4.12 CHAPTTR 33 



SUMMARY AND CONCLUSIONS 



Under typical circumstances, R.NA is synthesized in the chromosomes, and many — 

 but not all— polypeptides are synthesized in the cytoplasm. DNA-dependent RNA 

 polymerase transcribes the information in one strand of double-stranded DNA into 

 complementary, single-stranded, messenger RNA. This mRNA becomes attached, 

 probably at one end. to a ribosome. The ribosome functions to orient successive 

 groups of bases in the messenger RNA. one group at a time, so that they can pair 

 with unpaired complementary bases present in adapter RNA, each type of which 

 transports a single different amino acid. Since a protein-synthesizing ribosome only 

 accepts amino acid-adapter RNA molecules one at a time, the polypeptide chain grows 

 by single amino acid steps (beginning at the N-terminus). As the messenger RNA 

 is translated, it passes over (or through) the ribosomal surface, liberating the starting 

 point to accept another ribosome at that position. Consequently, a number of ribo- 

 somes, comprising a polysome, simultaneously translate the same messenger RNA. 



Ribosomes can be used a number of times; a given messenger RNA may be used 

 ten to twenty times, and under some circumstances, many more; once liberated from 

 the growing polypeptide chain, adapter RNA is free to accept another amino acid 

 for transport. 



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