Virus symmetry (cuboid) Most animal viruses have spherical or cuboid symmetry. Obtaining a true sphere is not possible for such structures and hence subunits come together to produce a cuboid structure which is very close to being spherical The 'closed shell capsid is usually based on the structure referred to as an icosahedron A regular icosahedron, formed from assembly of identical subunits, consists of 20 equilateral triangular faces, 30 edges and 12 vertices and exhibits 2-, 3-and 5-fold symmetry The minimum number of capsomers required to construct an icosahedron is 12, each composed of five identical subunits. Many viruses have more than 12
Most animal viruses have spherical or cuboid symmetry. Obtaining a true sphere is not possible for such structures and hence subunits come together to produce a cuboid structure which is very close to being spherical. Virus symmetry （cuboid） The 'closed shell' capsid is usually based on the structure referred to as an icosahedron. A regular icosahedron, formed from assembly of identical subunits, consists of 20 equilateral triangular faces, 30 edges and 12 vertices and exhibits 2-, 3- and 5-fold symmetry . The minimum number of capsomers required to construct an icosahedron is 12, each composed of five identical subunits. Many viruses have more than 12
Although in adenoviruses projecting fibers are also present, which distinguishes this capsid from that of other viruses. The maturation and assem bly of these structures is very complex; indeed, much of how it happens is unknown. Axes of rotation Diagrammatical representation of isosahedral symmet
Diagrammatical representation of isosahedral symmetry Although in adenoviruses projecting fibers are also present, which distinguishes this capsid from that of other viruses. The maturation and assembly of these structures is very complex; indeed, much of how it happens is unknown. Axes of rotation
Twofold axes of rotation Helical axis Helical axi Threefold axes Fivefold axes of rotation of rotation T=1 T=3
4. Virus envelopes Many viruses in addition to having a capsid also contain a virus-encoded envelope. Most enveloped viruses bud from a cellular membrane(plasma membrane, e.g. influenza virus, or nuclear membrane, e.g. herpes simplex virus). Within this virus lipid/ protein bilayer are a number of inserted virus- encoded glycoproteins. The envelopes can be amorphous (e.g. the herpes virion) or tightly bound to the capsid(e. g HIV). Thus, the lipid of the envelope is derived from the cell, the glycoprotein being encoded by the virus. Quite how the process of budding occurs is largely unknown
Many viruses in addition to having a capsid also contain a virus-encoded envelope. Most enveloped viruses bud from a cellular membrane (plasma membrane, e.g. influenza virus, or nuclear membrane, e.g. herpes simplex virus). Within this virus lipid / protein bilayer are a number of inserted virusencoded glycoproteins. The envelopes can be amorphous (e.g. the herpes virion) or tightly bound to the capsid (e.g. HIV). Thus, the lipid of the envelope is derived from the cell, the glycoprotein being encoded by the virus. Quite how the process of budding occurs is largely unknown. 4. Virus envelopes
ey Notes definitions Viruses are obligate intracellular parasites and ary from 20-200 nm in size. They have varied hape and chemical composition, but contain only RNA or DNA. The intact particle is termed a virion which consists of a capsid that may be enveloped further by a glycoprotein/lipid membrane Viruses are resistant to antibiotics
Viruses are obligate intracellular parasites and vary from 20-200 nm in size. They have varied shape and chemical composition, but contain only RNA or DNA. The intact particle is termed a 'virion' which consists of a capsid that may be enveloped further by a glycoprotein/lipid membrane. Viruses are resistant to antibiotics. Key Notes definitions