Online Biology Dictionary - O

o- Ortho-.

O Oxygen.

obligate aerobes /AIR-ohbs/ Organisms that must have oxygen to live.

obligate anaerobes /AN-uhr-ohbs/ Organisms that not only cannot use oxygen, but that are also poisoned by it.

occlusal surface That surface of a tooth that comes into contact with the opposing teeth when chewing takes place.

occlusion (1) closure; (2) the position of the teeth when the jaws are closed.

odynacusis A condition in which pain in the ears is caused by noises.

oligo Short for oligonucleotide.

Oligocene /oh-LIG-oh-seen/ An epoch of the Tertiary Period of the Cenozoic Era. It lasted from 33.9 to 23 mya. GEOLOGIC TIME SCALE

oligogenic trait A phenotypic trait produced by two or more genes working together.

oligonucleotide (often shortened to oligo) A short segment of RNA or DNA. An oligotide of a given length is indicated by adding the suffix -mer To a number indicating the number of nucleotides present. For example, an oligonucleotide containing 12 nucleotides is a 12-mer.

oligosaccharide A saccharide chain composed of several (usually 10 or fewer), simple sugars.

omnivorous Feeding on both plant and animal substances.

onchium (pl onchia) /AWNK-ee-uhm/ A hook found in or on the margin of the oral cavity of a certain nematodes.

oncology /awn-CALL-uh-jee/ The medical study and treatment of cancer.

oncospheres /AWNK-oh-sfeer/ Embryonic larvae of a tapeworm, which are armed with hooks serving as anchors.

oncogene /AWNK-uh-jeen, -oh-/ A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.

one gene/one enzyme hypothesis A hypothesis, first proposed by the English physician Archibald Garrod in 1909, which asserts that each gene codes for a single, specific enzyme. In the early 1940s American geneticists Edward Tatum and George Beadle evaluated Garrod's hypothesis using the fungus Neurospora. Their findings led to the acceptance of this view. Their key experiments involved exposing the fungus to x-rays, creating mutant strains that were then selected for their inability to grow on a medium that could support the non-mutated form of the fungus. To grow, each mutated strain required the addition of a specific nutritional element to the medium. They found each mutant was blocked at a particular metabolic step that required a particular enzyme that the strain could not produce. However, we now know that not all genes direct the construction of enzymes. Some code for structural proteins, such as collagen or keratin. Moreover, different RNA splicing options can produce distinct polypeptides from the same gene. In addition, some functional proteins, such as the various hemoglobins, are composed of two or more polypeptide chains, derived from two or more different genes. Some genes, such as those for tRNAs do not even code for a polypetide. However, Tatum and Beadle's work remains significant because it provided, for the first time, evidence that a particular gene produces a particular molecular product or set of molecular products.

ontogeny /on-TAWJ-uh-nee/ The course of an embryo's development.

oocyte /OH-uh-site/ The cell undergoing meiosis in females; often called "primary oocyte."

oogamous /oo-uh-GAM-uhs/ Having a small mobile male gamete and a large immobile female gameteembryo."

oogenesis (also ovogenesis) /oh-uh-JEN-uh-suhs/ The process producing female gametes.

ootheca /oh-uh-THEK-uh/ A sturdy protein case containing multiple, usually many, eggs; produced by certain insects and mollusks.

open reading frame (ORF) The sequence of DNA or RNA located between the start-code sequence (initiation codon) and the stop-code sequence (termination codon).

operon /AWP-er-on/ A set of adjacent genes with related functions. They are transcribed under the control of an operator gene.

orad /OH-rad, ORE-ad/ Toward the mouth.

orbit /ORE-buht/ The bony cavity that holds the eyeball.

orchidectomy /ORE-kuh-DECK-uh-mee/ Surgical removal of a testis.

order In taxonomy, a division of a class that itself contains one or more families. CHART OF RELATIVE TAXONOMIC RANKS

orexigenic /oh-REKS-uh-GEN-ik/ Stimulating or increasing appetite.

organelle /OR-guh-NELL/ One of the tiny organs with various specialized functions found in eukaryotic cells.

organic /ore-GAN-ik/ (1) of or relating to organisms or the chemical compounds made by them; (2) referring to a chemical compound containing carbon.

Origin of Species The shortened name usually used to refer to Charles Darwin's most famous book, On the Origin of Species by means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life. Although it has been the most influential book ever written on the subject of evolution, it fails to define its subject matter — the word species has no generally accepted meaning among biologists (READ DISCUSSION OF THIS SUBJECT). Moreover, it misrepresented the temporal dynamics of evolution by asserting that the fossil record shows that new forms of life typically arise gradually from their predecessors. Paleontology has clearly demonstrated that this is not the case (READ ABOUT THE EVIDENCE). On the contrary, fossil forms typically arise abruptly and remain unchanged thereafter up to the time of their extinction. These two oversights on Darwin's part have caused a wide variety of erroneous notions to become embedded in standard biological thought (LEARN MORE).

ornithine cycle (or urea cycle) /ORN-uh-theen/ A cycle of chemical reactions in which the toxic nitrogenous products of protein catabolism are converted to urea, which is nontoxic. In the cycle, ornithine is first converted to citrulline, which is then converted to argino-succinic acid, which is converted to arginine. Urea then splits off of arginine, which produces ornithine again. CHART OF CYCLE

ornithophily /ORE-nuh-THAWF-uh-lee/ Pollination by birds.

ornithology /ORE-nuh-THAWL-uh-gee/ The scientific study of birds.

oropharynx /ORE-oh-FAIR-inx/ The portion of the pharynx below the soft palate.

ortet /ORE-tet/ The ancestral individual that founds a clone. Compare: ramet.

Ortho-, meta- and para- positions of a phenyl group, with respect to a substituent group "R"

ortho- /ORE-thoh/ Chemistry: in ortho-substitution, two substituents occupy positions next to each other, which are usually numbered 1 and 2 (in the figure at right, these are marked R and ortho).

orthopedic /ORE-thoh-PEED-ik/ Correcting deformity.

Oryza sativa /ore-EYE-zuh suh-TIE-vuh/ The scientific name for rice.

osmoregulation /AWZ-moh-reg-yuh-LAY-shuhn/ Regulation of water balance.

osmosis /awz-MOH-suhs/ Regulation of water balance.

osmotic pressure /awz-MAWT-ick/ Pressure caused by a difference in concentration between solutions on the two sides of a selectively permeable membrane.

osteosarcoma (also osteogenic sarcoma) /OS-tee-oh-sar-KOH-muh/ A sarcoma of the bone; usually affects the large bones of the arm or leg; more common in young people and affects more males than females.

otitis /oh-TIGHT-is/ Inflammation of the ear (otitis media: inflammation of the middle ear).

ouabain A compound that blocks the transfer of ions in and out of cells.

outbreeding (also crossbreeding) The mating of unrelated individuals.

outcross (1) mating in which the male and female gametes are derived from different individuals; used in opposition to self-fertilization; (2) the pollination of a stigma with pollen from another plant (used in opposition to self-pollination); (3) outbreed.

outcrosser An individual, or a type of organism, that engages in outcrossing.

ova (sing ovum) An unfertilized egg; a female gamete.

ovary (1) in animals: a female gonad; (2) in plants: the portion of a carpel containing the ovules.

ovariectomy /oh-VER-ee-EK-tuh-mee/ Excision of an ovary.

overlapping inversion An inversion that involves a chromosomal region already affected by an inversion.

overwinding Positive coiling of DNA; results in the DNA double helix being wound more tightly.

oviduct /OH-vuh-duhkt/ An egg-carrying tube.

ovine /OH-vine/ Pertaining to or resembling sheep.

oviparous /oh-VIP-er-uhs/ Egg laying.

ovipositor /oh-vuh-PAWZ-uh-ter/ An organ for laying eggs.

ovogenesis /OH-voe-JEN-uh-sis/ See: oogenesis.

ovotestis An organ in hermaphrodites that serves both as a testis and an ovary.

ovule The structure that develops into a seed after fertilization.

ovum See: ova.

overlapping clones See: genomic library.

oxidant An oxidizing agent.

oxidation reduction reactions Chemical reactions involving the transfer of electrons from a reductant to an oxidant (which oxidizes the reductant and reduces the oxidant).

oxidation The gain of oxygen atoms, the loss of hydrogen atoms (or of electrons) by a compound.

oxidize (1) to remove electrons from or dehydrogenate; (2) to combine with oxygen.

oxygen (O) A gaseous chemical element; odorless, tasteless, colorless; atomic weight 15.9994, atomic number 8, valence 2^-. The second most abundant of the biologically important elements. Makes up 75% of an animal's weight, and 80% of a plant's. During plant respiration oxygen is produced from CO₂, a fact originally discovered by Jan Ingenhousz. PERIODIC TABLE

oxyhemoglobin Oxygenated hemoglobin.

ozone A form of oxygen (O₃); near the Earth's surface it is a pollutant produced by internal combustion engines, but at it also forms a protective high altitude atmospheric layer that blocks ultraviolet radiation.