> See also: > - Reference # Acellular Organisms ## Viruses The first step in the life cycle of a viral particle is attachment to the host. - This is mediated by ligands (Ex: spike proteins) embedded in the viral envelope which bind to receptors on the host cell H1N1 - H = Hemagluttinin Ligand/Spike Protein - N = Neuramidase Ligand/Spike Protein A nucleocapsid is composed of a protein shell surrounding the viruses’ nucleic acids Viruses have tropism, or a specific set of cells that they can infect based on the existing tissue and their receptor-ligand interactions Three common methods for animal virus entry: - Fusion of viral envelope with host cell membrane - Entry by endocytosis (invagination of membrane to form vacuole) - Possible for both enveloped and non-enveloped - Release of viral nucleic acids into cytoplasm of host cell Proteins encoded by viral nucleic acids are categorized in two groups: - Structural: - Non-structural: Play a role in the viral life cycle but are not part of the complete mature vision (whole viral particle)