430 



( II VPTER 33 



erlj exposed. The 50s subunit holds the 

 recently-made portion of a polypeptide 



which ends in an sRNA molecule in such 

 a way (see Figure 5, p. s-126) that: 



1 . The next correct, free amino acid- 

 sRNA combination can pair — prob- 

 ably via the nucleotides at the bend 

 of the sRNA molecule — with the next 

 sequence in the messenger RNA 



2. An enzyme can link the amino part 

 of the free amino acid-sRNA com- 

 plex to the carboxyl portion of the 



amino-acid sRNA complex at the end 

 of the polypeptide chain, liberating the 

 sRNA molecule formerly bound to the 

 end of the chain 

 3. The liberated sRNA is then free to 

 accept another one of its specific 

 amino acid molecules for transport. 



It is not known how the sRNA at the end 

 of a completed polypeptide is removed. It 

 should also be noted that since the growing 

 polypeptide chain appears attached to the 

 50s particle only at its growing end (this 



Base-Paired 



Amino Acid — s RNA complex 



30 s submit 

 50 s submi 



70 s Ribosome 



Growing 

 Polypeptide 

 (N- terminus) 



Messenger RNA 



figure 33—3. Schematic representation of the hypothesized re- 

 lation between messenger RNA, adapter RNA, rihosomes. and 

 growing polypeptide chains. 



