Lac operon: A structural unit in
the E.coli genome that consists of three structural genes (encoding different
enzymes involved in sugar metabolism) transcribed together and their common
promoter and operator genes. Provides a good model for studying the interactions
between promoters and repressors. Lacerate: With a deeply and irregularly incised margin. Lagoon: A shallow pond or channel linked to the ocean, but often separated by a reef or sandbar. lanceolate - Tapering at both ends. larva immature juvenile form of an animal Larvae: An early developmental stage of an organism that is morphologically different from the juvenile or adult form. See EMBRYONIC DEVELOPMENT. Last male sperm precedence: A situation that results in fertilization of the ovum by the sperm of the last male in multiply inseminated female. This is due to sperm incapacitation by the semen and sperm displacement. This well-documented form of sperm competition is best known in Drosophila. Lateral: In desmokonts oriented toward the left or right side of the cell. Lateral line: A pressure sensory system located in a line of pores under the skin on both sides of most fishes. The system is connected indirectly with the inner ear and senses water pressure changes due to water movement (including sound waves). Leader sequence: A sequence at the 5' (N-terminal) of the DNA and mRNA which leads the newly synthesized mRNA to the ribosome (it is not translated). It is also used to mean the signal sequence which is translated but is subject to post-translation cleavage when the final destination is reached or following secretion. Lek: A special site where communal courtship display takes place by swarms of animals. Lekking is best known in male birds to attract female mates. Rare form of lekking by females is seen in the African butterfly (Acraea encedon), the Dance fly (Empis borealis) and the European dotterel bird (Eudromias morinellus). Length Frequency: An arrangement of recorded lengths which indicates the number of times each length or length interval occurs. For example, 10 measurements of lengths are taken in the following order: 10, 12, 12, 14, 12, 15, 15, 19, 12, and 10. A length frequency would be: Length Length Occurrence Interval Occurrence 10 2 11 0 10-12 6 12 4 13-15 3 13 0 or 16-18 0 14 1 19-21 15 2 16 0 17 0 18 0 19 Lenticulate: Shaped like a double convex lens; shaped like a lentil. Librigenae: The "free cheeks"; separate, detachable portions of the trilobite cephalon. Life cycle: A continuum of phases and cell types in the reproduction and growth of a species. The life cycle usually contains at least an asexual phase in which a cell can divide by binary fission and produce two similar cells (1N). It may also contain a sexual phase in which gametes fuse to form zygotes (2N) and these zygotes produce 1N cells. Life history: The description of an organism's life cycle. Ligase: An enzyme which is of vital importance in recombinant DNA technology. It joins nucleotides together by a phosphodiester bond between the 5'-P end of a polynucleotide chain and the 3'-OH end of another one. Limpet: A herbivorous intertidal invertebrate. Lineage: Any continuous line of descent; any series of organisms connected by reproduction by parent of offspring. Lines (long interspersed elements): One of the abundant intermediate (6 to 7 kb) repeat DNA sequences in mammals (see also SINES). Linkage disequilibrium (LD): The tendency for two 'alleles' to be present on the same chromosome (positive LD), or not to segregate together (negative LD). As a result, specific alleles at two different loci are found together more or less than expected by chance. The same situation may exist for more than two alleles. Its magnitude is expressed as the delta (D) value and corresponds to the difference between the expected and the observed haplotype frequency. It can have positive or negative values. LD is decreased by recombination. Thus, it decreases every generation of random mating unless some process opposing the approach to linkage equilibrium. Permanent LD may result from natural selection if some gametic combinations result in higher fitness than other combinations. Link to a lecture on linkage disequilibrium; online linkage disequilibrium analysis. Linkage: The tendency of 'genes' on the same chromosome to segregate together. This means that linked genes are transmitted to the same gamete more than 50% of the time. Genetic linkage reflects a lack of meiotic crossovers between two genes. Liposome: Small spheres of phospholipid bilayers (just like cell membranes). They are used for gene transfer into cells. The DNA to be transferred is attached to them. The DNA-liposome complex fuses with the cell membrane to enter the cell and releases the DNA into the cell. List: Membranous thecal extensions of armoured dinoflagellates (often associated with the cingulum and sulcus); some extensions are curved or ribbed. littoral (intertidal) area of the shore between mean high water and mean low water; the intertidal zone Littoral: The shore area between the mean low and high tide levels. Water zones in this area include the littoral pelagic zone and the littoral benthic zone. Living fossil: An extant species which is morphologically very similar to a species from the ancient past. Despite apparent lack of change, they seem to have escaped extinction. Coelacanth (a 350 million-years-old lobe-finned fish), Horseshoe Crab (a 510 million-years-old marine arthropod), Amazon River Dolphin, Gingko (maidenhair tree, a gymnosperm), and Metasequoia (Metasequoia glyptostrobodes, a conifer) are examples. Lobe: A rounded projection on a structure. Lobe-finned fish: A group of fish that have bone and muscle in their limbs as opposed to simple fins as in most teleosts. The group includes coelacanth and lungfish. Locus: The position on a chromosome occupied by a particular gene (plural: loci). Lod score: Logarithm of the odds favoring linkage obtained from the statistical analysis of linkage. The lod score (Z) of +3 is considered evidence for linkage. Long-term Potential Catch: The largest annual harvest, in weight, that can be removed from a fish stock year after year, under existing environmental conditions. This can be estimated in various ways, from maximum values from production models to average observed catches over a suitable period of years. Lophophore: Complex ring of hollow tentacles used as a feeding organ. The tentacles are covered by cilia, which generate a current to bring food particles into the mouth. The structure is only found in the brachiopods, phoronids, and bryozoans. More info? Loss-of-heterozygosity (LOH): Refers to the disappearance of polymorphic marker alleles when constitutional DNA and tumor DNA from cancer patients are compared. The consequence is usually genomic deletion discarding the normal copies of tumor suppressor genes. Such deletion (or functional deletion through methylation) may uncover existing mutations in the homologue copy. Lyon hypothesis: The proposition by Mary F Lyon that random inactivation of one X chromosome in the somatic cells of mammalian females is responsible for dosage compensation and mosaicism. |