88 Martynas Yc^as 



The gene, and by inference DNA, is thought to contain the infoiTnation 

 which eventually appears as a sequence of amino acid residues in the corre- 

 sponding protein. As shown by a study both of the replacement process and of 

 the amino acid sequences, each residue has an independent genetic representa- 

 tion. These representations are presumably aligned in linear order on the DNA 

 molecule. There is in fact no evidence at present that the gene is anything other 

 than a linear sequence of amino acid determining sites, although the possibility 

 that it may also determine the structure of immunopolysaccharides in an 

 analogous fashion cannot yet be dismissed. 



Recent biochemical evidence (which I shall not discuss here) indicates that 

 it is RNA, not DNA, which is directly involved in the process of protein forma- 

 tion. Transfer of information therefore involves at least two steps : DNA to 

 RNA, and RNA to protein. 



The straightforward inference would thus be that DNA serves as a template 

 for the formation of RNA. Absence of cytoplasmic inheritance supports the 

 view that RNA is not a self-replicating structure. This is also supported by four 

 lines of biochemical evidence : 



1. The initial rate of incorporation of labeled precursors into nuclear RNA 

 is much greater than into cytoplasmic RNA (115). 



2. In Amoeba depleted of RNA, RNA only regenerates if a nucleus is 

 present (116). 



3. A one-way flow of RNA from nucleus to cytoplasm can be demonstrated 

 (117). 



4. The rate of RNA fomiation is minimal at the time DNA is replicating 

 (118). 



Unfortunately, this conclusion may be an oversimplification. There is no 

 lack of biochemical evidence pointing in the opposite direction: 



1. The composition of nuclear and cytoplasmic RNA is not identical (119). 



2. The time curves of precursor incorporation into RNA do not indicate 

 that the nuclear fraction is the precursor of the cytoplasmic (115). 



3. Radioactive precursor is incorporated into the RNA of enucleated 

 Acetabularia plants (120). 



4. Different strains of RNA viruses are self-replicating. This is difficult to 

 explain if RNA is the product of a DNA template. 



The problem is to reconcile these apparently discordant facts. Consider 

 first the determination of RNA structure by DNA. Since both DNA and RNA 

 are texts written in a four symbol alphabet, it is natural to suppose that the 

 coding problem is very simple. It is sufficient to assume that one nucleotide of 

 DNA determines one nucleotide of RNA (121). Recent evidence indicates, 

 however, that this is incorrect. 



It is possible to suppress protein synthesis in susceptible bacteria with 

 chloramphenicol. When this is done using amino acid-requiring strains, it can 

 be demonstrated that amino acids are required for RNA synthesis, even though 

 no protein synthesis is taking place (122, 123, 124). The natural inference, 

 supported by several converging lines of evidence, is that it is not the nucleotides 

 themselves which are the precursors of RNA, but rather compounds containing 

 both a nucleotide and an amino acid. This leads to a unitary picture of the 

 synthesis of RNA and of protein. When such precursors are lined up on a 



