Terpen. Mange terpener stammer fra harpiks, her et fyrretræ. Terpener er en stor og varieret gruppe af naturligt forekommende kulstofforbindelser, som primært produceres i mange planter, specielt i nåletræer, men også af nogle insekter, som f.eks. svalehalelarver, der udsender terpener, når de er truet. Terpener er hovedbestandelen i harpiks, og i terpentin fremstillet af harpiks ("vegetabilsk terpentin"). Navnet terpen er afledt af ordet terpentin. Når terpener ændres kemisk, som f.eks. ved oxidation eller omdannelse af kulstofskelettet, kaldes de resulterende forbindelser for terpenoider.
Dog bruges ordet terpen af og til som samlebetegnelse for alle terpener og terpenoider. Terpener og terpenoider udgør hovedbestanddelen af planters og blomsters æteriske olier. Æteriske olier bruges i vidt omfang som smagsstoffer i madvarer, som duftstoffer i parfumer samt i traditionel og alternativ medicin. Struktur og biosyntese[redigér | redigér wikikode] Isopren. Typer[redigér | redigér wikikode] Transposable element. A bacterial DNA transposon A transposable element (TE, transposon or retrotransposon) is a DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cell's genome size. Transposition often results in duplication of the TE. Barbara McClintock's discovery of these jumping genes earned her a Nobel prize in 1983.[1] TEs make up a large fraction of the C-value of eukaryotic cells.
They are generally considered non-coding DNA,[citation needed] although it has been unambiguously shown that TEs are important in genome function and evolution.[2] In Oxytricha, which has a unique genetic system, they play a critical role in development.[3] They are also very useful to researchers as a means to alter DNA inside a living organism. Discovery[edit] Barbara McClintock discovered the first TEs in maize, Zea mays, at the Cold Spring Harbor Laboratory. TEs are more common than usually thought. Classification[edit] Class I (retrotransposons)[edit] Phototropism.wmv. Gene Model Detail. Flowering plant.
The ancestors of flowering plants diverged from gymnosperms around 245–202 million years ago, and the first flowering plants known to exist are from 160 million years ago. They diversified enormously during the Lower Cretaceous and became widespread around 120 million years ago, but replaced conifers as the dominant trees only around 60–100 million years ago. Angiosperm derived characteristics[edit] Flowers The flowers, which are the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from the other seed plants. Flowers aid angiosperms by enabling a wider range of adaptability and broadening the ecological niches open to them. These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans.
Evolution[edit] In 2013 flowers encased in amber were found and dated 100 million years before present. Classification[edit] Gymnosperm. Classification[edit] In early classification schemes, the gymnosperms (Gymnospermae) were regarded as a "natural" group. There is conflicting evidence on the question of whether the living gymnosperms form a clade.[1][2] The fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology (the so-called "seed ferns" or pteridosperms.)[3] When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids are considered, it is clear that angiosperms are nested within a larger gymnosperm clade, although which group of gymnosperms is their closest relative remains unclear.
For the most recent classification on extant gymnosperms see Christenhusz et al. (2011).[4] Subclass Cycadidae Order CycadalesFamily Cycadaceae: CycasFamily Zamiaceae: Dioon, Bowenia, Macrozamia, Lepidozamia, Encephalartos, Stangeria, Ceratozamia, Microcycas, Zamia. Subclass Ginkgoidae. Callose. Callose is a plant polysaccharide. It is composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan.
It is thought to be manufactured at the cell wall by callose synthases and is degraded by β-1,3-glucanases. It is laid down at plasmodesmata, at the cell plate during cytokinesis and during pollen development. It is produced in response to wounding, infection by pathogens.,[1] aluminium and abscisic acid. Deposits often appear on the sieve plates at the end of the growing season.[2] Callose also forms immediately around the developing meiocytes and tetrads of sexually reproducing angiosperms but is not found in related apomictic taxa.[3] See also[edit] Curdlan References[edit]