C (1) In DNA or RNA, cytosine. (2) In proteins, cysteine. C-terminal (see carboxy terminal)

C value (genome size) The characteristic amount of DNA in the haploid genome of a species.

C-value paradox The apparent lack of correlation between the C value and the level of morphological complexity.

capping The modification of the 5' end of pre-mRNA in eukaryotes whereby a GTP is added to the molecule via a 5'–5' triphosphate bond.

cap site (transcription initiation site) (1) In the DNA, the site where transcription starts. (2) In the RNA, the site that is capped during the process of mRNA maturation.

carboxy terminal (C-terminal) The COOH end of a polypeptide.

carrying capacity The maximum number of individuals of a given species that can be sustained in a defined habitat.

catalyst A compound that lowers the energy necessary to activate a chemical reaction without being consumed or altered by it (see enzyme).

cDNA (see complementary DNA)

census population size (see population size)

character A variable feature that in any given taxonomic unit takes one out of a set of two or more different character states.

character polarity The inferred direction of change of a character in the phylogenetic tree, usually by reference to the character state in an outgroup.

character state The specific value taken by a character in a specific taxonomic unit.

chimeric protein (see mosaic protein)

chirochore Segments defined by a symmetry break in the racemic proportions of complementary nucleotides (mainly C and G) between the two strands.

chloroplast (plastid) A chlorophyll-containing, membrane-enclosed organelle which is the site of photosynthesis in the cells of plants and some protists.

chromatid (1) Each of the two copies produced by chromosome replication. (2) Each of the two DNA strands that comprise the chromosome.

chromosomal duplication (see aneuploidy)

chromosome (1) In prokaryotes, the DNA molecule containing the essential part of the genome (see plasmid and episome). (2) In eukaryotes, a linear DNA molecule complexed with proteins forming a thread-like structure containing the genetic information.

cis The arrangement of two sequences or genes on the same chromosome.

clade (monophyletic group, holophyletic group) (1) According to the rigorous definition, a taxon consisting of a single species and all its descendants representing a monophyletic branch on an evolutionary tree. (2) In looser usage, as above, except that some descendants are not represented. (3) In reference to extant organisms, a subgroup of organisms from among a larger group under consideration sharing a common ancestor not shared by the other organisms in the group.

cladogenesis (see speciation)

cladogram A graphic representation that portrays or attempts to portray the evolutionary relationships among a number of populations, species, or higher taxa.

classification (see taxonomy)

cleavage The removal of the signal peptide and other amino acid sequences from the preproprotein to form the active protein.

coding region All exon parts of a protein-coding gene that are ultimately translated.

codominance (genic selection) The equal contribution to fitness made by the two alleles at a locus in a diploid organism.

codon A triplet of adjacent nucleotides in mRNA that either codes for an amino acid carried by a specific tRNA or specifies the termination of the translation process.

codon family All the codons that code for the same amino acid and differ from each other only at the third position (e.g., among the six codons for leucine, UUA and UUG form one family, and CUU, CUC, CUA and CUG form another).

codon usage The frequency with which members of a codon family are used in protein-coding genes (see unequal codon usage).

coenzyme An organic nonprotein molecule that does not bind the enzyme but is required for the function of the enzyme by acting as an intermediate carrier of electrons, atoms, or groups of atoms.

cofactor An inorganic molecule required by an enzyme in order to function.

coincidental evolution (see concerted evolution)

coincidental substitution The occurrence of two substitutions at the same nucleotide site in two homologous sequences.

colinearity The exact correspondence between the DNA sequence of intronless genes and the amino acid sequence of the encoded protein.

complementarity The antiparallel pairing of nucleotides in double-stranded DNA, double-stranded RNA, or DNA-RNA duplexes.

complementary DNA (cDNA) DNA synthesized from an RNA template by the enzyme reverse transcriptase.

complex transposon (composite transposon) A transposon flanked by two complete, independently transposable insertion sequences.

compositional assimilation The accumulation of point mutations in a pseudogene that eventually obliterates its sequence similarity to the functional gene from which it has been derived and makes its nucleotide composition similar to neighboring DNA sequences.

concerted evolution (horizontal evolution, coincidental evolution) Maintenance of homogeneity of nucleotide sequences among members of a gene family in a species, although the nucleotide sequences change over time.

conditional fixation time The time until fixation of a mutant allele that will eventually become fixed in the population (see mean conditional fixation time).

consensus sequence A sequence that represents the most prevalent nucleotide or amino acid at each site in a number of aligned homologous sequences. Since it is an artificial construct, the consensus sequence may or may not be identical with any of the sequences from which it was inferred.

consensus tree A phylogenetic tree containing undecided multifurcations representing the points of agreement between two or more inferred phylogenetic trees.

conservative substitution The substitution of an amino acid by another with similar chemical properties.

conservative transposition The movement of a transposable element from one genomic position to another without replication of the element.

constant site or region (invariant site) A site or region within the DNA that is occupied by the same nucleotide in all homologous sequences under comparison.

convergence The independent evolution of similar genetic or phenotypic traits.

convergent substitution The substitution of two different nucleotides by the same nucleotide at the same nucleotide site in two homologous sequences.

crossing-over (crossover) The process of exchange of genetic material between two homologous chromosomes leading to recombination of linked genes. Presumed to occur through breakage of both chromosomes at homologous sites followed by reunion after exchange (see recombination).

cryptic gene A pseudogene that can revert to a functional state via a back mutation.

cryptogene A protein-coding or RNA-specifying gene that bears little resemblance to the resulting RNA due to extensive RNA editing.

cyanobacteria A type of photosynthetic eubacteria possessing the ability to photosynthesize. Formerly called blue-green algae.