Glossary of Scientific Terms

S

Sequence
The linear arrangement of building blocks in biological macromolecules like DNA, RNA, protein and polysaccharides. DNA and RNA macromolecules are linear polymers of nucleotides. Proteins are linear polymers of amino acids. Polysaccharides are linear and branched polymers of monosaccharides (sugars). While the sequence of RNA and proteins are encoded for by the nucleotide sequence in DNA (the genes and genomes), polysaccharides which play important roles in physiology are not encoded for by genetic information, but rather by the spatial and temporal activity of enzymes that synthesize these polysaccharides. 

Similarity
Similarity in biology refers to the relatedness of nucleic acid and amino acid sequences and protein structures. Similarity can be expressed in percent identity referring to the percentage of building blocks in any two or more sequences found in the same string or pattern. Similarity is used to infer homology, a term in evolutionary biology that indicates a common ancestry between sequences or structures, i.e., to modern genes or proteins, albeit not identical but highly similar (~72%) are evolutionarily related and have diverged and accumulated changes independently from each other after a speciation event. Similarities around 25% identity or lower can no longer indicate an evolutionary relationship.

Speciation
Speciation is the process of forming two new species from a common ancestor species. Speciation is the central process of macroevolution, the evolution of novel forms.

Species
A group of organisms (individuals) that can interbreed and reproduce with each other. Used to distinguish sexually reproducing organisms into groups. Individuals from two different species cannot have offspring. They are said to be reproductively isolated. The biologist Ernst Mayr formulated this definition of a species advancing our understanding of the mechanism of evolution of higher organisms. For microbes, the species definition does not properly apply, because they do not reproduce sexually, but have an efficient mechanism to exchange genetic material even between evolutionarily distant forms. This exchange of genes is known as horizontal gene transfer. Unlike sexual reproduction, it usually involves only a fraction of an organisms genome that is being transferred and is a mechanisms of increasing genetic variability among microorganisms that does not depend on cellular reproduction (cell division).

Splicing
A process during protein synthesis where the mRNA cuts out the intron sequences and strings together the exon (coding) sequences derived from a DNA template during transcription. Usually, eukaryotic genes consist of several exons/introns and sometimes not all exons are used in the final mature RNA. Ultimately, a single gene can be spliced into more than one mature mRNA producing splice variants. Splice variants give rise to proteins with different size and functionality.

Starch
The major complex carbohydrate of caloric value from plant products. Starch is a polymer made of thousands of glucose units.

Statistics
The mathematical procedure to describe probabilities and the random or non-random distribution of matter or occurrence of events.

Steady-state (equilibrium)
A process in biochemistry that refers to situations of high activity even though the overall structure and composition of cells seems not to change. Steady state processes are used to explain metabolic homeostasis. The flow of water molecules in a river or free flowing traffic are examples of steady state processes where the overall movement of water molecules (cars) does not appear to change, although the composition of particular molecules (cars) is constantly changing.

Stem Cell
Stem cells are specialized cells of animals and plants that have the ability to grow and divide by mitosis. Cell division results in self-regeneration as well as differentiation. This means that stem cells can maintain their characteristic as stem cells (self-regeneration), but also change into a different cell type (differentiation).

Stochastic
A stochastic event is based on random behavior. The occurrence of individual events cannot be predicted, although measuring the distribution of all observations usually follows a predictable pattern. These patterns can be described by statistical means. An example is the decay of radio active material, where a clump of matter has a measurable and thus predictable half-life time. It is impossible, however, to mark an individual atom and predict when it will decay and emit radiation. The latter process is a stochastic event.

Stomata
The pore openings underneath plant leaves that can open and close according to the metabolic needs of the plant. They are the ports for exchange of oxygen and carbon dioxide gas for photosynthesis, but also release excess water into the air. This process of water loss maintains a steady flow of water and minerals from the roots to the leaves. To minimize the water loss, many plants regulate the duration and time of day when stomatas are open.

Structure, high-resolution
The high resolution structure of a molecule refers to its atomic organization in three-dimensional space. It is either obtained from analysis of diffraction patterns of high energy radiation (X-rays, electron waves) or nuclear magnetic resonance spectra (NMR). Structural information has an important place in biological studies at the molecular level, because structures can be used to elucidate the detailed mechanism of a chemical reaction, a biological binding events such as hormone signaling or immunological defenses, or nutrient transport (absorption) across intestinal epithelial cell layers and cell membranes. The structural analysis of DNA in 1953 has helped understand the mechanism of replication of genetic information during reproduction as well as the mechanism of genetic encoding, reading (transcription), and synthesis (translation)of amino acid sequences in proteins and enzymes.

Synapse
The synapse is a specialized portion of a neuron or nerve cell that is used for cell to cell communication with other neurons and muscle cells. The chemical synapse contains packaged neurotransmitters that can be released upon an electrical signal (action potential) reaching the synapse from the dendrites and cell body of the neuron, where action potentials originate (where synapses of other neurons interact with the signaling cell). A neuron can have multiple synapses, often with different signaling properties being excitatory or inhibitory synapses. Multiple synapses signaling to a receiving neuron or muscle can strengthen a stimulus or inhibition by activating some or all of the synapses through addition of signaling strength. In addition to chemical synapses, electrical synapses are propagating an action potential signal without a neurotransmitter, but directly by coupling membranes of adjacent cells using gap junctions. The feature of both chemical and electrical synapses allows the signal to propagate unidirectional. The signal cannot reverse. However, feedback signals between the signaling and receiving cell to strengthen or weaken the synaptic interaction, a process called synaptic plasticity.