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Observations on the Structure of the Nucleocapsids of some Paramyxoviruses
SUMMARY The intact particles and nucteocapsids of mumps, Sendai and measles viruses are of closely similar appearance, size and structure. The intact parti- cles are about 15o nm. in diameter. The filamentous nucleocapsids have a modal length of about ~. I/Am., and are constructed of subunits arranged as a single start helix of pitch 5.0 nm. for Sendai virus, and about 6"o nm. for mumps and measles viruses. A (helical) projection of the structure of the Sendai nucleocapsid calculated from an electron micrograph showed that the structural subunits are hour-glass-shaped and are arranged in the helix with their long axes inclined at an angle of about 6o ° to the long axis of the particle. There are probably J ~ or I3 subunits in each turn of the basic helix. Optical diffraction patterns of electron micrographs of mumps and measles nucleo- capsids show that they have closely similar structures.