25
Dec
2009
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Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

S

secondary structure
In proteins and nucleic acids, the structure of the molecule brought about by the formation of hydrogen bonds between amino acids or nucleotides, respectively. In proteins, the localized structures in the polypeptide chain (e.g., alpha-helix, beta-sheet, turn). In single-stranded DNA or RNA, the localized double-stranded structures (e.g., hairpins).
segregation
(see Mendelian segregation)
segregator gene
A regulatory gene providing chromosomal attachment sites for the segregation machinery during meiosis and mitosis.
selection
(see natural selection)
selection intensity (stringency of selection)
The difference in the fitness values between the various genotypes in a population.
selective constraint
(see functional constraint)
self-splicing intron
An intron that can be cleaved out of the pre-mRNA without the aid of an external catalyst.
sense codon
A codon specifying an amino acid.
sense strand
The nontranscribed strand of a gene, the DNA sequence of which is identical to the RNA transcript.
sequence divergence (divergence)
The differences between two homologous sequences due to the independent accumulation of genetic changes in each lineage.
sex-linkage
The situation in which a gene is located on the sex chromosomes. Often the use of the term is restricted to genes on the X-chromosome.
sibling species
Species that are indistinguishable morphologically but are reproductively isolated.
signal peptide
A leader peptide that is cleaved out after the synthesis of the polypeptide and before the protein assumes its correct tertiary structure.
silencing
(see nonfunctionalization)
simple transposon
A transposon that does not contain insertion sequences.
SINE (Acronym of Short INterspersed Element)
Any interspersed repeated sequence shorter than 500 base pairs found in 105 or more copies in the genomes of multicellular eukaryotes.
single copy DNA
(see unique DNA)
somatic cell
A cell that is not destined to become a gamete.
somatic mutation
A mutation occurring in a somatic cell.
spacer DNA
The DNA found between two genes. Can be either transcribed or nontranscribed.
speciation (cladogenesis)
The splitting of one population into two or more populations that are reproductively isolated. The process by which new species arise.
species
A basic taxonomic category for which there are various definitions, among them: (1) a group of actually or potentially interbreeding individuals that is reproductively isolated from other such groups (the biological species concept); (2) a lineage evolving separately from others (the evolutionary species concept); and (3) a group of organisms resembling each other more than they resemble any other organism outside the group (the taxonomic species concept).
species tree
A phylogenetic tree that represents the evolutionary relationships of a group of species.
splicing
The removal of introns in the process of RNA maturation.
splicing site or junction
The border between exons and introns.
split gene
A gene containing introns.
SPR
Sub-tree Pruning Regrafting.
standard nucleotide
Adenine, cytosine, guanine, thymine, or uracil.
sticky ends
Single strands of DNA that protrude from opposite ends of a duplex. Usually, generated by staggered cuts of a double-stranded DNA by a restriction enzyme.
stochastic process
A process, the outcome of which cannot be predicted exactly from knowledge of initial conditions. However, given the initial conditions, each of the possible outcomes of the process can be assigned a certain probability.
stop
(see termination codon)
structural domain
(see module)
subspecies
A geographically or morphologically distinct population in a species.
substitution
(see gene substitution and nucleotide substitution)
substitution scheme
(see pattern of substitution)
symbiosis
The coexistence of two or more organisms in a mutually beneficial relationship.
symmetrical exon
An exon residing between two same-phase introns.
synonymous substitution (silent substitution)
A nucleotide substitution resulting in a codon specifying the same amino acid as before.
systematics
Taxonomy and phylogenetics.