The use of less conventional methods of virus preparation for electron microscopy (i.e. freeze-drying and freeze-etching) showed that the influenza virus particles are more uniform than has generally been assumed. Their shape resembles an icosahedron with an average ‘diameter’ of 1080 Å. The surface projections (spikes) are arranged mostly in equilateral triangles, often building hexons and, in a few cases, pentons as well. Freeze-etching revealed the inside of some particles, particularly in the case of long filaments. We arrived at the following general concept of virus structure: the nucleoprotein coil, either cylindrical or spherical, is enveloped by a membrane bearing spiky macromolecules, probably of different structure and function. The membrane with spikes is called the envelope. The centre-to-centre distance of the spikes ranges from 70 to 90 Å, depending on the mode of preparation for electron microscopy. The innermost space of the virus particle is probably filled with a liquid, as is apparent from the change in form after normal drying and also from freeze-etching.

The nucleoprotein helix consists of threads about 70 Å thick having an internal channel of about 10 Å. The number of threads is usually a multiple of 3, such as 6, 9, 12 or 30 etc.


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