According to wholevehicles, Deoxyribonucleic acid is also known as DNA, polymeric compound that is the carrier of genetic information in all living things and in some viruses.
Structure: Deoxyribonucleic acid is made up of nucleotides, which in turn consist of a sugar component, a base and a phosphoric acid residue. The sugar building block of DNA is deoxyribose; the purine bases are adenine (A) and guanine (G), the pyrimidine bases cytosine (C) and thymine (T). Some bases are modified, v. a. methylated, before.
With the exception of some viruses, DNA does not occur in living organisms as a single-stranded molecule, but as a coiled double-strand in the form of a double helix (Watson-Crick model). The genetic information is encoded in the order of the nucleotides (base sequence) ( genetic code).
The outer part of this double helix is formed by the phosphoric acid diester deoxyribose chains. The bases protrude perpendicular to the central axis into the double helix, with hydrogen bonds being formed between the bases of the two strands. For structural reasons, these bonds are always only possible between the bases referred to as complementary adenine and thymine and cytosine and guanine. In this arrangement, the base sequence on a strand of the molecule always clearly determines the base sequence (nucleotide sequence) of the complementary, antiparallel strand, a property that is of crucial importance for the identical duplication of the DNA. Corresponding to the complementary base pairing the same number of purine and pyrimidine bases are always present in double-stranded DNA; the quantitative ratio of guanine + cytosine to adenine + thymine, on the other hand, is variable and the GC content is a variable that is characteristic of every organism. In humans, the GC content is around 40%, but the distribution over the DNA of the chromosomes is not uniform because protein-coding genes usually have a higher content of guanine and cytosine.
The base pairing results in different spatial helical structures, which are referred to as A-DNA, B-DNA and Z-DNA. The B-DNA reproduces the classic structure of the double helix. As has been proven experimentally, most of the DNA in the genome is in this form.
DNA molecules can be linear (eukaryote chromosomes) or circular ( prokaryote, mitochondrial, and plastid DNA). Their size can vary considerably. The genome of the hepatitis B virus contains only around 3,200 base pairs, whereas the largest human chromosome 1 contains around 260 million. The entire human haploid chromosome set comprises around 3 billion base pairs, distributed over 23 DNA molecules.
In eukaryotic cells, most of the DNA is strongly condensed in the chromosomes in the cell nucleus. However, less complex genomes are also found in mitochondria and chloroplasts. Human circular mitochondrial DNA is fully sequenced and consists of 16,569 base pairs. Because of the large number of mitochondria in a cell, it can account for up to 0.5% of the total amount of DNA in a cell. The similarity of the DNA of mitochondria and chloroplasts with prokaryotic DNA is an indication of the development of these cell organelles from prokaryotic symbionts ( endosymbiont hypothesis). The genetic code of mitochondrial DNA differs from that of DNA in the nucleus.
Replication: During the identical duplication of DNA (DNA replication) during cell division, the two strands of DNA separate from each other by breaking the hydrogen bonds between the base pairs. Each strand then serves as a template for the synthesis of a complementary strand. The base sequence and the genetic information contained in it is preserved with the participation of highly specific enzyme systems that guarantee base pairing and recognize errors.
Protein biosynthesis: The genetic information is converted through the synthesis of specific RNA molecules (ribonucleic acid). In contrast to prokaryotes, only a relatively small part of the DNA in eukaryotes carries the information coding for proteins or necessary for reading and copying processes. The predominant part consists of sequences that are apparently functionless and sometimes repeat themselves many times (repetitive DNA). The apparent inoperability of these genes has led to the name junk DNA. Whether these areas of DNA actually only represent an unnecessary burden or whether they fulfill as yet unknown functions is a matter of controversy.
The activity of the genes depends largely on the structure of the DNA molecule. The methylation status of the respective gene has an important influence on the implementation of the genetic information. Methylations take place v. a. at the base cytosine. Molecular genetic observations have shown that strong methylation usually leads to the shutdown of a section of DNA (epigenetics).
Damage and repair: DNA molecules can be damaged by external influences such as increased temperature, radiation, chemical substances or viruses, whereby the damage often consists of strand breaks or a change in the type of bond between the nucleic acids (e.g. formation of pyrimidine dimers); if the function is restricted or lost, one speaks of denaturation of the DNA. The cells of all living things are characterized by the fact that they are able to repair minor damage (DNA repair) and thus avoid cell death or mutations.
Characterization and analysis: The prerequisite for all analytical methods is to generate samples that are as pure as possible, i. H. the DNA molecules present in a solution must be separated from other cell components. After the cells have been disrupted, the following methods are used: extraction, chromatography, electrophoresis, sedimentation (esp. Centrifugation) and filtration. The concentration of the DNA molecules can be measured using spectroscopic methods (especially photometry). With additional methods such as hybridization or the Molecular combs can be used to achieve particularly pure states for subsequent investigations. The molecular weight of DNA is v. a. derived from the results of separation methods such as gel electrophoresis or mass spectrometry (or a combination of these methods). The arrangement of the nucleotides within a DNA molecule is determined using various sequencing methods. For structural investigations, v. a. the X-ray analysis and fluorescence microscopy used. In a narrower sense, DNA analysis is the study of the genetic aspects of DNA (and the Chromosomes). In this context, the method of the polymerase chain reaction is of crucial importance.
History: The history of the discovery of nucleic acids goes back to the 19th century. The Swiss physiologist Friedrich Miescher discovered a substance in cell nuclei that differed from the proteins investigated up to now and which he called “Nuclein” (1869). In the following years it turned out that this substance can be separated into two fractions, protein and a protein-free part, the nucleic acid. However, it was not until 1944 that the transformation attempts by O. T. Avery and co-workers provided evidence that DNA is responsible for the transmission of genetic information. By E. Chargaff and co-workers found laws governing the quantitative base composition of DNA (1949) were an important preliminary achievement for the structure elucidation of DNA, which J. D. Watson and F. H. C. Crick finally succeeded in 1953 and which gave a decisive impetus to the development of molecular biology. Then between 1960 and 1964, v. a. driven by S. Ochoa and M. Nirenberg, the elucidation of the genetic code. The development of the polymerase chain reaction by K. B. Mullis was of decisive importance for molecular genetics and the burgeoning genetic engineering in 1983, which allows DNA to be reproduced almost at will. Since the 1980s, scientists have been pursuing projects that aim to decipher the sequence of bases in the genome of plants and animals. The prerequisite for this was the development of methods for the sequence analysis of DNA. The process developed by F. Sanger in the mid-1970s is still used today, but it is largely automated in so-called sequencers. The human genome project is at the center of the genome projects, which was dedicated to the complete structure elucidation of the human genetic material. After the genome sequences of numerous prokaryotes, eukaryotes and viruses are known, an initiative founded in 2003 describes the methylation status of the genome (human epigenome project).