486 DANIEL MAZIA 



apparatus as tubular, and suggesting the presence of structures reminiscent 

 of endoplasmic reticulum in the apparatus in some cases, we are now 

 reconsidering the possible significance of lipoproteins in the mitotic 

 apparatus. The newer method of isolation has made this practicable. 



2. The RNA content is relatively high. The last studies [27] give a 

 figure of about (yi[j. When the major proteins components are put into 

 solution, and partly purified, they behave as ribonucleoproteins. At one 

 time, it was thought that adenylic nucleotides predominated [8] but this 

 was erroneous. Better analyses recovered from the mitotic apparatus 

 as RNA having about the same nucleotide composition as the averaged 

 RNA of the whole cell (sea urchin egg) [27]. The present hypo- 

 thesis is that the structure proteins are in fact ribonucleoproteins. If 

 so, we are confronted with the question of function of the RNA ; there is 

 no reason to think that mitotic structure or action involves protein synthesis. 



My current speculations about the role of RNA in the structure of the 

 mitotic apparatus take the following form. If RNA carries information 

 regarding amino acid sequences in proteins, perhaps this information may 

 be used for the "recognition" of proteins as well as for their synthesis. 

 Could such a recognition function be involved in the assembly of a 

 structure by association of like molecules ? Of course, the RNAs need not 

 recognize proteins ; they might recognize each other. If the speculation is 

 unsupported, it does call attention to a question that is easily overlooked : 

 how does genetic information operate in governing the structure of the 

 cell in its larger sense, as well as the structure of its component molecules ? 



3. By electrophoretic and ultracentrifugal criteria [27] and by immuno- 

 logical criteria [28] the major protein composition of the mitotic apparatus 

 appears to be simple, probably deceptively so. Two or at most three 

 components can be detected, and one of them predominates. It seems to 

 be a protein whose molecular weight is r. 315 000 [27]. 



4. Amino acid analysis [29], data cited by Mazia [30], shows striking 

 similarities between the major protein of the mitotic apparatus and actin 

 from vertebrate muscles. This might be a matter of chance, but might 

 also point to some common properties of structure proteins involved in 

 biological movement. 



7. Origin of the mitotic apparatus 



It has already been mentioned that the mitotic apparatus occupies a 

 considerable part of the volume of a dividing cell. Analyses of sea urchin 

 eggs and mitotic apparatus isolated from these eggs shows that the mitotic 

 apparatus represents an investment of at least io°o of all the protein in 

 the cell. If the compositional studies cited above are not entirely deceptive, 

 this is mostly protein of one or a few kinds. The mitotic apparatus is not 



