240 



NUCLEIC ACIDS AND RELATED COMPOUNiDS 



Deoxyribonucleic acid (DNA) contains the bases adenine, guanine, cytosine and thy- 

 mine. Some plant DNA has been found to contain, in addition, 5-methylcytosine. Con- 

 trary to earlier ideas of structure there is no indication of a regular sequence of bases. 

 However to call the sequence random would be equally erroneous since the heredity of an 

 organism apparently depends on the specific sequence of bases present in its DMA. Fur- 

 ther discussion of this genetic coding is beyond the scope of the present work. DNA pre- 

 pared from various sources has been found to have molecular weights ranging from some- 

 what less than a million to several million. There is no reason to believe that all the 

 DNA even in a single nucleus is homogeneous, and a wide range of molecular weights may 

 be present. The accepted macromolecular structure of DNA based on the original pro- 

 posal of Watson and Crick (13) represents it as a two stranded helix with each strand con- 

 sisting of a chain of polynucleotides and the strands bound together by hydrogen bonds. 

 Adenine of one chain is always paired with thymine of the other, and guanine with cytosine. 

 The hydrogen bonding of the latter pair may be represented as: 



deoxyribosyl 

 GUANINE 



deoxyribosyl 



CYTOSINE 



The geometry of the helix allows for the presence of a third strand and it is possible that 

 in the cell protein occupies this position. Both DNA and RNA can be readily isolated as 

 a combination with protein, but the biological significance of these nucleoproteins is not 

 clear. Non-specific binding between the acidic nucleic acids and basic proteins readily 

 occurs so that the protein present in a nucleoprotein preparation may not be the same 

 protein originally combined with the nucleic acid in the cell. As would be predicted for 

 a long-chain, polar polymer, DNA forms very viscous solutions in water. 



Ribonucleic acid in addition to having ribose rather than deoxyribose and uracil 

 rather than thymine differs in some other properties from DNA. It generally is found to 

 have a somewhat lower molecular weight, ranging around one hundred thousand. There 

 has also been some suggestion that branched chain structures may be present but no cer- 

 tain evidence of this. There is also no evidence bearing on the macromolecular geometry 

 of RNA. Ribonucleic acid is apparently synthesized in the nucleus and then transferred to 

 the cytoplasm where it becomes part of the endoplasmic reticulum or microsome fraction 

 of the cell. Its primary function seems to be concerned with the assembly of amino acids 

 into specific proteins, and a brief discussion of this function will be found in Chapter 10. 

 There is also some evidence that nucleic acids may function as flowering hormones (14). 



