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Modern Genetic Analysis Glossary
O
ocher codon The codon UAA, a nonsense codon.
octad An ascus containing eight ascospores, produced in species in which the tetrad normally undergoes a postmeiotic mitotic division.
Okazaki fragment A small segment of single-stranded DNA synthesized as part of the lagging strand in DNA replication.
oligonucleotide A short segment of synthetic DNA.
oncogene A gene that contributes to the production of a cancer. Oncogenes are generally mutated forms of normal cellular genes.
opal codon The codon UGA, a nonsense codon.
open reading frame See ORF.
operator A DNA region at one end of an operon that acts as the binding site for repressor protein.
operon A set of adjacent structural genes whose mRNA is synthesized in one piece, plus the adjacent regulatory signals that affect transcription of the structural genes.
ORF (open reading frame) A section of a sequenced piece of DNA that begins with a start codon and ends with a stop codon; it is presumed to be the coding sequence of a gene.
organelle A subcellular structure having a specialized functionfor example, the mitochondrion, the chloroplast, or the spindle apparatus.
organogenesis The production of organ systems in animal embryogenesis.
origin of replication The point of specific sequence at which DNA replication is initiated.
overdominance A phenotypic relation in which the phenotypic expression of the heterozygote is greater than that of either homozygote.

© 1999 by W. H. Freeman and Company. All rights reserved.