C-value is the amount, in

picogram To help compare different '' orders of magnitude'', the following lists describe various ''mass'' levels between 10−67 kg and 1052 kg. The least massive thing listed here is a graviton, and the most massive thing is the observable univer ...

s, of

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

contained within a

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

nucleus (e.g. a

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

) 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. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

somatic cell In cellular biology, a somatic cell (), 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. Somatic cells compose the body of an organism ...

of a

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

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 or 10−12 of a gram, abbreviated pg) or less frequently in daltons, or as the tot ...

are used interchangeably; however, in

polyploids Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...

the C-value may represent two or more

genome 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 genes, other functional regions of the genome such as ...

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 - C-value

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 package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

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 ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

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 has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

contained in its genome; for example, some single-celled

protists A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...

have genomes much larger than that of

humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

. This observation was deemed counterintuitive before the discovery of repetitive DNA. 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 or apparently true premises, leads to a seemingly self-contradictor ...

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 and organism levels, including cell size,

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''; : 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 name and ...

, body size, metabolic rate, developmental rate,

organ Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...

complexity, geographical distribution, or

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce 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

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 the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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

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 reg ...

. 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 1,10 ...

, 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

, rather than

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

, 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 characterizes the behavior of a system or model whose components interact in multiple ways and follow local rules, leading to non-linearity, randomness, collective dynamics, hierarchy, and emergence. The term is generally used to c ...

of the

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

). 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 t ...

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 repetitive DNA in the late 1960s resolved the main question of the C-value paradox:

does not reflect

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

number in

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

since most of the excess DNA in many species appears to be

junk DNA Junk DNA (non-functional DNA) is a DNA sequence that has no known biological function. Most organisms have some junk DNA in their genomes—mostly pseudogenes and fragments of transposons and viruses—but it is possible that some organ ...

. The

human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as the DNA within each of the 23 distinct chromosomes in the cell nucleus. A small DNA molecule is found within individual Mitochondrial DNA, mitochondria. These ar ...

, for example, contains about 10% functional elements and the remaining 90% is thought to be junk. Species with larger genomes are thought to contain a higher proportion of junk DNA.

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

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 ...

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

University of Guelph The University of Guelph (abbreviated U of G) is a comprehensive Public university, public research university in Guelph, Ontario, Canada. It was established in 1964 after the amalgamation of Ontario Agricultural College (1874), the MacDonald I ...

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 package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...

, nuclei, cells, and

organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pr ...

? * 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 In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases in a DNA or RNA molecule that are either guanine (G) or cytosine (C). This measure indicates the proportion of G and C bases out of ...

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, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted ...

(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 chromosomes in many organisms, including mammals, and is found in both males and females. It is a part of the XY sex-determination system and XO sex-determination system. The X chromosome was named for its u ...

, while the Y gamete contains a

Y chromosome The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the ...

. 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

External links

Animal Genome Size DatabaseFungal Genome Size Database
{{Webarchive, url=https://web.archive.org/web/20130207120355/http://www.zbi.ee/fungal-genomesize/index.php , date=2013-02-07 DNA Paradoxes