| ochre 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 |
| Segment of newly replicated DNA produced
during discontinuous DNA replication. |
| oncogene |
| A gene that contributes to the production
of a cancer. Oncogenes are generally mutated forms of normal cellular
genes (proto-oncogenes). A gene capable, when activated, of transforming
a cell. Oncogenes are found in the oncogenically activated state
in retroviruses and transformed cells and in their normal non-oncogenically
activated state in non-transformed cells in which they are called
proto-oncogenes. |
| one-gene-one-enzyme hypothesis
|
| Hypothesis of Beadle and Tatum that one
gene controls the production of one enzyme. Later modified to the
concept that one cistron controls the production of one polypeptide.
|
| oocyte |
| The gamete in females. See ovum. |
| oogenesis |
| The process of ovum formation in female
animals. |
| oogonia |
| Cells in females that produce primary oocytes
by mitosis. |
| opal codon |
| The codon UGA, a nonsense codon. |
| operator |
| A DNA region at one end of an operon that
acts as the binding site for repressor protein. A DNA sequence that
is recognized by a repressor protein or repressor-corepressor complex.
When the operator is complexed with the repressor, transcription
is prevented. |
| operon |
| A set of adjacent structural genes in bacteria
whose mRNA is synthesized in one piece, together with the adjacent
regulatory signals that affect transcription of the structural genes.
A sequence of adjacent bacterial genes all under the transcriptional
control of the same operator. |
| ORF (open reading frame)
|
| A section of a sequenced piece of DNA that
begins with an initiation (methionine ATG) codon and ends with a
nonsense codon. ORFs all have the potential to encode a protein
or polypeptide, however many may not actually do so. |
| organelle |
| A subcellular structure having a specialized
function for example the mitochondrion, the chloroplast, or the
spindle apparatus. |
| organogenesis |
| The production of organ systems during animal
embryogenesis. |
| origin of replication
|
| The point of specific sequence at which
DNA replication is initiated. |
| outbreeding |
| The mating of genetically unrelated individuals.
|
| outside markers |
| loci on either side of another locus or
specified region. |
| overdominance |
| A relationship in which the phenotypic expression
of the heterozygote is greater than that of either homozygote. |
| ovum |
| Egg. The one functional product of each
meiosis in female animals. |
