DNA: Deoxyribonucleic Acid
According to ABBREVIATIONFINDER.ORG, the deoxyribonucleic acid is DNA or with DNA (for English ” D esoxyribo n ucleic a abbreviated cid”) and is a spirally wound giant molecule which contains the genetic information of an organism.
DNA is the blueprint of life and is contained in almost every cell – this is one of the reasons why it plays a major role in many crime novels and usually leads successfully to the murderer.
The DNA is out Nucleic acid built up, which consists of a chain of nucleotides. A certain sequence of nucleotides contains the information for a Gene. In the DNA molecule, two such nucleotide chains are usually connected to one another in the form of a ladder. The two halves of this “ladder” are wound around each other and thus form a double helix (Watson-Crick model). This means that very long DNA chains can be accommodated in the smallest of spaces.
The DNA has the same structure in all cells of an organism. It can duplicate identically in a process known as replication. Replication precedes every mitosis and meiosis (Cell division). The conversion of the genetic information (transcription) takes place via the synthesis of RNA molecules (Ribonucleic acid).
Inheritance of individual traits – the experiments of Gregor Mendel
Gregor Mendel dealt with the subject of inheritance as early as 1866 (1822-1884). The monk of the Augustinian monastery of St. Thomas in Brno (now Brno in the Czech Republic) planted peas in the garden of the monastery from 1854, which he used for cross-breeding experiments. In ten years he crossed at least 28,000 pea plants that had seeds with few but well-defined properties. He observed the following: when he crossed peas with smooth seeds and those with wrinkled seeds, the seeds of all direct offspring were smooth. If he crossed these offspring with each other again, he received three quarters of smooth-seeded offspring, but also a quarter with angular seeds. He was also able to find similar astonishing results in cross-breeding experiments with yellow and green peas. Mendel concluded that an organism does not inherit its characteristics as a whole, but passes on individual properties such as smooth and angular or green and yellow independently of one another. One of these characteristics sometimes prevails first, so that the first Generation of offspring can look uniform. Only in the second generation does the missing property have a chance again.
Gregor Mendel had discovered that the overall genetic information is composed of individual genes and that the mother and father of the organism each provide a set of the entire hereditary properties.
The nature of the genetic material
The American Thomas Hunt Morgan (1866–1945) discovered in 1910 exactly what this genetic material looks like, which parents pass on to their children: Chromosomes were the substance in the cell nucleus on which the genetic makeup was arranged.
What was not known was which part of the chromosomes carried the genetic material. Chromosomes are made up of nucleic acids and many different proteins. Initially, most researchers assumed proteins as hereditary substance, only a minority of scientists relied on nucleic acids, of which there are two types: ribonucleic acid (RNA, from English ribonucleic acid) and deoxyribonucleic acid (DNA).
In 1944, the Canadian Oswald Avery (1877–1955) solved the riddle about the genetic material with the help of experiments on two strains of pneumococci (bacteria that can cause pneumonia, among other things). The so-called S strain of these microorganisms had a protective capsule made of mucus, while the R strain managed without such a shell. The genetic information of the S-strain had to contain the information for the formation of a shell. The Canadian isolated the four different substances DNA, RNA, proteins and the sugar chains (polysaccharides, which are also abundant) from this strain). Then he added the individual substances to different cultures with R-strain pneumococci. In three of the experiments, the bacteria remained without a shell. But in the culture to which DNA had been added, at least some of the microorganisms without a mucous membrane suddenly grew a mucous membrane after all. The assumption was made that the DNA could carry the genetic information.
Oswald Avery provided proof of this with an additional experiment: When he provided R-stem pneumococci with DNA from S-stem pneumococci, he also added a substance that decomposed DNA. When not a single cell with a mucous membrane was formed, it was finally clear that the genetic information had to be in the DNA.
The Meselson-Stahl experiment
When the double helix structure was found, it suddenly became clear how organisms can duplicate their genetic make-up and pass a full half on to their offspring. The spirals separate so that each individual spiral can attach very specific nucleotide building blocks: A T is always attached to an A nucleotide, a G is attached to a C, an A is attached to every T, and a G connects to a C. In principle, the previously separated DNA strand has to be created again automatically.
The Americans Matthew Meselson (born 1930) and Franklin Stahl (born 1929) in 1958 provided evidence of this doubling through complete separation of the strands contained. The microorganisms therefore only built nitrogen-15 into their DNA. When the bacteria then continued to grow on a nutrient medium with nitrogen-14, half of the second generation bacteria had only nitrogen-14 in their DNA, while the other half contained both nitrogen isotopes (14 and 15) in almost equal amounts. There was only one explanation for this: the two strands of DNA had to separate from each other and each of them had to have formed a completely new partner.