On-Line Biology Book: GLOSSARY

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occipital lobe  The lobe of the cerebral cortex located at the rear of the head; is responsible for receiving and processing visual information. PICTURE

oils  Triglycerides that are liquid at room temperature.

oncogenes  Genes that can activate cell division in cells that normally do not divide or do so only slowly. A gene that when over-expressed leads to cancer, but which normally functions in cell division.

"one gene, one enzyme hypothesis Holds that a single gene controls the production, specificity, and activity of each enzyme in a metabolic pathway. Thus, mutation of such a gene changes the ability of the cell to carry out a particular reaction and disrupts the entire pathway.

"one gene one polypeptide hypothesis"  A revision of the one gene, one enzyme hypothesis. Some proteins are composed of different polypeptide chains encoded by separate genes, so the hypothesis now holds that mutation in a gene encoding a specifc polypeptide can alter the ability of the encoded protein to function and thus produce an altered phenotype.

oocyte A cell that will/is undergo/ing development into a female gamete. PICTURE

oogenesis  The production of ova. The development of a diploid cell into a haploid ovum or egg cell. PICTURE

open community  A community in which the populations have different density peaks and range boundaries and are distributed more or less randomly.

opposable  The capability of being placed against the remaining digits of a hand or foot; e.g., the ability of the thumb to touch the tips of the fingers on that hand.

opsins  Molecules in cone cells that bind to pigments, creating a complex that is sensitive to light of a given wavelength.

orders  Taxonomic subcategories of classes. PICTURE

Ordovician extinction Paleozoic-aged mass extinction possibly related to glaciation in the southern-hemisphere supercontinent Gondwana.

Ordovician Period Geologic period of the Paleozoic Era after the Cambrian Period between 500 and 435 million years ago. Major advances during this period include the bony fish and possibly land plants (during the late Ordovician). PICTURE

organelles  Cell components that carry out individual functions; e.g., the cell nucleus and the endoplasmic reticulum. Subcellular structures (usually membrane-bound and unique to eukaryotes) that perform some function, e.g. chloroplas