structure & classification of virus copy.pptx

structure & classification of virus copy.pptx

1. VIROLOGY Basics of Virus

2. INTRODUCTION Viruses are the smallest infectious agents (ranging from about 20 nm to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome. The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-containing membrane. They replicate only in living cells, being parasites at the genetic level.

3. SIZE & SHAPE OF VIRUSES Viruses range from 20 to 300 nm in diameter.  Their shapes are frequently referred to in colloquial terms (e.g., spheres, rods, bullets, or bricks), but in reality they are complex structures of precise geometric symmetry . The shape of virus particles is determined by the arrangement of the  repeating subunits  that form the protein coat ( capsid ) of the virus.

4. Shapes and sizes of medically important viruses

5. VIRAL NUCLEIC ACIDS Viruses contain a single kind of nucleic acid—either DNA or RNA—that encodes the genetic information necessary for replication of the virus . The genome may be single or double stranded, circular or linear, and segmented or non segmented. The type of nucleic acid, its strandedness , and its size are major characteristics used for classifying viruses into families.

6. Capsid: The protein shell, or coat, that encloses the nucleic acid genome. Capsomeres: Morphologic units seen in the electron microscope on the surface of icosahedral virus particles. Capsomeres represent clusters of polypeptides.

7. VIRAL CAPSID & SYMMETRY The nucleic acid is surrounded by a protein coat called a  capsid  made up of subunits called capsomers . Each capsomer , consisting of one or several proteins, can be seen in the electron microscope as a spherical particle. The structure composed of the nucleic acid genome and the capsid proteins is called the  nucleocapsid .  The arrangement of capsomers gives the virus structure its geometric symmetry. 

8. Types of Symmetry of Virus Particles Viral architecture can be grouped into three types based on the arrangement of morphologic subunits: (1) cubic / Icosahedral symmetry( eg , adenoviruses), (2) helical symmetry ( eg orthomyxoviruses ), and (3) complex structures ( eg , poxviruses).

9. Nucleocapsid: A structural unit consisting of the genetic material of a virus and the protein that surrounds it.

10. Envelope: A lipid-containing membrane that surrounds some virus particles. Virus encoded glycoproteins are exposed on the surface of the envelope. These projections are called peplomers.

11. Atypical Virus Like agents DEFECTIVE VIRUS : are composed of viral nucleic acid and proteins but cannot replicate without a helper virus, which provides the missing function. PSEUDOVIRONS: contains host cell DNA instead of viral DNA within the capsid . They are formed during infection with certain viruses. VIROIDS: consists of single molecule of circular RNA without protein coat or envelop.

12. PRIONS: Prions are infectious particle that are composed solely of protein ( i.e they contain no nucleic acid). They are implicated as the cause of certain slow disease called Transmissible Spongiform encephalopathies which include disease as Creutzfeidt -Jakob disease in humans and Scrapie in sheep.

13. BACTERIOPHAGE A virus which parasitizes a bacterium by infecting it and reproducing inside it. A virus that lives within a bacterium replicating itself and eventually destroying the bacterial cell. The length of a bacteriophage is between 24 and 200 nm.

14. The structure of a bacteriophage consists of a head and a tail. The genetic material of the bacteriophage is stored in the head or capsid region. From the capsid, a long string of proteins make up the tail structure of a bacteriophage. The tail is a hollow tube that serves as a passageway for the genetic material to pass from the capsid to the host bacteria.

15. CLASSIFICATION OF VIRUSES Virus classification is the process of naming viruses and placing them into a taxonomic system.

16. BASIS OF CLASSIFICATON 1. Virus capsid, its size and type of symmetry & enveloped or not 2. Virus nucleic acid ,its type (DNA or RNA), strandedness (single or double), whether linear or circular.

17. DNA VIRUSES

18. MEDICALLY IMPORTANT DNA VIRUSES

19. RNA VIRUSES