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Biology Definitions

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V SegmentAntibodies are created by the stochastic (random) splicing of three different gene sequences: a V Segment, a J Segment, and a D Segement. In this way, the immune system can create many different antibodies. Note that the joining of the V, J, and D segment is somewhat sloppy, resulting in even more mutations and variations in antibodies.Antibody, Antigen
VacuoleA small, membrane-bound cavity that is used by the cell for storage. Vacuoles are used for storing water, food, or metabolic waste. The membranes of vacuoles are typically made from a lipid bilayer. A vesicle is an example of a very small vacuole, often used for transport (e.g. in the Golgi Apparatus).Lipid, Vesicle, Lipid
VectorA scientist, wishing to insert some foreign DNA into a cell (thus "transforming" the cell), uses a stretch of DNA called a "vector" to do this. This stretch of DNA has replication initiation sites ("origins of replication", so that the inserted DNA can be replicated). Episomes are good candiates for vectors in bacteria.DNA Cloning, Transform, Episome
Vesiclea small, membrane-bound, bubble-like container that is used for storage and transport within the cytoplasm of the cell. For example, the Golgi Apparatus uses vesicles to store proteins. The membrane of the vesicle is created by a lipid bilayer. A vesicle is an example of a very small vacuole.Lipid, Golgi Apparatus, Vacuole, Cytoplasm, Protein
Viral EnvelopeSome viruses have an "envelope" consisting of a lipid bilayer with some transmembrane proteins. This envelope is typically "stolen" from the infected cell's membrane as the virus leaves the cell. When classifying viruses by structure, some viruses are classified as having an Envelope. For example, a virus may be classified as having Helical Symmetry with an Envelope (Sendai Virus, Rabies Virus), or may be classified as Helical without an Envelope (Tobacco Mosaic Virus).Virus, Helical Symmetry
Viral GenomesThere are seven known types of viral genome: double stranded DNA, gapped double-stranded DNA, single-stranded DNA, double-stranded RNA, single-stranded plus-polarity RNA, single-stranded minus-polarity RNA, and single-stranded plus-polarity RNA with a DNA intermediate. Note that RNA genomes are the most abundant viral genomes.Virus Classification, Virus Structure, Baltimore Scheme
Viral TegumentSome viruses have a nucleocapsid surrounded by an envelope. If the virus has a cluster of proteins lining the space between the nucleocapsid and the envelope, this cluster of proteins is called the Tegament. Herpes Virus includes a Tegument. Often, some of these proteins are transcriptional stimulators which induce the cell to transcribe viral genomes.Viral Structure
Viral TropismThe specificity of a virus for a particular cell type. Viruses typically invade a cell through a particular receptor, exhibiting a "tropism" ("facing towards") for that particular cell type. For example, the HIV virus has a tropism for T Helper Cells which have the CD4 receptor which the virus attaches to.AIDS, CD4, Xenotropic, Ecotropic, Amphitropic
VirionA virus particle.Virus
VirusA sub-cellular particle that doesn't have its own energy metabolism and that parasitize the macro-molecular machinery of a cell so that it can reproduce. The virus enters a cell by "fooling" a receptor on the cell's surface. The virus carries a genome, which can be either DNA or RNA. If the genome consists of RNA, the virus will bring Reverse Transcriptase (a protein) into the cell. Some viruses are cytopathic (causing cell death, lysis), while others are not. Viruses generally have a lipid-bilayer cell membrane encapsulating a protein capsid with an RNA or DNA genome (single or double-stranded) and some specialized proteins (e.g. integrase, reverse transcriptase). The layer of plasma membrane is often taken from an infected cell and added to the virion as it is pushed out of the infected cellDNA, RNA, Lysis, Integrase, Reverse Transcriptase, Virus Structure, Virus Classification
Virus ClassificationVirus classification uses a subset of Linnaean Classification (viruses aren't in the tree of life), and are classified according to families, genera, and species (with no domain, kingdom, phylum, class, or order). Examples of virus families include Phycodnaviridae, Orthomyxoviridae, and Coronaviridae. Examples of virus genera include Enteroviruses, Rhinoviruses, and Cardiovirus. Polio Type 1 Virus is an example of a species. Because viruses mutate so much, each virion can itself be considered a quasi-species. Virus Classification is somewhat controversial, and there are competing classification systems; plant virologists generally do not follow the family, genera, species classification, and some classifications include an "order" at the top of the hierarchy. Historically, virus classifcation was made based on the nature of the nucleic acid (RNA, DNA, single strand, double strand, plus or minus polarity), the symmetry of the shell, enveloped or not, the dimensions (big or small), host characteristics (tropism), antigenic character, and disease. Today, virus classification is mainly done by sequencing the genome.Linnaean Classification, Virus Structure, Species
Virus Entry Into CellsViruses need to get into cells so that they can hijack the cell's translation and transcription mechanisms to produce new copies of the virus. Most viruses are too big to enter directly through the cell membrane, so they usually bind to a receptor on the cell membrane, and "trick" the receptor into giving the virus entry. There are some exceptions to this rule. Viruses of Yeast and Fungi have no extra-cellular phases, and are transmitted vertically (from parent to child). Viruses do not enter plants through receptors (probably because of the protection afforded by the plant's cell wall which surrounds the cell membrane). Viruses of plants enter plants via insects and also mechanical damage (e.g. a temporary break in the plant's cell wall). Vincent Racaniello of TWIV describes two mechanisms of virus entry into cells: the "hook" and the "unzipper". A "hook" receptor holds the virus in place until the cell takes it up, and the cell helps remove the nucleic acid via a mechanism called "uncoating". An "unzipper" receptor binds to the virus, and also begins to take the virus apart, freeing its nucleic acid from the viral envelope and/or membrane, and releasing the nucleic acid into the cell. Polio virus is an example of a virus that attaches to an "unzipper" receptor.Virus, Yeast, Translation, Transcription
Virus StructureViruses are classified by structure as being helical (Tobacco Mosaic Virus, Rabies Virus, Sendai Virus), as having an envelope (Rabies Virus, Sendai Virus, Influenza), as having a Protein Shell (Polio Virus, SV40, Rotaviruses, Adenoviruses), as having an Envelope and a Nucleocapsid (yellow fever virus, Flavia viruses, Alpha Viruses, Toga Viruses), or as being "Complex Viruses" (Pox Viruses). Herpes Virus has an envelope with an icosehedral nucleocapsid. Examples: Meme virus (a virus that infects amoebae) is an example of a virus with an Icosahedral shell but no envelope.Helical Structure, Viral Envelope, Protein Shell, Icosahedral Symmetry, Viral Tegument

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