S Serine in proteins satellite DNA Repetitive DNA that forms a satellite band in a density
gradient scaffold attachment region An AT-rich segment of a eukaryotic genome that acts as an
attachment point to the nuclear matrix. scanning A system used during initiation of eukaryotic
translation, in which the preinitiation complex attaches to
the 5'-terminal cap structure of the mRNA and then scans
along the molecule unit it reaches an initiation codon. score matrix In a dynamic programming alignment, the score matrix
indicates the quality of the alignment ending at each
possible pair of residues. secondary structure The conformation, such as an alpha helix and beta sheet,
taken up by a polypeptide second messenger An intermediate in a certain type of signal transduction
pathway sedimentation analysis The centrifugal technique used to measure the
sedimentation coefficient of a molecule or structure sedimentation coefficient The value used to express the volocity at which a
molecule or structure sediments when centrifuged in a dense
solution (like sucrose or cesium chloride). segment polarity genes Developmental genes that provide greater definition to
the segmentation pattern of the Drosophila embryo
established by the action of the pair-rule genes segregation The separation of homologous chromosomes, or members of
allele pairs, into different gametes during meiosis segregator gene A regluatory gene providing chromosomal attachment sites
for the segregation machinery during meiosis and mitosis selection coefficient A quantitative measure of the reduction in fitness of a
genotype in comparison with the fittest genotype in the
population. A measure of selecitve disadvantage. selection intensity The difference in the fitness values between the various
genotypes in a population. selective medium A medium that supports the growth of only those cells
that carry a particular genetic marker (like antibiotic
resistance where the medium contains a specific
antibiotic) selfish DNA DNA that appears to ahve no function and apparently
contributes nothing to the cell in which it is found self splicing intron An intron that can be cleaved out of the pre-mRNA without
the aid of an internal catalyst. semiconservative replication The mode of DNA replication in which each daughter double
helix is made up of one polynucleotide from the parent and
one newly synthesized polynucloetide. 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 skimming A method for rapid sequence acquisition in which a few
random sequences are obtained from a cloned fragment, the
rationale being that if the fragment contains any genes,
then there is a good chance that at least some of them will
be revealed by these random sequences. sequence divergence The differences between two homologous seqeunces due to
the independent accumulation of genetic changes in each
lineage. sequence tagged site STS -Short (200 to 500 base pairs) DNA sequence that has
a single occurrence in the human genome and whose location
and base sequence are known. Detectable by PCR, STSs are
useful for localizing and orienting the mapping and sequence
data reported from many different laboratories and serve as
landmarks on the developing physical map of the human
genome. Expressed sequence tags (ESTs) are STSs derived from
cDNAs. sex cell A reproductive cell; a cell that divides by meiosis sex chromosome A chromosome which is involved in sex determination sex linkage The situation in which a gene is located on the sex (X or
Y) chromosomes. Shine-Dalgarno sequence The ribosome binding site upstream (towards the 5' end)
of an E. coli gene. short patch repair A nucleotide excision repair process of E. coli that
results in excision and resynthesis of about 12 nucleotides
of DNA short tandem repeat A type of simple sequence length polymorphism comprising
tandem copies of, usually, di-, tri-, or tetranucleotide
repeat units. Also called a microsatellite shotgun approach A genome sequenceing strategy in which the molecules to
be sequenced are randomly broken into fragments which are
then individually sequenced. sibling species Species that are indistinguishable morphologically but
are reproductively isolated. signal peptide A short sequence at the N-terminus of some proteins that
directs the protein across a membrane. It's often cleaved
off after translocation across the ER membrane, although
some proteins have an internal signal sequence that isn't
cleaved (e.g. ovalbumin). silencer A regulatory sequence that reduces the rate of
transcription of a gene or genes located some distance away
in either direction. silent mutation A change in the DNA sequence that has no effect on the
expression or function of any gene or gene product. similarity approach A rigorous mathematical approach to alignment of
nucleotide sequences Simple sequence length polymorphism An array of repeat sequences that display length
variations. SINE Short (less than 500 base pairs) Interspersed Nuclear
Element. A type fo genome-wide repeat, typified by the
Alu sequences found in the human genome. Single copy DNA A DNA sequence that is not repeated elsewhere in the
genome single gene disorder Hereditary disorder caused by a mutant allele of a single
gene (e.g., Duchenne muscular distrophy, sickle cell
disease). single nucleotide polymorphism (SNP) A point mutation that is carried by some individuals of a
population single orphan A single gene, with no homolog, whose function is
unknown. single-strand binding protein One of the proteins that attach to single-stranded DNA in
the regin of the replication fork, preventing base pairs
forming between the two parent strands before they have been
copied. single stranded A DNA or RNA molecule that comprises just a single
polynucleotide sister taxa The pair of species among a group of species under study
taht are evolutionarily the closest to each other. In a
phylogenetic tree, two taxa connected through a single
internal node. site directed mutagenesis Techniques used to produce a specified mutation in a
predetermined position in a DNA molecule site specific recombination Recombination between two double-stranded DNA molecules
which have only short regions of nucleotide sequence
similarity slippage The translocation of a ribosome along a short noncoding
nucleotide sequence between the termination codon of one
gene and the initiation codon of a second gene. small nuclear ribonucleoprotein Structures involved in splicing GU-AG and AU-AC introns
and other RNA processing events, comprising on e or two
snRNA molecules complexed with proteins small nuclear RNA (snRNA) A type of short eukdayotic RNA molecules involved in
splicing GU-AG and AU-AC introns and other RNA processing
events. S1 nuclease An enzyme that degrades single-stranded DNA or RNA
molecules including single-stranded regions in predominantly
double-stranded molecules. somatic cell A nonreproductive cell, a cell that divides by
mitosis somatic mutation A mutation occurring in a somatic cell SOS response A series of biochemical changes that occur in E. coli in
response to damage to the genome and other stimuli Southern hybridization A technique used for detection of a specific restriction
fragment against a background of many other restriction
fragments. Named for Edwin Southern. spacer DNA The DNA found between two genes. Can be either
transcribed ro nontranscribed. speciation 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: species tree A phylogenetic tree that shows the evolutionary
relationships between a group of species. S phase The stage of the cell cycle when DNA synthesis occurs spliceosome The protein-RNA complex involved in splicing GU-AG or
AU-AC introns. splicing The removal of introns from the primary transcript of a
discontinuous gene splicing enhancer A nucleotide sequence that plays a regulatory role during
splicing of GU-AG introns. split gene A gene containing introns spontaneous mutation A mutation that arises from an error in replication SQL Structured Query Language - The most important query
language based on the relational model for extracting data
from a relational database. SR protein A protein that plays a role in splice-site selection
during splicing of GU-AG start codon Codon that signals the start of a sequence to
betranslated to protein. Frequently AUG, but it can vary in
different organisms. stem-loop structure A structure made up of base-paired stem and
nonbase-paired loop, which can form in a single-stranded
polynucleotide that contains an inverted repeat pattern of
bases steric hindrance Atoms in a molecule are forced into certain
configurations because other parts of the molecule are
occupying nearby regions in space and physically blocking
certain atoms from exploring those regions. steroid hormone A type of extracellular signaling compound. steroid receptor A protein that binds a steroid hormone after the latter
has entered the cell, as an intermediate step in modulation
of genome activity. sticky end An end of a double-straneded DNA molecule where there is
a single-stranded extension. Usually generated by digestion
with a restriction enzyme. stop codon Codon that signals the end of a sequence to be translated
to protein. Frequently UAA, UAG or UGA. stringency When a statistic or score is calculated for a window of
residues, the term stringency is used to refer to the
minimum score which will be save dor considered to be a
match. For example, with a window of 10 and a stringency of
6, 6 out of 10 bases must be identical. 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. strong bond In reference to double-stranded nucleic acids or
segments, the three hydrogen bonds between C and G. Confers
increased stability and high melting temperature. strong promoter A promoter that directs a relatively large number of
productive initiations per unit time structural domain A segment of a polypeptide that folds independently of
other segments. Also, a loop of eukaryotic DNA,
predominantly in the form of the 30nm chromatin fiber,
attached to the nuclear matrix. structural gene A DNA nucleotide sequence that codes for a protein or
specifies an RNA molecule. STS mapping A physical mapping procedure that locates the positions
of sequence tagged sites (STSs) in a genome. stuffer fragment A DNA fragment contained within a lambda vector that is
replaced by the DNA to be cloned. subspecies A geographically or morphologically distinct population
in a species substitution matrix The representation of the pattern of substitution in the
form of a matrix, the elements of which denot ethe relative
rate of substitution between any two nucleotides substitution mutation A mutation in one gene that reverses the effect of a
mutation in a second gene. superfamily A collection of genes, all products of gene duplication,
that have diverged from each other to a considerable extent
( in protein-coding genes, usually a similarity of less than
50% at the amino acid level). S value The unit of measurement for a sedimentation coefficient
(Svedberg unit). symbiosis The coexistence of two or more organisms in a mutually
beneficial relationship. symmetrical exon An exon residing between two same-phase introns syncytium A cell-like structure comprising a mass of cytoplasm and
many nuclei. synonymous mutation A mutation that changes a codon into a second codon that
spedcifies the same amino acid. synteny refers to a pair of genomes in which the least some of
the genes are located at similar map positions. systematics Taxonomy and phylogenetics
