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World Wide Web (www)
World Wide Web, library of resources available to computer users through the global Internet. It enables users to view a wide variety of information, including magazine archives, public and college library resources, and current world and business news. World Wide Web (WWW) resources are organized so that users can easily move from one resource to another. The connections to different source computers, or servers, on the network are made automatically without being seen by the user. These connections are made with the use of hypertext and hypermedia. Users generally navigate through information on the WWW with the aid of a program known as a WWW browser, or client. The browser presents text, images, sound, or other information objects on the user's computer screen in the form of a page, which is obtained from a WWW server. The user can navigate through information by pointing to specially designated text or other objects on the screen. These objects link the user to other WWW pages on the same server or on any other accessible WWW server on the network. The WWW links exist across the global Internet, forming a large-scale, distributed, multimedia knowledge base through related words, phrases, and images. Smaller-scale implementations are present on the enterprise internets used by businesses.
WWW pages are formatted using Hypertext Markup Language (HTML), and WWW communication among computers uses the Hypertext Transfer Protocol (HTTP), or Wireless Access Protocol (WAP) for mobile phones. This communication is usually through the Internet via Transmission Control Protocol (TCP) connections, but almost any kind of connection can be used.
The WWW was developed in 1989 by Timothy Berners-Lee, a British computer scientist at the CERN research facility near Geneva, Switzerland, to allow information-sharing among internationally dispersed teams of high-energy physics researchers. It subsequently became a platform for related software development, and the numbers of linked computers and users grew very rapidly to support a wide variety of Endeavour’s, including a large business marketplace. Its further development is guided by the WWW Consortium based at Massachusetts Institute of Technology. Future concerns include the efficiency of search engines, the security of transactions and privacy of users, as well as preventing Internet piracy.
Source:
Microsoft Encarta
Human Immunodeficiency Virus (HIV)
Human Immunodeficiency Virus (HIV), member of the Retroviridae family of viruses (commonly known as retroviruses), and classified in the subfamily lentiviruses. Human infection with HIV results in a complex clinical disease known as acquired immune deficiency syndrome (AIDS), which may take ten years or more to develop. HIV was isolated in 1983 almost simultaneously by three groups of scientists: Luc Montagnier's group at the Pasteur Institute in Paris, Robert Gallo's group at the National Cancer Institute, and a group headed by Jay Levy at the University of California, San Francisco. Initial infection with HIV may cause a brief flu-like illness, which is typically followed by a long asymptomatic period during which progressive damage to the immune system occurs, resulting eventually in the
HIV is closely related to viruses that cause similar immunodeficiency diseases in a range of animal species. Its origin in humans is widely accepted to have resulted from cross-species transfer of a simian immunodeficiency virus (SIV) from the chimpanzee, Pan troglodytes troglodytes, in central Africa, probably centuries ago. Changing social mores and urbanization are believed to have provided the conditions necessary for the emergence of HIV as a pandemic during the latter decades of the 20th century. In 2000, close attention to alternative aetiologic (causative) theories, such as the suggestion that HIV spread to humans from contaminated polio vaccines developed in Africa during the 1950s, concluded that there is little evidence to support them.
There are two main types of HIV, known as HIV-1 and HIV-2. HIV-1 is responsible for the majority of infections in the West, while both HIV-1 and HIV-2 are prevalent in Africa. HIV-2 is associated with a less aggressive disease course than HIV-1. HIV-1 has been further classified into several subtypes, known as clades, whose geographical distribution varies from country to country. No preventative vaccine for HIV infection exists, although several candidate vaccines have entered human trials. It remains unclear whether a single vaccine is ever likely to be able to provide protection against all HIV clades, or whether different vaccines may be required for each clade.
HOW HIV CAUSES INFECTION
HIV infects certain human cells by binding its envelope glycoproteins gp120 and gp41 to specific molecules on the surface of the cells. Only cells that carry the appropriate molecules are susceptible to infection by HIV. In the 1980s, scientists quickly recognized that a molecule called CD4, which is found particularly on certain T-lymphocytes (a type of white blood cell), was the primary binding site, but it was only in 1996 that other co-receptors that are also required for infection were identified. Fusion of the virus with the cell membrane permits the viral nucleoid to enter the cell.
One of the co-receptors is called chemokine receptor 5 (CCR5). Because of their inherited genetic make-up, about 14 per cent of Caucasians have unusually small numbers of these receptors on the surface of their cells and a smaller proportion do not express CCR5 on their cells at all, rendering these cells less susceptible to infection with HIV. Studies suggest that this may help to explain why some people appear to be resistant to HIV infection and remain HIV-negative despite multiple exposures to the virus, and why some HIV-positive people experience slower disease progression than others.
As HIV disease progresses, HIV variants called syncytium-inducing (SI) strains evolve within the individual's body. SI variants can use an additional co-receptor on human cells, called CXCR4. This may allow HIV to infect a wider range of cells and may help to explain why the emergence of SI variants is associated with a worse prognosis. Again, a small proportion of Caucasians (about 1 per cent) do not produce this co-receptor.
Once fusion has taken place, reverse transcription then occurs to convert the viral genomic RNA into double-stranded DNA. The viral DNA is transported to the cell nucleus and is integrated, or inserted, into the normal cellular chromosomal DNA. When the right activation signals are present, the process of making new virions begins. Using the replication machinery of the host cell, the integrated viral DNA is transcribed to make messenger RNA (mRNA) and new strands of viral genomic RNA. The viral mRNA is then translated into a protein string that is cleaved into specific viral proteins. Assembly of new virions then takes place within the cell, and the new HIV particles are released by budding from the cell surface, taking a piece of the cell membrane as their envelope.
HIV replication can directly kill CD4+ T-lymphocytes. The loss of these cells paralyses the immune system and is one mechanism by which HIV infection causes AIDS.
A number of anti-HIV drugs have been developed, each targeting a different stage in this viral life cycle. By 2001, nine reverse transcriptase inhibitors and six protease inhibitors had been developed, with more in development. The widespread use of combinations of these agents in the developed world has resulted in dramatic reductions in rates of HIV-related illness and death. Several members of a new class of drugs, which inhibit the binding or fusion of HIV to host cells, are now in clinical development.
Source:
Microsoft Encarta
World Wide Web, library of resources available to computer users through the global Internet. It enables users to view a wide variety of information, including magazine archives, public and college library resources, and current world and business news. World Wide Web (WWW) resources are organized so that users can easily move from one resource to another. The connections to different source computers, or servers, on the network are made automatically without being seen by the user. These connections are made with the use of hypertext and hypermedia. Users generally navigate through information on the WWW with the aid of a program known as a WWW browser, or client. The browser presents text, images, sound, or other information objects on the user's computer screen in the form of a page, which is obtained from a WWW server. The user can navigate through information by pointing to specially designated text or other objects on the screen. These objects link the user to other WWW pages on the same server or on any other accessible WWW server on the network. The WWW links exist across the global Internet, forming a large-scale, distributed, multimedia knowledge base through related words, phrases, and images. Smaller-scale implementations are present on the enterprise internets used by businesses.
WWW pages are formatted using Hypertext Markup Language (HTML), and WWW communication among computers uses the Hypertext Transfer Protocol (HTTP), or Wireless Access Protocol (WAP) for mobile phones. This communication is usually through the Internet via Transmission Control Protocol (TCP) connections, but almost any kind of connection can be used.
The WWW was developed in 1989 by Timothy Berners-Lee, a British computer scientist at the CERN research facility near Geneva, Switzerland, to allow information-sharing among internationally dispersed teams of high-energy physics researchers. It subsequently became a platform for related software development, and the numbers of linked computers and users grew very rapidly to support a wide variety of Endeavour’s, including a large business marketplace. Its further development is guided by the WWW Consortium based at Massachusetts Institute of Technology. Future concerns include the efficiency of search engines, the security of transactions and privacy of users, as well as preventing Internet piracy.
Source:
Microsoft Encarta
Human Immunodeficiency Virus (HIV)
Human Immunodeficiency Virus (HIV), member of the Retroviridae family of viruses (commonly known as retroviruses), and classified in the subfamily lentiviruses. Human infection with HIV results in a complex clinical disease known as acquired immune deficiency syndrome (AIDS), which may take ten years or more to develop. HIV was isolated in 1983 almost simultaneously by three groups of scientists: Luc Montagnier's group at the Pasteur Institute in Paris, Robert Gallo's group at the National Cancer Institute, and a group headed by Jay Levy at the University of California, San Francisco. Initial infection with HIV may cause a brief flu-like illness, which is typically followed by a long asymptomatic period during which progressive damage to the immune system occurs, resulting eventually in the
HIV is closely related to viruses that cause similar immunodeficiency diseases in a range of animal species. Its origin in humans is widely accepted to have resulted from cross-species transfer of a simian immunodeficiency virus (SIV) from the chimpanzee, Pan troglodytes troglodytes, in central Africa, probably centuries ago. Changing social mores and urbanization are believed to have provided the conditions necessary for the emergence of HIV as a pandemic during the latter decades of the 20th century. In 2000, close attention to alternative aetiologic (causative) theories, such as the suggestion that HIV spread to humans from contaminated polio vaccines developed in Africa during the 1950s, concluded that there is little evidence to support them.
There are two main types of HIV, known as HIV-1 and HIV-2. HIV-1 is responsible for the majority of infections in the West, while both HIV-1 and HIV-2 are prevalent in Africa. HIV-2 is associated with a less aggressive disease course than HIV-1. HIV-1 has been further classified into several subtypes, known as clades, whose geographical distribution varies from country to country. No preventative vaccine for HIV infection exists, although several candidate vaccines have entered human trials. It remains unclear whether a single vaccine is ever likely to be able to provide protection against all HIV clades, or whether different vaccines may be required for each clade.
HOW HIV CAUSES INFECTION
HIV infects certain human cells by binding its envelope glycoproteins gp120 and gp41 to specific molecules on the surface of the cells. Only cells that carry the appropriate molecules are susceptible to infection by HIV. In the 1980s, scientists quickly recognized that a molecule called CD4, which is found particularly on certain T-lymphocytes (a type of white blood cell), was the primary binding site, but it was only in 1996 that other co-receptors that are also required for infection were identified. Fusion of the virus with the cell membrane permits the viral nucleoid to enter the cell.
One of the co-receptors is called chemokine receptor 5 (CCR5). Because of their inherited genetic make-up, about 14 per cent of Caucasians have unusually small numbers of these receptors on the surface of their cells and a smaller proportion do not express CCR5 on their cells at all, rendering these cells less susceptible to infection with HIV. Studies suggest that this may help to explain why some people appear to be resistant to HIV infection and remain HIV-negative despite multiple exposures to the virus, and why some HIV-positive people experience slower disease progression than others.
As HIV disease progresses, HIV variants called syncytium-inducing (SI) strains evolve within the individual's body. SI variants can use an additional co-receptor on human cells, called CXCR4. This may allow HIV to infect a wider range of cells and may help to explain why the emergence of SI variants is associated with a worse prognosis. Again, a small proportion of Caucasians (about 1 per cent) do not produce this co-receptor.
Once fusion has taken place, reverse transcription then occurs to convert the viral genomic RNA into double-stranded DNA. The viral DNA is transported to the cell nucleus and is integrated, or inserted, into the normal cellular chromosomal DNA. When the right activation signals are present, the process of making new virions begins. Using the replication machinery of the host cell, the integrated viral DNA is transcribed to make messenger RNA (mRNA) and new strands of viral genomic RNA. The viral mRNA is then translated into a protein string that is cleaved into specific viral proteins. Assembly of new virions then takes place within the cell, and the new HIV particles are released by budding from the cell surface, taking a piece of the cell membrane as their envelope.
HIV replication can directly kill CD4+ T-lymphocytes. The loss of these cells paralyses the immune system and is one mechanism by which HIV infection causes AIDS.
A number of anti-HIV drugs have been developed, each targeting a different stage in this viral life cycle. By 2001, nine reverse transcriptase inhibitors and six protease inhibitors had been developed, with more in development. The widespread use of combinations of these agents in the developed world has resulted in dramatic reductions in rates of HIV-related illness and death. Several members of a new class of drugs, which inhibit the binding or fusion of HIV to host cells, are now in clinical development.
Source:
Microsoft Encarta