* Viruses are extremely small compared to bacterial, archaean and eukaryotic cells, but are diverse in size and shape Viral genome contains all the information necessary to use host cells for production of new viral particles; expression of viral genome in host cell directs host cell to use its metabolic machinery to duplicate the virus Bacteriophage T4 infecting cell by injecting DNA Bacteriophage (EM) * Basic viral structure. All viruses have the same basic structure; a core of nucleic acid – ss or ds RNA or DNA -- surrounded by protein layer called a capsid Some viruses, especially animal viruses, have an outer membrane envelope derived from a membrane of its host Many virus capsids also contain DNA or RNA polymerase enzymes required for genome replication, and perhaps other enzymes. Receptor-specific glycoprotein “spikes” of viral origin, with recognition and adhesion functions ; envelope consists of phospholipids and membrane proteins of host origin & glycoproteins of viral origin * The outer membrane of enveloped viruses such as HIV is added to new virions as they "bud" from the human host cell, typically without rupturing the cell Non-enveloped viruses are released by rupturing of the cell membrane; bacteriophage T4 directs synthesis of phage-encoded lysozyme that ruptures bacterial cell wall * Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage; involves translation of a transcription prevention gene New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycle Lytic cycle or Prophage Bacterial chromosome Phage Phage DNA The lytic and lysogenic cycles of phage , a temperate phage * Structural motifs The shapes of viruses vary. Forms include rods, polyhedrons, spheres, and complex shapes with heads and tails. Most viruses are helical or polyhedral; both enveloped helical and enveloped polyhedral viruses (e.g., HIV) are known Helical Polyhedron Sphere Complex * Viral Diversity, Classification Classified into orders, families, genera & species (>4000) based on; size & structure; type and number of nucleic acid molecules; method of genome replication in host cells; host range, infective cycle Host range Host range, which depends on evolved recognition system, limited to one or a few related species for most viruses; emerging infectious diseases may arise when mutation expands host range to include humans >20 of the viral families include species that cause human disease * Source: Linda Stannard;s website; http://web.uct.ac.za/depts/mmi/stannard/herpes.html Members of the family herpesviridae are found in a wide range of host systems. At least 7 “species” are known to infect humans, including herpes simplex virus (HSV); Herpesviruses have an envelope surrounding an spherical (icosahedral) capsid, which contains a dsDNA genome. Envelope is derived from inner nuclear membrane of host cell. Virus-encoded glycoproteins in virion envelope are visible as spikes that project from its surface Herpesvirus * Viruses of Animals are characterized by their host range, and are usually tissue specific polio virus particle adenovirus Some viruses have host ranges broad enough to include several species. * Classification of Animal Viruses Many variations on the basic scheme of viral infection and reproduction represented among the animal viruses. Two key variables are whether the genome is DNA or RNA, whether its double-stranded or single stranded; The nature of the genome is the basis for the classification of viruses. * Most viruses contain a polymerase that copies the viral genome inside the infected host cell, using nucleotides and ATP provided by the host cell * Single stranded RNA viruses are further classified into three classes (IV-VI) according to how the RNA genome functions in a host cell. * Classes IV and V: RNARNA, via a viral RNA replicase (an RNA polymerase) * Classes VI; retroviruses: RNADNA, via a viral reverse transcriptase (a DNA polymerase) * Viral Infection & Replication Cycle - host entry - genome replication - protein production - particle assembly - exit host Important traits wrt nature of replication cycle -nucleotide type -number of strands -strandedness (+ or -) -replication enzymes Basic elements of viral replication cycle-example with ds-DNA virus * Human Immunodeficiency Virus (HIV) is an emerging retrovirus in the lentivirus group that apparently arose from Simian Immunodeficiency Viruses in Africa (SIV) Phylogeny of HIV Strains and Types * HIV virus is shown crossing the mucosa of the genital tract to infect CD4+ T-lymphocytes. Major modes of spread: As a sexually transmitted disease Through injection drug use As a perinatal infection HIV-AIDS Overview of Human Immune System * HIV gains entry into cells by making use of host cell surface receptor proteins that participate in normal host immune responses. The main receptor for HIV is CD4, and requires a co-receptor. Helper T Cell * CD4 T-Cell count and Viral load for typical patient infected with HIV. “AIDS” ensues when CD4 T-cell count drops below ~200 cells per mm3; immune system no longer capable of eliminating pathogens – hosts typically die of pneumonia, fungal infection, or certain forms of cancer * Mycobacterial infections, shown in lung, are frequently seen with AIDS. Pneumonia in human lung caused by Pneumocystis carinii, a ubiquitous fungus Visceral involvement with Kaposi's sarcoma in AIDS is common, here seen as multiple reddish irregular masses in the liver. People with AIDS are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse.