Deoxyribonucleic acid - DNA


Genetic material, genes, genetic fingerprint

Desoxyribonucleic acid (DNS)


The DNA is the construction manual for the body of every living being (mammals, bacteria , fungi, etc.). It corresponds in its entirety to our genes and is responsible both for the general characteristics of a living being, such as the number of legs and arms, as well as for individual characteristics such as the hair color.
Similar to our fingerprint, the DNA of each human is different and dependent on the DNA of our parents. Identical twins are the exception: they have identical DNA.

Rough construction of the DNA

In humans, the DNA is contained in every cell-nucleus cell. In organisms that have no cell nucleus, such as bacteria or fungi, the DNA is free in the cell space ( cytoplasm ). The nucleus, which measures only about 5-15 microns, is seen the heart of our cells. In it, our genes are housed in 46 chromosomes in the form of DNA. To pack the approximately 2m long DNA into the tiny cell nucleus, it is compressed via stabilizing proteins and enzymes in spirals, loops and coils.

Thus, multiple genes on a strand of DNA yield one of 46 X-shaped chromosomes. Half of the 46 chromosomes are made up of mother's chromosomes and half of her father's chromosomes. Activation of the genes, however, is much more complicated, so that the characteristics of the child is not exactly 50% attributable to each parent.

Apart from the DNA in the form of chromosomes in the cell nucleus, there is more circular DNA in the "power plants" of the cells, the mitochondria.
This DNA circle is only inherited from the mother to the child.

Illustration of a DNA

Figure construction of the DNA

Construction of DNA, DNA
Deoxyribonucleic acid
deoxyribonucleic acid
Double strand (helix)

  1. cytosine
  2. thymine
  3. adenine
  4. guanine
  5. phosphate
  6. sugar
  7. Hydrogen
  8. base pairs
  9. nucleotide
    a - pyrimidine bases
    b - purine bases
    A - T: 2H - bridges
    G - C: 3H - bridges

Detailed structure of the DNA

One can imagine the DNA as a double strand, which is constructed like a spiral staircase. This double helix is ​​slightly uneven, so that there is always a larger and a smaller distance between the steps of the spiral staircase ( large and small furrows ).

The handrail of these ladders alternately form:

  • a sugar residue ( deoxyribose ) and
  • a phosphate radical.

The handrails are each one of four possible bases. Thus, two bases form one step. The bases themselves are connected by hydrogen bonds.

From this structure, the name DNA is now explained: deoxyribose (= sugar ) + nucleic (= from the nucleus ) + acid / acid (= total charge of the sugar-phosphate backbone).

Bases are ring-shaped different chemical structures with correspondingly dissimilar chemical bonding functions. There are only four different bases in DNA.

  • Cytosine and thymine (replaced in the RNA by uracil) are so-called pyrimidine bases and have a ring in their structure.
  • Purine bases, on the other hand, have two rings in their structure. In DNA these are called adenine and guanine.

For the combination of the two bases, which together form a step, there is only one possibility.

There is always a purine base linked to a pyrimidine base. Due to the chemical structures, cytosine with guanine and adenine always form base pairs with thymine.

DNA bases

There are 4 different bases in the DNA.
These include the pyrimidine-derived single-ring bases (cytosine and thymine) and the purine-derived bases with two rings (adenine and guanine).

These bases are each linked to a sugar and a phosphate molecule and are then also referred to as adenine nucleotide or cytosine nucleotide. This coupling to the sugar and the phosphate is necessary so that the individual bases can be connected to a long DNA strand. In the DNA strand, namely sugar and phosphate alternate, they form virtually the lateral elements of the DNA ladder. The ladder stages of the DNA are formed by the four different bases pointing inward.
It always go adenine and thymine, respectively. Guanine and cytosine a so-called complementary base pairing.
The DNA bases are linked via so-called hydrogen bonds. There are two for the adenine-thymine pair and three for the guanine-cytosine pair.

DNA polymerase

DNA polymerase is an enzyme that binds the nucleotides together to create a new strand of DNA.
However, the DNA polymerase can only work if a so-called "primer", ie a starting molecule for the actual DNA polymerase, has been produced by another enzyme (another DNA polymerase).
The DNA polymerase then starts at the free end of a sugar molecule within a nucleotide and links that sugar to the phosphate of the next nucleotide.
In the context of DNA replication (DNA replication in the process of cell division), DNA polymerase produces new DNA molecules by reading the already existing DNA strand and synthesizing the correspondingly opposite daughter strand. In order for the DNA polymerase to reach the "parent strand", the DNA double replication actually needs to be unraveled by preparatory enzymes.

In addition to the DNA polymerases involved in the amplification of DNA, there are DNA polymerases that can repair broken or incorrectly copied sites.

DNA as a material and its products

In order to ensure the growth and development of our body, the inheritance of our genes and the production of the necessary cells and proteins, a cell division (meiosis, mitosis) must take place. The necessary processes that our DNA has to go through are shown in the overview:


The goal of replication is the doubling of our genetic material (DNA) in the cell nucleus, before cell division. The chromosomes are piecewise removed, allowing enzymes to attach to the DNA.
The opposing DNA double strand is opened, so that the two bases are no longer connected. Each side of the handrail or base is now read by a variety of enzymes and complemented by the complementary base including handrail. This results in two identical DNA double strands, which are distributed to the two daughter cells.


Just like replication, transcription also takes place in the cell nucleus. The aim is to rewrite the base code of the DNA into a mRNA (messenger ribonucleic acid). In this case, thymine is exchanged with uracil and DNA components that do not code for proteins, similar to a blank, cut out. As a result, the mRNA, which is now transported out of the nucleus, is a lot shorter than the DNA and only single-stranded.


Once the mRNA has arrived in the cell room, the key is read from bases. This process takes place on ribosomes. Three bases ( base triplet ) give the code for one amino acid. A total of 20 different amino acids are installed. Once the mRNA has been read, the strand of amino acids yields a protein that is either used in the cell itself or sent to the target organ.


Increasing and reading the DNA can lead to more or less serious errors. In a cell, there are about 10, 000 to 1, 000, 000 damage per day, which can usually be corrected by repair enzymes, so that the defects have no effect on the cell.

If the product, so the protein, despite mutation unchanged, so there is a silent mutation. However, if the protein is altered, the disease often develops. For example, UV radiation (sunlight) causes damage to a thymine base can not be repaired. The consequence can be skin cancer.
However, mutations do not necessarily have to be linked to a disease. You can also change the organism to its advantage. Thus, mutations are a major component of evolution because organisms can only adapt to their environment in the long term through mutations.

There are several types of mutations that can occur spontaneously during different cell cycle phases. For example, if a gene is defective, it is called a gene mutation. However, if the defect affects certain chromosomes or chromosomal parts, then it is a chromosome mutation. If the number of chromosomes is affected, then it leads to a gene mutation.

DNA replication

The goal of DNA replication is the duplication of existing DNA.
During cell division, the DNA of the cell is exactly doubled and then distributed to both daughter cells.

The doubling of the DNA takes place according to the so-called semiconservative principle, which means that after the initial unwinding of the DNA, the original DNA strand is separated by an enzyme (helicase) and each of these two "original strands" as a template for a new DNA strand serves.

DNA polymerase is the enzyme responsible for the synthesis of the new strand. Since the opposite bases of a DNA strand are complementary to one another, the DNA polymerase can use the present "original strand" to arrange the free bases in the cell in the correct order, thus forming a new DNA double strand.

After this exact doubling of the DNA, the two daughter strands, which now contain the same genetic information, are split between the two cells that developed during cell division. Thus, two identical daughter cells have emerged.

History of DNA

For a long time it was unclear which structures in the body are responsible for the transmission of our genome. Thanks to the Swiss Friedrich Miescher, in 1869 research focused on the content of the cell nucleus.

In 1919, Lithuania's Phoebus Levene discovered the bases, the sugar and phosphate residues as the building material of our genes. The Canadian Oswald Avery was able to prove the evidence that DNA and no proteins are actually responsible for the transfer of genes in 1943 with bacterial experiments.
The American James Watson and the British Francis Crick put an end to the 1953 research marathon, which had spread across many nations. They were the first to produce DNA and X-ray DNA with the help of Rosalind Franklin's ( Britin ) DNA double-helical model including purine and pyrimidine bases. Rosalind Franklin's X-rays were not released by her, but by her colleague Maurice Wilkins for research. Wilkins was awarded the Nobel Prize for Medicine in 1962 together with Watson and Crick. Franklin was already dead and could not be nominated anymore.

Today's significance of the discovery of DNA

Some blood at the scene can convict the offender.


Will suspicious material like

  • Blood,
  • Sperm or
  • hair

Found at a crime scene or a victim, so you can win the DNA. Apart from the genes, the DNA contains more sections that consist of frequent repeats of bases and do not code for a gene. These cutscenes serve as a genetic fingerprint because they are highly variable. The genes, however, are almost identical in all humans.

If one then cuts the DNA obtained with the help of enzymes, then many small DNA parts, also called microsatellites are formed. If one compares the characteristic pattern of the microsatellites (DNA fragments) of a suspect (eg from a saliva sample) with that of the existing material, one identifies with high probability the offender with agreement. The principle is similar to that of the fingerprint.

Paternity Test:

Again, the length of the child microsatellites is compared with those of the potential father. If they agree, paternity is very likely (see also: Forensics).

Human Genome Project (HGP):

In 1990, the human genome project was launched. Initially, James Watson led the project with the goal of decoding the entire code of the DNA. Since April 2003, the human genome is considered completely decrypted. 3.2 billion base pairs could be assigned to approximately 21, 000 genes. The sum of all genes, the genome, is responsible for hundreds of thousands of proteins.

DNA sequencing

In DNA sequencing, biochemical methods are used to determine the order of nucleotides (DNA base molecule with sugar and phosphate) in a DNA molecule.

The most widely used method is the Sanger chain termination method.
Since the DNA is composed of four different bases, four different approaches are made. In each approach is the DNA to be sequenced, a primer (starting molecule for sequencing), DNA polymerase (enzyme that prolongs the DNA) and a mixture of all four nucleotides needed. However, in each of these four approaches, another base is chemically altered so that it can be incorporated, but provides no target for the DNA polymerase. Thus, it comes then to chain termination.
By this method arise different lengths of DNA fragments, which are then separated by the so-called gel electrophoresis according to their length chemically. The resulting sort can be translated into the order of the nucleotides in the sequenced DNA segment by labeling each base with a different fluorescent dye.

DNA hybridization

DNA hybridization is a molecular genetic method that is used to detect the similarity between two DNA single strands of different origin.

In this method, it is made use of that a DNA double strand is always composed of two complementary single strands.
The more similar both single strands are to one another, the more bases form a firm connection (hydrogen bonds) with the opposite base or the more base pairings are formed.

There will be no base pairing between sections on the two DNA strands that have a different base sequence.

The relative number of compounds can now be determined by determining the melting point at which the newly formed DNA duplex is separated.
The higher the melting point, the more complementary bases have formed hydrogen bonds to each other and the more similar are the two single strands.

This method can also be used to detect a particular sequence of bases in a DNA mixture. For this purpose, artificially formed DNA pieces can be labeled with (fluorescence) dye. These then serve to identify the corresponding base sequence and thus make it visible.

research objectives

After completion of the human genome project, the researchers are now trying to assign the individual genes of their importance to the human body.
On the one hand, they try to draw conclusions about pathogenesis and therapy, on the other hand, by comparing human DNA with the DNA of other living beings, there is the hope of being better able to represent the evolutionary mechanisms.

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