Genome

A genome or genetic material of an organism or virus is referred to both in the abstract meaning the entirety of the hereditary information of a cell or a virus particle, and in the real sense, the whole of the material carriers of hereditary information: chromosomes, deoxyribonucleic acid (DNA ) or ribonucleic acid ( RNA) in RNA viruses in which the RNA is used instead of DNA as the information carrier.

The term genome was coined in 1920 by Hans Winkler. The branch of genetics that deals with the study of the structure of genomes and the interaction between genes is called genomics (English genomics ).

Basics

Required for the inheritance of properties and characteristics and forwarded at the level of cells and individuals, information is included in the DNA, in the sequence ( sequence ) of the DNA bases, adenine (A ), guanine (G), cytosine ( C) and thymine (T). Using ribonucleic acids in place of thymine uracil (U). Each three consecutive bases mean by the rule of the genetic code one amino acid.

We distinguish coding and non-coding sections of DNA. In accordance with the base sequence of the coding portions ( genes ) are formed in the course of protein expression of amino acids. But also non-coding regions may have important functions, such as in gene regulation. There is also the so-called pseudogenes: by mutations become inoperative and no longer read from the organism genes.

Most organisms have, in addition to the chromosomal DNA of the nucleus ( so called Karyom ) additional genetic material in other parts of the cell. In these eukaryotes (animals, plants, fungi and protists ), the mitochondria of plants and algae further the plastids own small genomes. Prokaryotes (bacteria and archaea ) contain many additional, relatively short, self-contained DNA molecules called plasmids.

Organization of genomes

Eukaryotes

In eukaryotes, the nuclear genome is ( Karyom ) from several to many string-shaped chromosomes. The number of chromosomes is species-specific and may vary between two different (on Pferdespulwurm ) to several hundred (for some ferns ). In addition, the chromosome number when changing the core phase (meiosis and karyogamy ) changes. A characteristic feature of eukaryotic genomes is still a high proportion of non-coding DNA ( in humans about 95 %) and the intron - exon structure of genes.

Prokaryotes

In prokaryotes, the DNA is present as a long, self-contained molecule. In addition, shorter, also self-contained DNA molecules, called plasmids, may be present in variable number. These can be reproduced independently of the main DNA and passed on to other prokaryotic cells are (conjugation ), even across species barriers. They usually contain only a few genes that mediate resistance to antibiotics, for example.

Prokaryotic genomes are much smaller than eukaryotic generally. They contain relatively small non-coding portions (5-20 %) and only few or no introns.

Organelles

The genomes of mitochondria and plastids are like prokaryotic genomes organized (see endosymbiotic theory ). However, they contain only a small portion of the time required for the function of these organelles genes, which is why these organelles are referred to as "semi- autonomous".

Viruses

Viral genomes are very small, because in them only very few proteins are encoded, and the genetic information is also highly compressed by about different genes overlap or some portions may act in both reading directions than genes at the same time. They may consist of DNA or RNA and can be double-stranded and single-stranded or linear, circular, or segmented. Particularly interesting are the retroviruses, whose RNA genome can be generated by reverse transcription into DNA "translated " and integrated into the host genome. The properties of the genomes of viruses are important criteria in their classification (Virus taxonomy).

Genome sizes

As the size of the genome present in a genome amount of DNA is called. In eukaryotes, this statement usually refers to the haploid set of chromosomes, it is referred to as a C - value. It is either given the number of existing base pairs ( bp) or the mass of DNA in the unit pg ( picograms ). 1 pg of double-stranded DNA consists of about 0,978 by 109 bp, ie from almost one billion base pairs. Are also common designations kilo base pair ( kbp or kb) of 1000 base pairs and mega base pair ( Mbp or Mb) for one million base pairs.

According to recent studies of the South American lungfish ( Lepidosiren paradoxa ) has with 80 pg ( 7.84 × 1010 bp), the largest known animal genome. Older, but probably less accurate studies show with about 133 pg even larger genomes also in lungfish, but in the African style Ethiopian lungfish ( Protopterus aethiopicus ) were found. With 0.04 pg (less than 50 million base pairs) which has the primitive phylum Placozoa belonging, living on algae, about 2 mm in size, poorly differentiated Trichoplax adhaerens the smallest previously known animal genome. The number of base pairs of the intestinal bacteria, Escherichia coli is smaller only by a factor of 10. The smallest bacterial genome has previously quantified the psylla - endosymbiont Carsonella ruddii: Its circular DNA molecule contains only about 160,000 base pairs, in which all information is stored, it needs to live.

The DNA of a single human cell is lined up about 1.80 m long. A base on one strand of DNA theoretically has an information content of 2 bits, since they = 4 states (A / T / G / C) can take 22. With about 3.27 billion base pairs in the human genome would therefore have a maximum possible information content of 6.54 billion bits or 780 MiB. The actual information content is probably significantly lower, as large parts of the DNA have non-coding sequences, however, have at least partially regulatory functions.

A comparison of genome size with the complexity and the degree of organization of the organism results in no clear relationship. So have larger genomes than salamanders reptiles, birds and mammals. Lung fish and cartilaginous fish have larger genomes than bony fish, and within taxa, such as the flowering plants or protozoa varies the size of the genome in a high degree. This is referred to as "C - value paradox". The largest amount of DNA have simple eukaryotes such as amoebae and some Urfarne to around a trillion base pairs. These types include single genes than a thousand multiple copies and long time protein-coding sections.

Sequenced genomes

The DNA genomes of different organisms that are relevant to the medical- pharmaceutical or applied research, or for basic research either, was almost completely " sequenced " (this is sometimes erroneously referred to as "Decrypt " ), that is their base sequence was determined (DNA sequencing ). The base sequences are, inter alia, on the Internet provided by the NCBI.

  • Quick Guide to Sequenced Genomes ( GNN ) ( excellent overview page, in alphabetical order and well organized place you previously sequenced organisms with pictures, memos, responsible for sequencing institution and relevant literature with links)
  • Genome Atlas
  • Archaea - Archaea
  • Bacteria - Bacteria Escherichia coli - E. coli bacteria
Reverse transcriptase Tetraodontidae Brassica oleracea C-value enigma Nucleic acid sequence National Center for Biotechnology Information Epigenetics Digital Object Identifier Ernst-Ludwig Winnacker
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