390 CELL HEREDITY 



troiii consideration tor this rok'. (^Iiromtjsonial HNA is a possible candi- 

 date, since it has been reported as a constituent of the chromosomes of 

 higlu'r forms, although there is no evidence for or against its presence on 

 the bacterial chromosome. However, it is a pure specuhition that 

 RNA may be involved in gene expression as a mapable part of the 

 chromosome. 



With respect to the nonchromosomal genetic factors, most likely they 

 are not DNA since no trace of nonchromosomal DNA has been detected 

 in most cell tvpes. Manv classes of RNA molecules surely exist, in addi- 

 tion to those already recognized, and some of them may carry primary 

 genetic information. In addition to the pathway of protein synthesis 

 described in Chapter 11, other pathways probably exist, for example, 

 concerned with the synthesis of insoluble, membrane-bound proteins. 

 Which molecules carry the code for those proteins? 



A somewhat less speculative question concerns the possible genetic 

 role of RNA molecules which are known to be involved in protein 

 synthesis: transfer RNA and microsomal RNA. The transfer RNA pre- 

 sumably carries specificity for combining with specific amino acid acti- 

 vating enzymes. Whether that specificity is transmitted by replication 

 from pre-existing RNA or by synthesis from DNA is not known. 



The ribosomal RNA is a candidate for the carrier of some primary 

 genetic information. Since proteins are synthesized on the ribosomes, 

 it has been widely postulated that ribosomal RNA carries the genetic 

 code by means of which the amino acids are polymerized in proper 

 sequence. If this is correct, then the question becomes one of estab- 

 lishing the origin of the ribosomal RNA. Is it replicated and there- 

 fore genetic, or is it synthesized on a DNA template? From existing 

 information obtained in Neurospora and in higher forms, it seems likely 

 that ribosomal RNA originates in the nucleus, but is it of chromosomal 

 or of nucleolar origin? 



Two principal possibilities have been considered: (1) that ribosomal 

 RNA is replicated in the nucleolus and, consequently, carries primary 

 genetic information; or (2) that it is synthesized on a DNA template, and 

 is a gene product, carrying the DNA code into the cytoplasm where 

 proteins are synthesized. Although many investigators favor the second 

 alternative, there is little direct support as yet beyond the fact of amino 

 acid substitution in mutant hemoglobins. 



Alternatively, the ribosomal RNA could be the product of the com- 

 bined action of a chromosomal gene of class S, giving rise to an RNA 

 gene product, and a genetically autonomous RNA replicating in the 

 nucleolus. On this hypothesis, part of the primary genetic code for 

 protein specificity would reside in the RNA and part in the DNA; ribo- 



