C-value
Encyclopedia
The term C-value refers to the amount of DNA
contained within a haploid nucleus
(e.g. a gamete
) or one half the amount in a diploid somatic cell
of a eukaryotic organism, expressed in picograms (1pg = ). In some cases (notably among diploid organisms), the terms C-value and genome size
are used interchangeably, however in polyploids
the C-value may represent two genome
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 have yet to be used by other authors. C-values are reported in picograms.
" (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
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.
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
contained in its genome, an observation that was deemed wholly counterintuitive before the discovery of non-coding DNA and which became known as the C-value paradox as a result. However, although there is no longer any paradox
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
rate, and, depending on the taxon
, body size, metabolic rate, developmental rate, organ
complexity, geographical distribution, and/or extinction
risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).
The relative molecular weight may be converted to an absolute value by multiplying it by the atomic mass unit
(1 u), which equals one-twelfth of a mass of 12C, i.e., 1.660539 × 10-27 kg. Consequently, the mean weight of one nucleotide pair would be 1.023× 10-9 pg, and 1 pg of DNA would represent 0.978 × 109 base pairs.
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 Mb
The current estimates for human female and male diploid genome sizes are 6.406 × 109 bp and 6.294 × 109 bp, respectively (female diploid genome sizes are larger than males because they have two X chromosomes, whereas males have one X and one Y chromosome
and the Y chromosome is much smaller than the X chromosome). By using the conversion formulas given above, diploid human female and male nuclei in G1 phase of the cell cycle
should contain 6.550 and 6.436 pg of DNA, respectively.
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
contained within a haploid nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
(e.g. a gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
) or one half the amount in a diploid somatic cell
Somatic cell
A somatic cell is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell...
of a eukaryotic organism, expressed in picograms (1pg = ). In some cases (notably among diploid organisms), the terms C-value and genome size
Genome size
Genome size is the total amount of DNA contained within one copy of a single genome. It is typically measured in terms of mass in picograms or less frequently in Daltons or as the total number of nucleotide base pairs typically in megabases . One picogram equals 978 megabases...
are used interchangeably, however in polyploids
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
the C-value may represent two genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
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 have yet to be used by other authors. C-values are reported in picograms.
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 genotypeGenotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
" (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
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
). Swift's study of this topic related specifically to variation (or lack thereof) among chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
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. ProtistProtist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
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
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
contained in its genome, an observation that was deemed wholly counterintuitive before the discovery of non-coding DNA and which became known as the C-value paradox as a result. However, although there is no longer any paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...
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-value enigma
The C-value enigma or C-value paradox is a term used to describe the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species...
. C-values correlate with a range of features at the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
and organism levels, including cell size, cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
rate, and, depending on the taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, body size, metabolic rate, developmental rate, organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
complexity, geographical distribution, and/or extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally 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 recover may have been lost before this point...
risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).
Calculating C-values
By using the data in Table 1, relative weights of nucleotide pairs can be calculated as follows: AT = 615.383 and GC = 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 AT to GC pairs is 1:1, the mean relative weight of one nucleotide pair is 615.8771.The relative molecular weight may be converted to an absolute value by multiplying it by the atomic mass unit
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
(1 u), which equals one-twelfth of a mass of 12C, i.e., 1.660539 × 10-27 kg. Consequently, the mean weight of one nucleotide pair would be 1.023× 10-9 pg, and 1 pg of DNA would represent 0.978 × 109 base pairs.
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 Mb
The current estimates for human female and male diploid genome sizes are 6.406 × 109 bp and 6.294 × 109 bp, respectively (female diploid genome sizes are larger than males because they have two X chromosomes, whereas males have one X and one Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
and the Y chromosome is much smaller than the X chromosome). By using the conversion formulas given above, diploid human female and male nuclei in G1 phase of the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
should contain 6.550 and 6.436 pg of DNA, respectively.
See also
- Animal Genome Size DatabaseAnimal Genome Size DatabaseThe Animal Genome Size Database is a comprehensive catalogue of published genome size estimates for vertebrate and invertebrate animals. It was created in 2001 by Dr. T. Ryan Gregory of the University of Guelph in Canada. As of September 2005, the database contains data for over 4,000 species of...
- Cell nucleusCell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
- Comparative genomicsComparative genomicsComparative genomics is the study of the relationship of genome structure and function across different biological species or strains. Comparative genomics is an attempt to take advantage of the information provided by the signatures of selection to understand the function and evolutionary...
- C-value enigmaC-value enigmaThe C-value enigma or C-value paradox is a term used to describe the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species...
- GenomeGenomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
- Genome sizeGenome sizeGenome size is the total amount of DNA contained within one copy of a single genome. It is typically measured in terms of mass in picograms or less frequently in Daltons or as the total number of nucleotide base pairs typically in megabases . One picogram equals 978 megabases...
- Human genomeHuman genomeThe human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...
- Junk DNA
- Noncoding DNANoncoding DNAIn genetics, noncoding DNA describes components of an organism's DNA sequences that do not encode for protein sequences. In many eukaryotes, a large percentage of an organism's total genome size is noncoding DNA, although the amount of noncoding DNA, and the proportion of coding versus noncoding...
- Plant DNA C-values DatabasePlant DNA C-values DatabaseThe Plant DNA C-values Database is a comprehensive catalogue of C-value data for land plants and algae. The database was created by Prof. Michael D. Bennett and Dr. Ilia J. Leitch of the Royal Botanic Gardens, Kew, UK...
- Selfish DNASelfish DNASelfish DNA refers to those sequences of DNA which, in their purest form, have two distinct properties: the DNA sequence spreads by forming additional copies of itself within the genome; and it makes no specific contribution to the reproductive success of its host organism.This idea was sketched...
- Transposable elementsashkan