504 



(HAP I IK 41 



some reduction in chromosome number — 

 probablj because o\' nondisjunction. Al- 

 though the /'// vivo frequency o\' somatic cell 

 mating in mammals is unknown, one possible 

 example of somatic cell mating has been 

 reported in cattle." This case involved a 

 pair of twins, whose members both showed 

 erythrocyte mosaicism due to the presence 

 of genetically different tissues which formed 

 antigenicallv different blood cells. At three 

 years of age one twin had blood 10 per cent 

 of which represented his own genotype and 

 90 per cent the genotype of his co-twin. At 

 eight years of age, however, this twin had 

 three blood types: the two "parental" types, 

 each representing two per cent, and a "hy- 

 brid'* type representing 96 per cent of the 

 cell population. 



Somatic cell mating is known to occur in 

 filamentous fungi such as Aspergillus and 

 Penicillium. 14 This parasexuality involves 

 the formation of diploid nuclei by rare, prob- 

 ably accidental, nuclear fusions in a multi- 

 nucleate mycelium containing haploid nuclei. 

 The diploid nuclei formed multiply side by 

 side with haploid nuclei and undergo chro- 

 mosome loss or "segregation" by means of 

 mitotic crossing over and or nondisjunction. 



Further study of somatic cell mating and 

 subsequent chromosome segregation may 

 provide valuable information with regard to 

 differentiation. 



RNA and Antibodies ' 



In the rat, the production of antibodies in- 

 volves the following stages. Young plasma- 

 blasts, which divide about every ten hours, 

 have free-floating ribosomes and a poorly 

 developed endoplasmic reticulum. After 

 exposure to antigens these cells begin to 

 synthesize ribosomes and mRNA at a high 



•'•See W. H. Stone. J. Friedman, and A. Fregin 



(1964). See also H. Harris and J. F. Watkins 



(1965i. 



14 See G. Pontecorvo (1958). 



'-See G. J. V. Nossal (1964). 



rate, and the endoplasmic reticulum under- 

 goes extensive development. Each plasma- 

 blast undergoes about nine successive divi- 

 sions — each successive division taking longer 

 — to produce a clone of mature plasma cells 

 which do not divide. Whereas the plasma- 

 blasts produce a great deal of RNA and pro- 

 tein — mainly structural proteins and enzymes 

 — the mature plasma cell synthesizes mainly 

 protein, 90 to 95 per cent of which is anti- 

 body. The plasma cell nucleus is shrunken 

 and dense and the nucleoli seem to disap- 

 pear. 



The first antibody molecules a given cell 

 produces have a molecular weight of about 

 a million (19s); later ones are smaller, with 

 a molecular weight of only 160,000 (7s). 

 The small antibodies are 7s gamma globulins 

 — tetramers composed of a pair of identical 

 B chains of 20,000 molecular weight and a 

 pair of identical A chains of 50,000 to 

 60,000 molecular weight. With rare ex- 

 ceptions each cell makes one type of anti- 

 body, even when other plasma cells in the 

 lymph nodes are synthesizing other anti- 

 bodies. It is not known how antibody is 

 released from the cell to neutralize an anti- 

 gen. Since little or no antigen enters the 

 antibody-producing cell, it is possible that 

 mere surface contact with the antigen is suffi- 

 cient to start a cell into antibody synthesis. 

 It is still too early to specify the detailed 

 roles of the antigen and the genotype in the 

 production of specific antibodies. 



RNA in Differentiation and Learning 



Developmental changes can be induced in a 

 growing cell by the introduction of RNA or 

 RNA-containing compounds. Although se- 

 rum albumin is not produced by mouse as- 

 cites tumor cells in vitro, such cells acquire 

 the ability to manufacture this protein after 

 exposure to RNA isolated from normal 

 mouse or calf liver. Using RNA, several 

 strains of cancer cells can be induced to 

 synthesize such enzymes as tryptophan pyr- 



