required amino acids into a 
protein. Each ribosome in a 
single E. coli , for example, 
can link 15 amino acids in a 
second. The speed and 
efficiency of translation means 
that each gene is capable of 
directing the manufacture of 
very large quantities of protein. 
For instance, in each cell of a 
sdkworm’s silk gland there is 
a single gene that codes for 
the protein fibroin, the chief 
component of silk. Each time 
it is activated, the gene can 
make 10,000 copies of its 
specific mRNA, and each 
copy of mRNA can direct the 
synthesis of 100,000 molecules 
of fibroin. In 4 days, a silk 
gland cell can manufacture a 
billion molecules of fibroin! 
This end 
translated first 
Ribosomes fall into two cate- 
gories: those that are free in 
the cytoplasm and those that 
are bound to membranes. The 
two kinds of ribosomes play 
similar roles in the manufac- 
ture of proteins. But while 
the free ribosomes leave the 
proteins equally free to float 
in the cytoplasm, the bound 
ribosomes transfer their 
finished proteins into a large, 
cobwebby organelle — the 
endoplasmic reticulum. 
Beginning of protein 
Nearly completed protein 
i RNA 
In the nucleus, DNA’s 
instructions are tran- 
scribed (below left) into 
a messenger molecule of 
ribonucleic acid (RNA). 
The code in a strand of 
messenger RNA is 
translated into a protein 
(below right) in tiny 
organelles, called 
ribosomes, in the 
cytoplasm. 
27 
Ribosome 
