A virus has no ribosomes, no enzymes for respiration and no supply of ATP. It therefore cannot reproduce on its own. To make new virus particles it must get inside a living host cell and take over the cell's machinery.Inside the host cell, the virus uses:
• host ribosomes to translate viral mRNA into viral proteins
• host nucleotides to make new copies of the viral genome
• host amino acids to build capsid proteins and enzymes
• host ATP as the energy source
• host enzymes, such as RNA polymerase, in many virusesThe first step is always attachment. Proteins on the virus (capsid proteins, tail fibres or envelope glycoproteins) bind to specific receptor molecules on the host cell surface. The shapes must be complementary, so each virus can only infect certain species and cell types. This is called host specificity. For example, HIV binds to CD4 receptors, so it infects T helper cells, and influenza binds to receptors on cells lining the airways.How the genome gets in depends on the virus:
• Bacteriophages inject only their nucleic acid through the bacterial wall; the empty capsid stays outside.
• Animal viruses enter whole, either by endocytosis or, for enveloped viruses, by fusion of the envelope with the host cell membrane. The capsid is then removed (uncoating).
• Plant viruses usually enter through wounds or are carried in by insects such as aphids, because the cellulose cell wall is a barrier.Once inside, the viral genome directs the cell to make viral components instead of its own.
• host ribosomes to translate viral mRNA into viral proteins
• host nucleotides to make new copies of the viral genome
• host amino acids to build capsid proteins and enzymes
• host ATP as the energy source
• host enzymes, such as RNA polymerase, in many virusesThe first step is always attachment. Proteins on the virus (capsid proteins, tail fibres or envelope glycoproteins) bind to specific receptor molecules on the host cell surface. The shapes must be complementary, so each virus can only infect certain species and cell types. This is called host specificity. For example, HIV binds to CD4 receptors, so it infects T helper cells, and influenza binds to receptors on cells lining the airways.How the genome gets in depends on the virus:
• Bacteriophages inject only their nucleic acid through the bacterial wall; the empty capsid stays outside.
• Animal viruses enter whole, either by endocytosis or, for enveloped viruses, by fusion of the envelope with the host cell membrane. The capsid is then removed (uncoating).
• Plant viruses usually enter through wounds or are carried in by insects such as aphids, because the cellulose cell wall is a barrier.Once inside, the viral genome directs the cell to make viral components instead of its own.