INTRODUCTION TO VIRUSES Characteristics of Viruses Virus Minuscule

INTRODUCTION TO VIRUSES

Characteristics of Viruses • Virus – Minuscule, acellular infectious agent having either DNA or RNA – Causes many infections of humans, animals, plants, and bacteria – Causes most of the diseases that plague the industrialized world © 2012 Pearson Education Inc.

Characteristics of Viruses • Cannot carry out any metabolic pathway • Neither grow nor respond to the environment • Cannot reproduce independently • Recruit the cell’s metabolic pathways to increase their numbers • No cytoplasmic membrane, cytosol, organelles (with one exception) © 2012 Pearson Education Inc.

Characteristics of Viruses • Virus Structure – Called virion – Protein coat (capsid) surrounding nucleic acid – Nucleic acid and capsid also called nucleocapsid – Some have phospholipid envelope – Outermost layer provides protection and recognition sites for host cells © 2012 Pearson Education Inc.

Figure 13. 1 Virions-overview

Characteristics of Viruses • Genetic Material of Viruses – Show more variety in nature of their genomes than do cells – Primary way scientists categorize and classify viruses – May be DNA or RNA, but never both • ds. DNA, ss. DNA, ds. RNA, ss. RNA – Single or segmented and linear and circular – Much smaller than genomes of cells © 2012 Pearson Education Inc.

Characteristics of Viruses • Hosts of Viruses – Most viruses infect only particular host’s cells • Affinity of viral surface proteins for proteins on host cell – May be so specific they infect only particular kind of cell in a particular host – Generalists – infect many kinds of cells in many different hosts © 2012 Pearson Education Inc.

Figure 13. 3 Hosts of viral infections-overview

Figure 13. 4 Sizes of selected virions E. coli (bacterium) (1000 nm 3000 nm) Red blood cell (10, 000 nm in diameter) Bacterial ribosomes (25 nm) Poliovirus (30 nm) Bacteriophage MS 2 (24 nm) Smallpox virus (200 nm 300 nm) Bacteriophage T 4 (50 nm 225 nm) Tobacco mosaic virus (15 nm 300 nm)

Characteristics of Viruses • Capsid Morphology – Capsids • Provide protection for viral nucleic acid • Means of attachment to host’s cells • Composed of proteinaceous subunits called capsomeres – Capsomere made of single or multiple types of proteins © 2012 Pearson Education Inc.

Characteristics of Viruses • Viral Shapes – Three basic shapes • Helical • Polyhedral • Complex © 2012 Pearson Education Inc.

Figure 13. 5 The shapes of virions-overview

Figure 13. 6 Bacteriophage T 4 -overview

Characteristics of Viruses • The Viral Envelope – Acquired from host cell during viral replication or release • Envelope is portion of membrane system of host – Composed of phospholipid bilayer and proteins • Some proteins are virally coded glycoproteins (spikes) – Envelope’s proteins and glycoproteins often play role in host recognition © 2012 Pearson Education Inc.

Figure 13. 7 Enveloped virion-overview

Table 13. 2 Families of Human Viruses

Viral Replication • Dependent on hosts’ organelles and enzymes to produce new virions • Lytic replication – Replication cycle usually results in death and lysis of host cell • Stages of replication cycle – Attachment – Entry – Synthesis – Assembly – Release © 2012 Pearson Education Inc.

Viral Replication • Replication of Viruses – Attachment of animal viruses • Chemical attraction • Have glycoprotein spikes or other attachment molecules that mediate attachment © 2012 Pearson Education Inc.

Figure 13. 12 Three mechanisms of entry of animal viruses-overview

Viral Replication • Replication of Animal Viruses – Synthesis of animal viruses • Requires different strategy depending on its nucleic acid • DNA viruses often enter the nucleus • RNA viruses often replicate in the cytoplasm • Must consider – How m. RNA is synthesized – What serves as template for nucleic acid replication © 2012 Pearson Education Inc.

Figure 13. 13 Synthesis of proteins and genomes in animal RNA viruses-overview

Viral Replication • Replication of Animal Viruses – Assembly and release of animal viruses • Most DNA viruses assemble in nucleus • Most RNA viruses develop solely in cytoplasm • Number of viruses produced depends on type of virus and size and initial health of host cell • Enveloped viruses cause persistent infections • Naked viruses are released by exocytosis or lysis © 2012 Pearson Education Inc.

Figure 13. 14 The process of budding in enveloped viruses Enveloped virion Budding of enveloped virus Viral glycoproteins Viral capsid Cytoplasmic membrane of host

Number of infective virions in medium Figure 13. 15 Pattern of virion abundance in persistent infections Entry Synthesis and Release of virions assembly Time Attachment

Viral Replication • Replication of Animal Viruses – Latency of animal viruses • When animal viruses remain dormant in host cells • May be prolonged for years with no viral activity • Some latent viruses do not become incorporated into host chromosome • Incorporation of provirus into host DNA is permanent © 2012 Pearson Education Inc.

The Role of Viruses in Cancer • Animal’s genes dictate that some cells can no longer divide or are prevented from unlimited division • Genes for cell division “turned off” or genes inhibiting division “turned on” • Neoplasia – Uncontrolled cell division in multicellular animal – Mass of neoplastic cells is tumor • Benign vs. malignant tumors – Metastasis – Cancers © 2012 Pearson Education Inc.

Figure 13. 16 The oncogene theory of the induction of cancer in humans Normal state: DNA Protooncogene Represses Gene for repressor m. RNA Repressor Result: No cancer First “hit”: Virus inserts promoter DNA Oncogene Represses Gene for repressor m. RNA Repressor Result: Still no cancer Second “hit”: Virus inserts into represssor gene DNA Oncogene m. RNA Protein No repressor protein because gene is segmented Causes cell division Result: Cancer

The Role of Viruses in Cancer • Environmental factors that contribute to the activation of oncogenes – Ultraviolet light – Radiation – Carcinogens – Viruses © 2012 Pearson Education Inc.

The Role of Viruses in Cancer • Viruses cause 20– 25% of human cancers – Some carry copies of oncogenes as part of their genomes – Some promote oncogenes already present in host – Some interfere with tumor repression – Specific viruses are known to cause ~15% of human cancers • • Burkitt’s lymphoma Hodgkin’s disease Kaposi’s sarcoma Cervical cancer © 2012 Pearson Education Inc.