REPLICATION AND CODC-SCRIPTS 151 



the proper order if the correct protein is to result, in what way can the four 

 be arranged so that they carry, and can transfer, information on how the 

 20 amino acids should be organized to form such a great variety of proteins? 



The answer to the question is not so simple. There are several theori< s, 

 but just a tew definitive facts, and information is accumulating. The evi- 

 dence is now that it is RNA which actually acts as the template or die lor 

 protein synthesis. The RNA in turn obtains its exact configuration, before its 

 job, by contact with the code-bearing DNA molecule. Its structure lias to 

 be well fixed, for it must guide without error the condensation or linking 

 of (of the order of) 100 amino-acid residues in even a smallish protein mole- 

 cule of molecular weight ~1000. For if one of the components falls into the 

 wrong slot, the whole molecule may be biochemically useless to the living 

 system — a "bad molecule." There are many pitfalls, for the number of 

 possible arrangements in a chain of even 100 units made up of 20 different 

 kinds is enormous. 



During protein synthesis the RNA is located in the cytoplasm primarily 

 in the microsomes (ribosomes) (see Figure 6-12), and it is here that the bulk 

 of the protein synthesis take place. Energy for the synthesis is provided by 

 the adsorption on RNA of the amino acids, the "mobile power supply," 

 ATP, and an enzyme, there being one specific enzyme (site) for each amino 

 acid. 



The replication process is supposed to go as follows: Sometime in the late 

 stages of the period between cell divisions, during the early part of the 

 prophase when the mitotic apparatus is collecting in preparation for division 

 of the cell, the DNA molecules — which have been depolymerized and dis- 

 persed throughout the cell and are presumably attending to the business of 

 synthesis of big molecules — begin to polymerize and collect into thread-like 

 bodies called chromosomes. (There is some evidence that this process itself 

 is controlled by a large protein.) During this collection process, the DNA's 

 intercoiled helical strands are pulled apart or unwound, and each acts as the 

 template for the condensation of another helical partner, formed from 

 nucleic acid residues in the fluid of the cytoplasm. The process is completed 

 as the resulting pairs of chromosomes are lined up (by contractile protein?) 

 midway between the asters of the mitotic apparatus, and perpendicular to 

 the spindles which join the asters, just before the actual division takes place. 



Replication of DNA and of the whole chromosome requires the action ol 

 subtle physical forces: the DNA helices must be pulled apart for individual 

 replication, before they are polymerized to form the chromosomes, which in 

 turn are lined up in a predetermined fashion in the mitotic apparatus; and 

 this is then forced to split in two. The nature of the forces which do these 

 jobs, and of the guiding principle which controls the order and speed with 

 which they are done, are essentially unknown. However, contractile forces 

 of molecular origin are now well known in myosin, and m.i\ be important 



