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Graphic of Viral Entry into Animal Cells: virus attachment to cell surface (adsorption) and virus entry into cell. Picture shows translocation, pore formation, receptor mediated endocytosis using clathrin coated vesicles and membrane fusion. Some viruses can use more than one strategy. Other means are also employed. The above image is 500 pixels across, the original measures 4,000 pixels across.
1. NAKED VIRUS - TRANSLOCATION: particle crosses cell membrane intact (cf Principles of Molecular Virolgy, 3rd Edition, Alan J. Cann, Academic Press p 117)
2. NAKED VIRUS - GENOME INJECTION: virus attaches to cell surface and releases its genome which penetrates the cytoplasm via a pore that has been created in the plasma membrane. (Bacteriophages, which attack bacterial cells, also inject their genomes and may use molecular "syringes" to do so, please see our diagram of T4 phages injecting).
3. NAKED VIRUS - ENDOCYTOSIS: virus attaches to cell surface receptor molecules and sinks into a clathrin coated pit. The pit invaginates and finally closes off creating a clathrin coated vesicle (drawn as a cage like sphere) and so the contained virus particle is drawn into the cytoplasm. The clathrin molecular cage soon dissociates into component triskelions (the propeller like objects) which leave a vesicle. The resulting uncoated vesicle transports the contained virion to an endosome. At some stage thereafter, the viral components are released. The virus shown in this example is an Adenovirus.
4. ENVELOPED VIRUS - ENDOCYTOSIS & MEMBRANE FUSION: virus enters cell by receptor mediated endocytosis. The cell membrane merges (fuses) with the endosome membrane and so the virus components are released. The virus shown here is an influenza virus, please see our Influenza virus life cycle illustration.
5. ENVELOPED VIRUS - MEMBRANE FUSION: virus enters the cell when its outer membrane fuses with the plasma membrane at the cell surface. The viral contents are then spilled into the cytoplasm of the cell. This example is HIV, which is unusual in having a conical core (most viral cores tend to be more spherical). Please see our HIV illustrations.
GENERAL POINTS: This diagram is only intended as an overview. The viruses are depicted to show general principles. NAKED VIRUSES have an exposed protein capsid, whilst ENVELOPED VIRUSES are cloaked in cell membrane, added during budding from the host cell. The fates of viral proteins and viral genetic material vary with the type of virus. Some viruses release their genetic material almost immediately (see entry 2) whereas others transport it to the cell nucleus still contained in a protein vehicle (see entry 5).