanley Miller had mixed various molecules that presumably existed on the primordial earth in a "primordial atmosphere apparatus". Using electrical gas discharges, he was able to manufacture the building blocks of DNA, among other things.

The first DNA snippets with their random nucleotide sequence did not, of course, contain any information for the structure of a living being. The complex system of genetic encryption as we know it today only developed in an evolutionary process that lasted many hundreds of millions of years.

After the first DNA molecules had emerged on the primordial earth, they were enclosed by membranes. These so-called micelles were the precursors of the cells. Ultimately, the first cells came into being with the cell membranes. It took another two billion years for the so-called eukaryotes to enter the stage of life. These are cells that have a nucleus in which the DNA is enclosed by an extra membrane. Almost all living things today are made up of such cells, except for bacteria. In eukaryotes, the DNA is wrapped around proteins called histones and is present in the cell in the form of chromosomes. This compresses the DNA and takes up less space.

The question was answered by Prof. Tim Clark from the Computer Chemistry Center of the University of Erlangen-Nuremberg.