3 The structure of virus particles

All virus genomes are surrounded by proteins which:

• Protect nucleic acids from nuclease degradation and shearing.

 • Contain identification elements that ensure a virus recognizes an appropriate target cell (but plant viruses do not, and enter the cell directly by injection or injury).

 • Contain a genome-release system that ensures that the virus genome is released from a particle only at the appropriate time and location.

 • Include enzymes that are essential for the infectivity of many, but not all, viruses.

 • Are called structural proteins, as they are part of the virus particle. All viruses contain protein and nucleic acid with at least 50%, and in some cases up to 90%, of their mass being protein. At first sight it would appear that there are many ways in which proteins could be arranged round the nucleic acid. However, viruses use only a limited number of designs. The limitation on the range of structures is due to restrictions imposed b


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