134 - The Cell 



served and perpetuated mainly by the mech- 

 anism of protein synthesis. In these processes 

 both DNA and RNA play roles of paramount 

 importance. 



How the cell achieves an exact replication 

 of its DNA, or genie components, is a prob- 

 lem that will not be considered until later 

 (Chap. 27). Here, however, it is important to 

 know that such replication operates to pre- 

 serve the sequence of organic bases (Fig. 7-5) 

 in every part of every DNA unit. This means 

 that the adenine (A), cytosine (C), guanine 

 (G), and thymine (T) are arranged in a pre- 

 cisely determined, though widely varying, 

 order along the sugar-phosphate skeleton in 

 each segment of the elongate nucleic acid 

 molecule. 



ACIDS TRANSPORT 

 RNA 



The base sequence in the DNA of a cell 

 likewise determines the sequence of bases in 

 its RNA (Fig. 7-5). The RNA is synthesized— 

 mainly, at least — in the nucleus, although 

 later much of it is transported to the cyto- 

 plasm, where protein synthesis must also 

 occur. In the nucleus, presumably, when a 

 new RNA unit is formed, the basic constitu- 

 ents of a prospective RNA molecule are care- 

 fully lined up in relation to a master tem- 

 plate, provided by some segment of the DNA 

 (Fig. 7-5). This alignment determines the 

 base sequence in the RNA product because 

 it always follows a definite rule, namely the 

 base-pair rule. According to this rule, only 

 certain pairs of the purine (adenine and 

 guanine) and pyrimidine (cytosine, thymine, 



TEMPLATE 

 RNA 



MASTER TEMPLATE 

 DNA 



ACTIVATED 

 AMINO ACIDS 

 NOT YET CODED 



ACTIVATED'*) 

 PHENYLALANINE 

 C0DE=AAA 



ACTIVATED 

 ASPARAGINE 

 C0DE=UUA 



ACTIVATED! 

 ISOLEUCINE 

 C0DE=AAU 



FREE IN 

 CYTOPLASM 



CHROMOSOME 



Fig. 7-5. Postulated mechanisms of protein synthesis. The sequence of organic bases in template RNA 

 derives from that of the chromosomal DNA, in obedience to the base-pair rule. This rule states that 

 only certain base-pairs can affiliate with one another: cytosine with guanine (C — G), adenine with thy- 

 mine (A — T), and adenine with uracil (A — U). The pick-up of particular amino acids by transport RNA is 

 determined by a partly known triplet code; e.g., AAA deals with phenylalanine; UUA with asparagine; 

 AAU with isoleucine; etc. Thymine, although present in DNA, is absent in RNA; and uracil, present in 

 RNA, is not present in DNA. Dotted lines indicate points of affinity among the specified constituents of 

 the system. 



