C-value enigma
Encyclopedia
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. 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 humans.
, rather than protein
, was the substance of which genes
are composed. The term C-value
reflects this observed constancy. However, it was soon found that C-value
s (genome size
s) vary enormously among species and that this bears no relationship to the presumed number of genes (as reflected by the complexity
of the organism
). For example, the cells of some salamanders may contain 40 times more DNA than those of humans. Given that C-value
s were assumed to be constant because DNA is the stuff of genes, and yet bore no relationship to presumed gene number, this was understandably considered paradox
ical; the term C-value paradox was used to describe this situation by C.A. Thomas, Jr. in 1971.
The discovery of non-coding DNA in the early 1970s resolved the main question of the C-value paradox: genome size
does not reflect gene
number in eukaryotes since most of their DNA is non-coding and therefore does not consist of genes. The human genome
, for example, comprises only about 1.5% protein-coding genes, with the other 98.5% being various types of non-coding DNA (especially transposable elements) (International Human Genome Sequencing Consortium 2001).
" (Swift 1950) in reference to haploid nuclear
DNA
contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory
of the University of Guelph
in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount of non-coding DNA found within the genome
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:
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...
among eukaryotic 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
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
have genomes much larger than that of humans.
C-value paradox history
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 DNADNA
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...
, rather than protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, was the substance of which 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...
are composed. The term C-value
C-value
The term C-value refers to the amount of DNA contained within a haploid nucleus or one half the amount in a diploid somatic cell of a eukaryotic organism, expressed in picograms...
reflects this observed constancy. However, it was soon found that C-value
C-value
The term C-value refers to the amount of DNA contained within a haploid nucleus or one half the amount in a diploid somatic cell of a eukaryotic organism, expressed in picograms...
s (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...
s) vary enormously among species and that this bears no relationship to the presumed number of genes (as reflected by the complexity
Complexity
In general usage, complexity tends to be used to characterize something with many parts in intricate arrangement. The study of these complex linkages is the main goal of complex systems theory. In science there are at this time a number of approaches to characterizing complexity, many of which are...
of the organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
). For example, the cells of some salamanders may contain 40 times more DNA than those of humans. Given that C-value
C-value
The term C-value refers to the amount of DNA contained within a haploid nucleus or one half the amount in a diploid somatic cell of a eukaryotic organism, expressed in picograms...
s were assumed to be constant because DNA is the stuff of genes, and yet bore no relationship to presumed gene number, this was understandably considered 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; the term C-value paradox was used to describe this situation by C.A. Thomas, Jr. in 1971.
The discovery of non-coding DNA in the early 1970s resolved the main question of the C-value paradox: 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...
does not reflect gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
number in eukaryotes since most of their DNA is non-coding and therefore does not consist of genes. The human genome
Human genome
The 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...
, for example, comprises only about 1.5% protein-coding genes, with the other 98.5% being various types of non-coding DNA (especially transposable elements) (International Human Genome Sequencing Consortium 2001).
C-value enigma origin
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-valueC-value
The term C-value refers to the amount of DNA contained within a haploid nucleus or one half the amount in a diploid somatic cell of a eukaryotic organism, expressed in picograms...
" (Swift 1950) in reference to haploid nuclear
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...
DNA
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...
contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory
T. Ryan Gregory
Dr. T. Ryan Gregory is a Canadian evolutionary biologist and genome biologist and a tenured Associate Professor in the and the at the University of Guelph in Guelph, Ontario, Canada....
of the University of Guelph
University of Guelph
The University of Guelph, also known as U of G, is a comprehensive public research university in Guelph, Ontario, Canada. It was established in 1964 after the amalgamation of Ontario Agricultural College, the Macdonald Institute, and the Ontario Veterinary College...
in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount of non-coding DNA found within the 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 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, nucleiCell 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...
, cells, and organisms? - Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?
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-valueC-valueThe term C-value refers to the amount of DNA contained within a haploid nucleus or one half the amount in a diploid somatic cell of a eukaryotic organism, expressed in picograms...
- 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 elements