C-value is the amount, in

picogram To help compare different orders of magnitude, the following lists describe various mass levels between 10−59 kg and 1052 kg. The least massive thing listed here is a graviton, and the most massive thing is the observable universe. ...

s, of DNA contained within a haploid

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

(e.g. a

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...

) or one half the amount in a

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

somatic cell A somatic cell (from Ancient Greek σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells compo ...

of a

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

organism. In some cases (notably among diploid organisms), the terms C-value and

genome size Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths (10−12) of a gram, abbreviated pg) or less frequently in daltons, or as the total ...

are used interchangeably; however, in polyploids the C-value may represent two or more

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

s contained within the same nucleus. Greilhuber ''et al.'' have suggested some new layers of terminology and associated abbreviations to clarify this issue, but these somewhat complex additions are yet to be used by other authors.

Origin of the term

Many authors have incorrectly assumed that the 'C' in "C-value" refers to "characteristic", "content", or "complement". Even among authors who have attempted to trace the origin of the term, there had been some confusion because Hewson Swift did not define it explicitly when he coined it in 1950. In his original paper, Swift appeared to use the designation "1C value", "2C value", etc., in reference to "classes" of DNA content (e.g., Gregory 2001, 2002); however, Swift explained in personal correspondence to Prof. Michael D. Bennett in 1975 that "I am afraid the letter C stood for nothing more glamorous than 'constant', i.e., the amount of DNA that was characteristic of a particular

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

" (quoted in Bennett and Leitch 2005). This is in reference to the report in 1948 by Vendrely and Vendrely of a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species" (translated from the original French). Swift's study of this topic related specifically to variation (or lack thereof) among

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

sets in different cell types within individuals, but his notation evolved into "C-value" in reference to the haploid DNA content of individual species and retains this usage today.

Variation among species

C-values vary enormously among species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000.

Protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...

genomes have been reported to vary more than 300,000-fold in size, but the high end of this range ( ''Amoeba'') has been called into question. Variation in C-values bears no relationship to the complexity of the organism or the number of

genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

contained in its genome; for example, some single-celled protists have genomes much larger than that of

humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

. This observation was deemed counterintuitive before the discovery of

non-coding DNA Non-coding DNA (ncDNA) sequences are components of an organism's DNA that do not encode protein sequences. Some non-coding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, microRNA, piRNA, ribosomal RNA, and regul ...

. It became known as the C-value paradox as a result. However, although there is no longer any

paradox A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...

ical aspect to the discrepancy between C-value and gene number, this term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested to more accurately comprise a complex but clearly defined puzzle known as the C-value enigma. C-values correlate with a range of features at the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

and organism levels, including

cell size Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis (production of biomolecules or anabolism) is greater than ...

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

rate, and, depending on the

taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, body size,

metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ce ...

, developmental rate, organ complexity, geographical distribution, or

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005). The or is the complex puzzle surrounding the extensive variation in nuclear

among

species. At the center of the C-value enigma is the observation that genome size does not correlate with organismal complexity; for example, some single-celled protists have genomes much larger than that of

. Some prefer the term C-value enigma because it explicitly includes all of the questions that will need to be answered if a complete understanding of

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

is to be achieved (Gregory 2005). Moreover, the term

implies a lack of understanding of one of the most basic features of eukaryotic genomes: namely that they are composed primarily of

. Some have claimed that the term paradox also has the unfortunate tendency to lead authors to seek simple one-dimensional solutions to what is, in actuality, a multi-faceted puzzle. For these reasons, in 2003 the term "C-value enigma" was endorsed in preference to "C-value paradox" at the Second Plant Genome Size Discussion Meeting and Workshop at the

Royal Botanic Gardens, Kew Royal Botanic Gardens, Kew is a non-departmental public body in the United Kingdom sponsored by the Department for Environment, Food and Rural Affairs. An internationally important botanical research and education institution, it employs 1,100 ...

, UK, and an increasing number of authors have begun adopting this term.

C-value paradox

In 1948, Roger and Colette Vendrely reported a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species", which they took as evidence that DNA, rather than

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

, was the substance of which

are composed. The term C-value reflects this observed constancy. However, it was soon found that C-values (

s) vary enormously among species and that this bears no relationship to the ''presumed'' number of genes (''as reflected by'' the

complexity Complexity characterises the behaviour of a system or model whose components interaction, interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence. The term is generall ...

of the

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...

). For example, the cells of some

salamanders Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...

may contain 40 times more DNA than those of humans. Given that C-values were assumed to be constant because genetic information is encoded by DNA, and yet bore no relationship to presumed gene number, this was understandably considered

ical; the term "C-value paradox" was used to describe this situation by C.A. Thomas Jr. in 1971. The discovery of

in the early 1970s resolved the main question of the C-value paradox:

does not reflect

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

number in

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

since most of their DNA is non-coding and therefore does not consist of genes. The

human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the n ...

, for example, comprises less than 2% protein-coding regions, with the remainder being various types of non-coding DNA (especially

transposable elements A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Trans ...

C-value enigma

The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...

DNA contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory of the

University of Guelph , mottoeng = "to learn the reasons of realities" , established = May 8, 1964 ()As constituents: OAC: (1874) Macdonald Institute: (1903) OVC: (1922) , type = Public university , chancellor ...

in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount of

found within the

s of different eukaryotes. The C-value enigma, unlike the older C-value paradox, is explicitly defined as a series of independent but equally important component questions, including: * What types of non-coding DNA are found in different eukaryotic genomes, and in what proportions? * From where does this non-coding DNA come, and how is it spread and/or lost from genomes over time? * What effects, or perhaps even functions, does this non-coding DNA have for

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

, nuclei, cells, and

organisms In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...

? * Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?

Calculating C-values

†Source of table: Doležel ''et al.'', 2003

The formulas for converting the number of nucleotide pairs (or base pairs) to picograms of DNA and vice versa are: genome size (bp) = (0.978 x 10⁹) x DNA content (pg) DNA content (pg) = genome size (bp) / (0.978 x 10⁹) 1 pg = 978 Mbp By using the data in Table 1, relative masses of nucleotide pairs can be calculated as follows: A/T = 615.383 and G/C = 616.3711, bearing in mind that formation of one phosphodiester linkage involves a loss of one H₂O molecule. Further, phosphates of nucleotides in the DNA chain are acidic, so at physiologic pH the H⁺ ion is dissociated. Provided the ratio of A/T to G/C pairs is 1:1 (the GC-content is 50%), the mean relative mass of one nucleotide pair is 615.8771. The relative molecular mass may be converted to an absolute value by multiplying it by the

atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

(1 u) in picograms. Thus, 615.8771 is multiplied by 1.660539 × 10⁻¹² pg. Consequently, the mean mass per nucleotide pair would be 1.023 × 10⁻⁹ pg, and 1 pg of DNA would represent 0.978 × 10⁹ base pairs (978 Mbp). No species has a GC-content of exactly 50% (equal amounts of A/T and G/C nucleotide bases) as assumed by Doležel ''et al.'' However, as a G/C pair is only heavier than an A/T pair by about 1/6 of 1%, the effect of variations in GC content is small. The actual GC content varies between species, between chromosomes, and between isochores (sections of a chromosome with like GC content). Adjusting Doležel's calculation for GC content, the theoretical variation in base pairs per picogram ranges from 977.0317 Mbp/pg for 100% GC content to 978.6005 Mbp/pg for 0% GC content (A/T being lighter, has more Mbp/pg), with a midpoint of 977.8155 Mbp/pg for 50% GC content.

Human C-values

The

varies in size; however, the current estimate of the nuclear haploid size of the reference human genome is 3,031,042,417 bp for the X gamete and 2,932,228,937 bp for the Y gamete. The X gamete and Y gamete both contain 22 autosomes whose combined lengths comprise the majority of the genome in both gametes. The X gamete contains an

X chromosome The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...

, while the Y gamete contains a

Y chromosome The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abse ...

. The larger size of the X chromosome is responsible for the difference in the size of the two gametes. When the gametes are combined, the XX female zygote has a size of 6,062,084,834 bp while the XY male zygote has a size 5,963,271,354 bp. However, the base pairs of the XX female zygote are distributed among 2 homologous groups of 23 heterologous chromosomes each, while the base pairs of the XY male zygote are distributed among 2 homologous groups of 22 heterologous chromosomes each plus 2 heterologous chromosomes. Although each zygote has 46 chromosomes, 23 chromosomes of the XX female zygote are heterologous while 24 chromosomes of the XY male zygote are heterologous. As a result, the C-value for the XX female zygote is 3.099361 while the C-value for the XY male zygote is 3.157877. The human genome's GC content is about 41%. Accounting for the autosomal, X, and Y chromosomes, human haploid GC contents are 40.97460% for X gametes, and 41.01724% for Y gametes. Summarizing these numbers:

References

{{Reflist, 2

External links

Animal Genome Size DatabaseFungal Genome Size Database
DNA Paradoxes