C value (genetics)

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The C value is a basic unit in genetics . It indicates how much deoxyribonucleic acid (DNA) a simple , in technical language haploid , genome of a living being contains; this value is denoted by 1 C in each case . There are two ways to display the corresponding amount of DNA:

  • as the mass of the DNA contained in the haploid genome, given in picograms (pg);
  • as the number of DNA base pairs (bp) contained in the haploid genome .

The C-value is largely stable within a biological species and therefore a species-specific characteristic. It can be very different between species. The values ​​range from 10 6 base pairs (1 Mbp) for Mycoplasma to the 10 11 base pairs (100 Gbp) for some plants and amphibians. When comparing it, it was found that more complex organisms usually have a higher C-value. The fact that this is sometimes not the case is also called the C-value paradox .

Comparison of species

The C-value is often used to measure the genomes of different species. There are open sources from which 1 C values ​​for plants and animals can be obtained.

plants

The UK botanical database (Version 6.0, December 2012) contains C values ​​for 8510 species. Hardly more than 1 percent of the (known) angiosperms are listed there.

In the case of plants in particular, it must be considered whether the germ cell nuclei of a species (breed) have each chromosome single, double or triple. If such complexity of a genetic material is to be emphasized, the relevant genome size can be noted as Cx (monoploid), C2x (diploid) or C3x (triploid). Without considering the complexity, one writes the usual, blank C-value for the whole ( holoploid ) genome. Hans Winkler has already experimentally investigated the problem of constitutive polyploidy and its effect on cell size. He pointed out that "tetraploidy and doubling of the diploid chromosome number are not necessarily identical to one another" [p. 517].

Mushrooms

Since 2005 there has been a database in Estonia with 2336 entries on the C values ​​of mushrooms (as of November 2018).

Animals

The database for C values ​​of 6222 animal species is offered from Canada. The smallest animal genome with 0.02 pg DNA has the nematode Pratylenchus coffeae ; the other extreme, 132.83 pg DNA, is presented by the lungfish Protopterus aethiopicus .

A report on 67 species of mammals with 51 new genome sizes at the time indicates the compactly small genomes of bats.

Analysis in a way

The C value forms the basis for comparing cell nuclei of different sizes in the tissues of the same organism (or a species). For this purpose, the DNA amounts are given as a multiple of the C value.

Mitosis

In a Diplont , the DNA content of a cell nucleus is 4 C from the completed S phase and remains at this level up to and including the next mitotic metaphase. The two daughter cell nuclei receive 2 C through the anaphase and the subsequent nuclear division; they hold this amount of DNA until the beginning of the next S-phase. Through DNA synthesis ( replication ) of all chromosomes, the DNA content increases again from 2 C via intermediate values ​​to 4 C.

meiosis

For this process in the cell nucleus, it is crucial that DNA synthesis does not take place between the first and second step of a meiosis . Only in this way is the nuclei of the germ cells 1 C due to anaphase II . This reduction to a single genome (a simple = haploid set of chromosomes) is the necessary purpose of meiosis. The DNA content of 1 C characterizes a germ cell (sperm cell or unfertilized egg cell).

Only the C value makes it possible to combine the reduction in nuclear DNA to the haploid value 1 C with the number of chromosomes that can be determined under the light microscope and to describe it unambiguously. For long stretches of meiosis the chromosomes (as show Bivalent ) united to haploid. The following table illustrates the problem:

Cell time phase feature Chromosomes DNA
LAST mito cycle Interphase S-phase completed not countable 4 C
MEIOSIS I Prophase I Leptotene, zygotene not countable 4 C
Prophase I Pachytan, diploma 1 n bivalent 4 C
Prophase I Diakinesis 1 n bivalent 4 C
Metaphase I. 1 n bivalent 4 C
Anaphase I. 1 n + 1 n 2 C + 2 C
Telophase I. not countable 2 C + 2 C
Interkinesis no S-phase Core division not countable 2 C + 2 C
MEIOSIS II Prophase II not countable 2 C
Metaphase II 1 n ; Chromatid gap 2 C
Anaphase II 1 n + 1 n 1 C + 1 C
Telophase II Core division not countable 1 C + 1 C
GAMET Egg or sperm not countable (1 n) 1 C

C value of humans

The human genome size is currently listed as 1 C = 3.50 pg DNA. This C value is based on microphotometric examinations and corresponds to 3423 Mbp. The conversion follows the equation 1 pg DNA = 978 Mbp. Nuclear DNA measurements are accurate enough to distinguish sperm with an X chromosome from sperm with a Y chromosome in many mammals. This consequently applies to an X-dependent versus a Y-dependent C value. The human "reference genome" is based on data obtained by DNA sequencing. The current edition "GRCh38.p12" sums the X-containing genome to 3200.10 Mbp and the Y-containing genome to 3101.29 Mbp. It is not a utopia: after such cell sorting, targeted artificial insemination or in vitro fertilization can decide the sex of the embryo.

DNA endoreplication

See main article endoreplication .

This not uncommon cell process consists of a series of DNA syntheses within the same cell nucleus. With such endocycles, the nucleus is not divided after every S phase , so that the C value increases. Particularly high C-values ​​are known from polytene chromosomes or polyploid cell nuclei in high-performance organs. In such cases, endoreplication is part of healthy, normal development. Examples are the Malpighi vessels , salivary glands and spinneret glands of flies and mosquitoes, of butterflies and spiders. The largest cell nuclei of the spinneret glands in the silk moth larvae reach a record value of around 300,000 C with 17 endocycles.

Endoreplication is also important for humans. The trophoblasts with endoreplicated cell nuclei serve the blastocyst in the early embryonic stage so that it can implant itself in the uterus .

Nuclear pathology

Cancers are based on genetic defects, on the wrong distribution of chromosomes in mitosis. These ideas go back to David Paul von Hansemann and Theodor Boveri . The pioneers could only estimate the amount of DNA and - difficult with human mitoses - count chromosomes. Hansemann would have liked to be able to precisely quantify the DNA content in nuclei during the interphase .

It was only with the advent of microscope photometry that it was possible to determine deviations in the nuclear DNA from the regular values ​​of 2 C, 4 C, also 8 C and beyond. Such successes opened the field of quantitative cytogenetics and cytopathology in order to detect abnormal DNA contents, the aneuploidy of the cell nuclei, in tumors.

history

The definition of the C value as the haploid DNA content of a biological species was made by Hewson Swift after he had weighed cell nuclei from maize with a self-designed microphotometer.

technology

A microscope photometer is used to measure the loss of light through the cell nuclei, which are fixed on glass supports and stained specifically for DNA using the Feulgen reaction . Although time-consuming, this method has the advantage that each individual nucleus can be morphologically assessed and repeated measurements are accessible. For the determination of 1 C it is not absolutely necessary to use (compact) sperm. The result is more reliable if spermatids (1 C) are offset against identified nuclei from prophase  I (4 C).

The other method is flow cytometry of cells whose nuclear DNA is registered by means of a specific fluorescent dye . This method of measuring light has the advantage of being able to evaluate very large samples.

With both methods it is necessary to measure an external DNA standard with a known, recognized DNA content in addition to the sample. From the device values ​​of the standard, the device values ​​of the sample are converted into pg or bp . Ellen Rasch introduced the 2 C cell nuclei from red blood cells of a chicken as a DNA standard with 2.53 pg DNA.

See also

Web links

Individual evidence

  1. ↑ Hans Winkler : Distribution and cause of Parthenogenesis in the plant and animal kingdom. Gustav Fischer, Jena: 1920.
  2. ↑ Jochen Graw: Genetics . Springer-Verlag, Berlin / Heidelberg 2010, ISBN 978-3-642-04998-9 , pp. 7 , doi : 10.1007 / 978-3-642-04999-6 .
  3. ↑ a b c Wilfried Janning, Elisabeth Knust: Genetics. General Genetics - Molecular Genetics - Developmental Genetics . 2nd Edition. Thieme, Stuttgart 2008, ISBN 978-3-13-128772-4 , p. 132 .
  4. ↑ Michael D Bennett, Ilia J Leitch (2012): Plant DNA C-values ​​database (release 6.0, December 2012). Database.
  5. ↑ Michael D Bennett: Plant genome values: How much do we know? In: PNAS 95, 5, 1998: 2011–2016. PDF
  6. ↑ Johann Greilhuber, Jaroslav Doležel, Martina A Lysák, Michael D Bennett: The origin, evolution and Proposed stabilization of the terms 'genome size' and 'C-value' to describe nuclear DNA contents. In: Ann Bot 95, 1, 2005: 255-260. PMC 4246724 (free full text)
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  9. ↑ T Ryan Gregory: Animal genome size database. Release 2.0 Guelph (Ontario) 2005. Database
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  11. ↑ August Weismann : Amphimixis or: The mixing of individuals. Fischer, Jena 1891.
  12. ↑ August Weismann: The need to halve the germplasm. (This is the first section of Chapter VIII. Modification of the germ plasm by Amphimixis. ) In: The germ plasm. A theory of inheritance. Fischer, Jena 1892; there pp. 308-330. To understand this masterpiece of theoretical genetics, put "Idant" = chromosome and "Id" = gene . Digitized.
  13. ↑ Jochen Graw: Genetics . Springer-Verlag, Berlin / Heidelberg 2010, ISBN 978-3-642-04998-9 , pp. 222 , doi : 10.1007 / 978-3-642-04999-6 .
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