PHYSICAL AND CHEMICAL PROPERTIES OF VIRUSES _ §5 
The top component is therefore the first native virus protein which 
has been obtained free of nucleic acid. 
The fact that the top component is non-infectious is direct evidence 
that nucleic acid is essential for the multiplication of plant viruses 
since the two components seem otherwise identical. 
The origin of the top component is at present obscure; it may 
represent a stage in the synthesis or the breakdown of the nucleo- 
protein, or might even be the result of an abortive attempt at 
multiplication (Markham and Smith, 1949). 
Plant and animal viruses have a common chemical background 
in that both appear to contain nucleic acid and protein; plant viruses, 
however, contain ribonucleic acid whereas those animal viruses 
examined so far contain either ribonucleic or desoxyribonucleic acid. 
Some animal viruses appear to be more complex and also contain fats. 
Influenza Virus 
There are two main strains of influenza virus, known as A and B, 
and they do not immunize against each other. There is a third strain, 
swine influenza virus, which is related to A; this is thought to be a 
survival of the virus which caused the great influenza pandemic of 
1918-19, having become adapted to the pig and persisting in it 
ever since. 
The methods of purifying influenza virus have been briefly des- 
cribed in Chapter V; there are two main techniques, one by sedi- 
menting out of solutions in the Sharples centrifuge and the other by 
adsorbing the virus on to chicken red cells and eluting with phosphate 
at pH 7-1. The yield is fairly small; with virus A it is 40 mg per litre 
of extra-embryonic fluid and with virus B about 60 mg per litre. 
Investigations on highly purified preparations of influenza virus 
have revealed the presence of an amount of carbohydrate apparently 
ereater than that accountable for in the nucleic acids. Knight (1947) 
obtained carbohydrate-rich fractions from highly purified samples of 
PR 8 and Lee influenza viruses and subjected them to analysis. In 
each case, the carbohydrate appeared to be a polysaccharide composed 
of mannose, galactose, and glucosamine units. 
Electrophoretically, the virus is not very homogeneous; it is 
insoluble at its isoelectric point of pH 4:5-5°s. 
The elementary composition is C, 52 per cent; N, 9°7 per cent; 
P, 0-85 per cent; Ribose, 0-71 per cent; CH,O, 6-1 per cent or 13 
per cent; Fat, 24 per cent. According to Taylor (1944) there is 2 per 
5—(T.502) 
