Random Motility Non-directed movement of a cell not controlled by a gradient. See Bacterial Motility.
Receptor Proteins that bind external signaling molecules (see: ligand) causing a change in the cell. Receptors are on the cell surface, and internal receptors respond to chemicals that cross the cell membrane. More in HIV Infection and Attachment and the Eukaryotic Cell Model.
Red Blood Cell (erythrocyte) A blood cell that contains the oxygen-carrying protein called hemoglobin. Solve the Red Blood Cell Jigsaw.
Reduction Division The process in eukaryote meiosis I resulting in a diploid (2N) cell producing haploid (1N) gametes. Learn more about Meiosis.
Replication Making an exact duplicate of something. For example , DNA replicating before mitosis occurs. There is more about this in the Cell Cycle.
Repolarize, Repolarization After a nerve pulse (action potential) depolarizes a neuron or muscle cell (makes the membrane change from negative to positive) the cell will renew the negative charge (repolarize) to the resting potential by the redistrubution of ions across the cell membrane. Read about membrane depolarizationin Ion CHannels.
Reverse Transcription The production of DNA from the reading of RNA. For example, HIV RNA is reverse transcribed to DNA within a host cell.
Rheinberg illumination A technique in light microscopy that adds color contrast to an otherwise colorless subject. It involves direct illumination with one color to produce a background of that color, and oblique illumination with an additional color or colors. See a further description.
RNA (Ribonucleic acid) RNA carries the genetic code from DNA (link to term) and defines the order of amino acids in the translation of the genetic code to proteins. Some viruses including HIV contain RNA rather than DNA. See also the definition for DNA.
Rod-Shaped Bacteria One of the three common shapes of bacteria that includes E. coli, Pseudomonas, and Salmonella. View more examples in the Bacteria Model.
Scanning Electron Microcope A microscope that uses a beam of electrons to image the surface of a subject resulting in an image that appears 3-D. There are several examples of scanning electron micrographs in the Gallery.
Secretory Vesicle A cellular vacuole that transports cell products from the golgi apparatus to the cell membrane where the contents are released to the outside (exocytosed) by fusion of the vesicle with the cell membrane. More in the Eukaryotic Cell Model.
Sister Chromatids Duplicate copies of the same DNA from a single chromosome connected to each other at the centromere following DNA replication prior to mitosis.
Spindle Fibers In eukaryote mitosis and meiosis microtubules and associated proteins that form a intracellular fiber structure that pulls the chromosomes apart during cell division. See more in Mitosis.
Spirillum (plural: Spirilla) A genus of helical (spiral) shaped bacteria.
Stationary Phase That stage in the bacterial growth curve when nutrients begin to be depleted and population size remains constant. Read more about bacterial growth stages.
Stem Cell An unspecialized cell that can continue to divide and has the potential to differentiate into more than one mature specialized end cell type.
Storage Granule Bacterial cytoplasmic inclusion that contains reserves of glycogen, lipids, polyphosphate and sometimes sulfur or nitrogen. Explore the Bacteria Model.
Streptolysin Hemolytic blood cell lysing exotoxin (secreted poison) produced by Streptococi causes self-destruction of the neutrophil by necrosis (link to term) rather than apoptosis (link to term). Watch the dramatic effects of streptolysin-s.
Stroma The colorless matrix within a chloroplast rich in enzymes where the later light-independent stages of photosynthesis occur. Learn Plant Cell Organelles.
Telophase In eukaryote mitosis when the daughter chromosomes arrive at the poles and the spindle fibers that have pulled them apart disappear.
Telophase I In eukaryote meiosis first division when the cleavage furrow forms beginning the process of cytokinesis (cell division). The resulting daughter cells are haploid (1N)
Telophase II In eukaryote meiosis second division when the cleavage furrow forms beginning cytokinesis.
Thylakoid Flattened sac-like forms within chloroplasts that are arranged in stacks called grana where the light-dependent photosynthetic reactions occur. Learn Plant Cell Organelles.
Time-lapse A sequence of photographic frames taken at given intervals of normally slow processes. When shown at normal speed the sequence appears speeded up.
Tissue Culture Growing cells or tissues out of the organism under artificial conditions in the laboratory.
Transcription The reading of the DNA genetic code into RNA code which then acts as a template for protein synthesis.
Translation The reading of RNA code to determine the order of amino acids in a protein that is being synthesized.
Trypsin A protease enzyme that breaks down proteins (proteolytic) at specific locations in the protein chain.
Tumbling (bacterial) When flagellar rotation abruptly changes to clockwise from counterclockwise bacteria will stop swimming and tumble in place and appear incapable of going anywhere. Explore bacterial motility.
Turgid Swollen from fluid pressure. The large vacuole in a plant cell contains fluid which keeps the cell rigid.