Most of the Bacteriophage capsid exhibits which symmetry?
ANSWER: (B) IcosahedralREF: BRS 4th ed page 22, Textbook of Microbiology & Immunology Subhash Parija 2nd Ed Ch- 54In geometry, an icosahedron is a polyhedron with 20 facesBACTERIOPHAGEViruses are called bacteriophages (or phages) if they have a bacterial hostAre obligate intracellular parasites.Are host-specific infectious agents for bacteria.Contain protein and RNA or DNA as major components.Morphologic classes of bacteriophagesPolyhedral phages: Are usually composed of an outer polyhedral/ Icosahedral -shaped protein coat (capsid) that surrounds the nucleic acid. May contain a lipid bilayer between two protein capsid layers. Have either circular double-stranded or single-stranded DNA or linear single-stranded RNA as their genetic material, although one phage that has three pieces of double-stranded RNA has been described.Filamentous phages: have a filamentous protein capsid that surrounds a circular single- stranded DNA genome. Are male bacteria specific in that infection occurs through the pili, which are only present on male bacteria. Do not lyse their host cells during the replication process.Complex phages: 95% of all phages. They are tailed phages. T even (T2, T4, T6...) are tadpole shaped with polyhedral / Icosahedral head symmetry containing linear double- stranded DNA and a Helical symmetry protein tail. Thus shows a combination of Icosahedral and helical symmetry (Binary symmetry) e.g. T and lambda phages of E coli.Genetic classes of bacteriophagesi, RNA phages: refer to all phages with RNA as their genetic material. Are specific for bacteria with male pili (male-specific phages). Contain single-stranded RNA, which can act as polycistronic mRNA. E.g. Phage MS-2ii. DNA phages: Most bacteriophage contains only DNA. Contain nucleic add bases that are frequently glucosylated or methylated. May contain some unusual nucleic add bases, such as 5-hydroxymethyl cytosine or 5-hydroxymethyl uracil. Are classified as virulent or temperate, depending on whether their pattern of replication is strictly lytic (virulent) or alternates between lytic and lysogenic (temperate).
Helical
Icosahedral
Spherical
Filamentous
Bacteriophage