Sunday, June 29, 2008

Molecular basis

Several types of molecules are particularly important during morphogenesis. Morphogens are soluble molecules that can diffuse and carry signals that control cell differentiation decisions in a concentration-dependent fashion. Morphogens typically act through binding to specific protein receptors. An important class of molecules involved in morphogenesis are transcription factor proteins that determine the fate of cells by interacting with DNA. These can be coded for by master regulatory genes and either activate or deactivate the transcription of other genes; in turn, these secondary gene products can regulate the expression of still other genes in a regulatory cascade. Another class of molecules involved in morphogenesis are molecules that control cell adhesion. For example, during gastrulation, clumps of stem cells switch off their cell-to-cell adhesion, become migratory, and take up new positions within an embryo where they again activate specific cell adhesion proteins and form new tissues and organs. Several examples that illustrate the roles of morphogens, transcription factors and cell adhesion molecules in morphogenesis are discussed below.

Sunday, June 22, 2008


Cytokinesis is the process whereby the cytoplasm of a single cell is divided to spawn two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next. In animal cells, one notable exception to the normal process of cytokinesis is oogenesis (the creation of an ovum in the ovarian follicle of the ovary), where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.

During normal proliferative divisions, animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring, made of non-muscle myosin II and actin filaments, assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible down a light microscope. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed and the process of abscission then physically cleaves this midbody into two. Abscission depends on septin filaments beneath the cleavage furrow, which provide a structural basis to ensure completion of cytokinesis. After cytokinesis, non-kinetochore microtubules reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase (see also cell cycle).

Sunday, June 15, 2008


The final step in the development process, starting in the 1980s and continuing through the present, was "Very Large-Scale Integration" (VLSI). This could be said to start with hundreds of thousands of transistors in the early 1980s, and continues beyond several billion transistors as of 2007.

There was no single breakthrough that allowed this increase in complexity, though many factors helped. Manufacturing moved to smaller rules and cleaner fabs, allowing them to produce chips with more transistors with adequate yield, as summarized by the International Technology Roadmap for Semiconductors (ITRS). Design tools improved enough to make it practical to finish these designs in a reasonable time. The more energy efficient CMOS replaced NMOS and PMOS, avoiding a prohibitive increase in power consumption. Better texts such as the landmark textbook by Mead and Conway helped schools educate more designers...

In 1986 the first one megabit RAM chips were introduced, which contained more than one million transistors. Microprocessor chips passed the million transistor mark in 1989 and the billion transistor mark in 2005. The trend continues largely unabated, with chips introduced in 2007 containing tens of billions of memory transistors .

Sunday, June 08, 2008

Popular trance

By the mid-1990s trance, specifically progressive trance, which emerged from acid trance much as Progressive house had emerged from Acid house, had emerged commercially as one of the dominant genres of dance music. Progressive trance has set in stone the basic formula of modern trance by becoming even more focused on the anthemic basslines and lead melodies, moving away from hypnotic, repetitive, arpeggiated analog synth patterns and spacey pads. Popular elements and anthemic pads became more widespread. Compositions continued to contain incremental changes (aka progressive structures), sometimes composed in thirds (as BT frequently does). Meanwhile, a different type of trance, generally called uplifting trance was becoming popular. Uplifting trance had buildups and breakdowns that were longer and more exaggerated, being more direct and less subtle than progressive, with more easily identifiable tunes and anthems. Many such trance tracks follow a set form, featuring an introduction, steady build, a breakdown, and then an anthem, a form aptly called the "build-breakdown-anthem" form. Uplifting vocals, usually female, were also becoming more and more prevalent, adding to trance's popular appeal.

Immensely popular, trance found itself filling a niche that was 'edgier' than house, more soothing than drum and bass, and more melodic than techno, which made it accessible to a wide audience. Artists like Tiƫsto, Armin van Buuren, Paul van Dyk, Robert Miles, Above & Beyond, Darren Tate, Ferry Corsten, Johan Gielen, ATB and Paul Oakenfold came to the forefront as premier producers and remixers, bringing with them the emotional, "epic" feel of the style. Many of these producers also DJ'd in clubs playing their own productions as well as those by other trance DJs. By the end of the 1990s, trance remained commercially huge, but had fractured into an extremely diverse genre. Some of the artists that had helped create the trance sound in the early and mid-1990s had, by the end of the decade, abandoned trance completely in favor of more underground sounds - artists of particular note here include Pascal F.E.O.S. and Oliver Lieb.

As trance entered the mainstream it alienated many of its original fans. As the industry became bigger, record labels, Ibiza based producers, clubs (most notably Ministry of Sound) and DJs continued to alter their sound to more of a pop based one, so as to make the sound more accessible to an even wider and younger audience.

Monday, June 02, 2008


A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus, hence the word mushroom is most often applied to fungi (Basidiomycota, Agaricomycetes) that have a stem (stipe), a cap (pileus), and gills (lamellae, sing. lamella) on the underside of the cap just as do store-bought white mushrooms. However, "mushroom" can also refer to a wide variety of gilled fungi, with or without stems, and the term is used even more generally to describe both the fleshy fruiting bodies of some Ascomycota and the woody or leathery fruiting bodies of some Basidiomycota, depending upon the context of the word. Forms deviating from the standard form usually have more specific names, such as "puffball", "stinkhorn", and "morel", and gilled mushrooms themselves are often called "agarics" in reference to their similarity to Agaricus or their placement in the order Agaricales. By extension, "mushroom" can also designate the entire fungus when in culture or the thallus (called a mycelium) of species forming the fruiting bodies called mushrooms.