Sunday, June 29, 2008

Gene

A gene is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions. The physical development and phenotype of organisms can be thought of as a product of genes interacting with each other and with the environment.A concise definition of a gene, taking into account complex patterns of regulation and transcription, genic conservation and non-coding RNA genes, has been proposed by Gerstein et al. "A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products".
Colloquially, the term gene is often used to refer to an inheritable trait which is usually accompanied by a phenotype as in ("tall genes" or "bad genes") -- the proper scientific term for this is allele.

In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and coding and non-coding sequence. Coding sequence determines what the gene produces, while non-coding sequence can regulate the conditions of gene expression. When a gene is active, the coding and non-coding sequence is copied in a process called transcription, producing an RNA copy of the gene's information. This RNA can then direct the synthesis of proteins via the genetic code. But some RNAs are used directly, for example as part of the ribosome. These molecules resulting from gene expression, whether RNA or protein, are known as gene products.

Genes often contain regions that do not encode products, but regulate gene expression. The genes of eukaryotic organisms can contain regions called introns that are removed from the messenger RNA in a process called splicing. The regions encoding gene products are called exons. In eukaryotes, a single gene can encode multiple proteins, which are produced through the creation of different arrangements of exons through alternative splicing. In prokaryotes (bacteria and archaea), introns are less common and genes often contain a single uninterrupted stretch of DNA, called a cistron, that codes for a product. Prokaryotic genes are often arranged in groups called operons with promoter and operator sequences that regulate transcription of a single long RNA. This RNA contains multiple coding sequences. Each coding sequence is preceded by a Shine-Dalgarno sequence that ribosomes recognize.
The total set of genes in an organism is known as its genome. An organism's genome size is generally lower in prokaryotes, both in number of base pairs and number of genes, than even single-celled eukaryotes. However, there is no clear relationship between genome sizes and complexity in eukaryotic organisms. One of the largest known genomes belongs to the single-celled amoeba Amoeba dubia, with over 670 billion base pairs, some 200 times larger than the human genome. The estimated number of genes in the human genome has been repeatedly revised downward since the completion of the Human Genome Project; current estimates place the human genome at just under 3 billion base pairs and about 20,000–25,000 protein-coding genes, with a article from 2007 giving a number of 20,488 plus perhaps 100 more yet to be discovered. The gene density of a genome is a measure of the number of genes per million base pairs (called a megabase, Mb); prokaryotic genomes have much higher gene densities than eukaryotes. The gene density of the human genome is roughly 12–15 genes per megabase pair.

Albinism

Albinism (from Latin albus, "white"; see extended etymology) is a form of hypopigmentary congenital disorder, characterized by a partial (in hypomelanism, also known as hypomelanosis) or total (amelanism or amelanosis) lack of melanin pigment in the eyes, skin and hair (or more rarely the eyes alone). Albinism results from inheritance of recessive alleles. The condition is known to affect mammals (including humans), reptiles, and amphibians. While the most common term for an organism affected by albinism is "albino" (noun and adjective), the word is sometimes used in derogatory ways towards people; more neutral terms are "albinistic" (adjective) and "person with albinism" (noun). Additional clinical adjectives sometimes used to refer to animals are "albinoid" and "albinic".

Sunday, June 22, 2008

Embryophyte

The embryophytes are the most familiar group of plants. They include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes with specialized reproductive organs. With very few exceptions, embryophytes obtain their energy through photosynthesis (that is, by absorbing light); and they synthesize their food from carbon dioxide. Embryophyta may be distinguished from chlorophyll-using multicellular algae by having sterile tissue within the reproductive organs. Furthermore, embryophytes are primarily adapted for life on land, although some are secondarily aquatic. Accordingly, they are often called land plants or terrestrial plants.

Embryophytes developed from complex green algae (Chlorophyta) during the Paleozoic era. The Charales or stoneworts appear to be the best living illustration of that developmental step. These alga-like plants undergo an alternation between haploid and diploid generations (respectively called gametophytes and sporophytes). In the first embryophytes, however, the sporophytes became very different in structure and function, remaining small and dependent on the parent for their entire brief life. Such plants are informally called 'bryophytes'. They include three surviving groups:

  • Bryophyta (mosses)
  • Anthocerotophyta (hornworts)
  • Marchantiophyta (liverworts)

All of the above 'bryophytes' are relatively small and are usually confined to moist environments, relying on water to disperse their spores. Other plants, better adapted to terrestrial conditions, appeared during the Silurian period. During the Devonian period, they diversified and spread to many different land environments, becoming the vascular plants or tracheophytes. Tracheophyta have vascular tissues or tracheids, which transport water throughout the body, and an outer layer or cuticle that resists drying out. In most vascular plants, the sporophyte is the dominant individual, and develops true leaves, stems, and roots, while the gametophyte remains very small.

Many vascular plants, however, still disperse using spores. They include two extant groups:

  • Lycopodiophyta (clubmosses)
  • Pteridophyta (ferns, whisk ferns, and horsetails)

Other groups, which first appeared towards the end of the Paleozoic era, reproduce using desiccation-resistant capsules called seeds. These groups are accordingly called spermatophytes or seed plants. In these forms, the gametophyte is completely reduced, taking the form of single-celled pollen and ova, while the sporophyte begins its life enclosed within the seed. Some seed plants may even survive in extremely arid conditions, unlike their more water-bound precursors. The seed plants include the following extant groups:

  • Cycadophyta (cycads)
  • Ginkgophyta (ginkgo)
  • Pinophyta (conifers)
  • Gnetophyta (gnetae)
  • Magnoliophyta (flowering plants)

The first four groups are referred to as gymnosperms, since the embryonic sporophyte is not enclosed until after pollination. In contrast, among the flowering plants or angiosperms, the pollen has to grow a tube to penetrate the seed coat. Angiosperms were the last major group of plants to appear, developing from gymnosperms during the Jurassic period, and then spreading rapidly during the Cretaceous. They are the predominant group of plants in most terrestrial biomes today.

Note that the higher-level classification of plants varies considerably. Some authors have restricted the kingdom Plantae to include only embryophytes, others have given them various names and ranks. The groups listed here are often considered divisions or phyla, but have also been treated as classes, and they are occasionally compressed into as few as two divisions. Some classifications, indeed, consider the term Embryophyta at the superphylum (superdivision) level, and include Land Plants and some Charophyceae in a subkingdom named Streptophyta.

On a microscopic level, embryophyte cells remain very similar to those of green algae. They are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. These usually take the form of chloroplasts, which conduct photosynthesis and store food in the form of starch, and characteristically are pigmented with chlorophylls a and b, generally giving them a bright green color. Embryophytes also generally have an enlarged central vacuole or tonoplast, which maintains cell turgor and keeps the plant rigid. They lack flagella and centrioles except in certain gametes.

Sunday, June 15, 2008

Application-specific integrated circuit

An application-specific integrated circuit (ASIC) is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC.
In contrast, the 7400 series and 4000 series integrated circuits are logic building blocks that can be wired together for use in many different applications. Intermediate between ASICs and standard products are application specific standard products (ASSPs).

As feature sizes have shrunk and design tools improved over the years, the maximum complexity (and hence functionality) possible in an ASIC has grown from 5,000 gates to over 100 million. Modern ASICs often include entire 32-bit processors, memory blocks including ROM, RAM, EEPROM, Flash and other large building blocks. Such an ASIC is often termed a SoC (System-on-a-chip). Designers of digital ASICs use a hardware description language (HDL), such as Verilog or VHDL, to describe the functionality of ASICs.

Field-programmable gate arrays (FPGA) are the modern day equivalent of 7400 series logic and a breadboard, containing programmable logic blocks and programmable interconnects that allow the same FPGA to be used in many different applications. For smaller designs and/or lower production volumes, FPGAs may be more cost effective than an ASIC design. The Non-recurring engineering cost (the cost to set up the factory to produce a particular ASIC) can run into the millions of dollars.

The general term application specific integrated circuit includes FPGAs, but most designers use ASIC only for non field programmable devices (e.g. standard cell or sea of gates) and make a distinction between ASIC and FPGAs.

Sunday, June 08, 2008

Breakbeat

This article is about the electronic dance music genre. For the technique and the meaning of the term with hip-hop and funk music, see Break (music). For the record label, see Breakbeat Kaos

Breakbeat (sometimes breakbeats or breaks) is a term used to describe a collection of sub-genres of electronic music, usually characterized by the use of a non-straightened 4/4 drum pattern (as opposed to the steady beat of house or trance). These rhythms may be characterised by their intensive use of syncopation and polyrhythms.

In the late 1960s and early 1970s, hip-hop DJs (starting with Kool DJ Herc) began using several breaks (the part of a funk or jazz song in which the music "breaks" to let the rhythm section play unaccompanied) in a row to use as the rhythmic basis for hip-hop songs. Kool DJ Herc's breakbeat style was to play the same record on two turntables and play the break repeatedly by alternating between the two records (letting one play while spinning the second record back to the beginning of the break). This style was copied and improved upon by early hip hop DJs Afrika Bambaataa and Grand Wizard Theodore.[1] This style was extremely popular in clubs and dance halls because the extended breakbeat was the perfect backdrop for breakdancers to show their skills.

Monday, June 02, 2008

Human Systemic circulation

Systemic circulation is the portion of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart.

Arteries always take blood away from the heart, regardless of their oxygenation, and veins always bring blood back. In general, arteries bring oxygenated blood to the tissues; veins bring deoxygenated blood back to the heart. In the case of the pulmonary vessels, however, the oxygenation is reversed: the pulmonary artery takes deoxygenated blood from the heart to the lungs, and oxygenated blood is pumped back through the pulmonary vein to the heart. As blood circulates through the body, oxygen and nutrients diffuse from the blood into cells surrounding the capillaries, and carbon dioxide diffuses into the blood from the capillary cells.

The release of oxygen from red blood cells or erythrocytes is regulated in mammals. It increases with an increase of carbon dioxide in tissues, an increase in temperature, or a decrease in pH. Such characteristics are exhibited by tissues undergoing high metabolism, as they require increased levels of oxygen.