THE RIBOSOMES, PROTEIN FACTORIES 
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ibosomes, which 
were discovered 
in the mid-1950’s, 
are extremely 
tiny — less than 
30 nanometers in diameter. 
However, due to their crucial 
role in protein manufacture, 
ribosomes can also be 
extremely numerous. In E. 
coli , for example, ribosomes 
account for one-fourth of the 
cell’s mass. A ribosome is 
made of two unequally sized 
subunits, each of which is 
composed of at least 40 
different proteins and a form 
of RNA called ribosomal RNA. 
message of the mRNA not one 
nucleotide at a time, hut 
rather in groups of three. 
These groups, called codons, 
are like words. Each word 
specifies one of the 20 different 
amino acid subunits of a 
protein or is a signal to start 
or stop making a protein. For 
example, the codon AGC in 
mRNA is translated into the 
amino acid serine, whereas 
nucleotides in a different order, 
say GCA, code for alanine. 
The amino acids called for by 
the mRNA are brought from 
the cytoplasm to the ribosome 
by a third kind of RNA, 
transfer RNA (tRNA). This 
small molecule is a connector: 
One end carries three 
nucleotides, known as the 
anticodon, which will join to 
a codon in the mRNA 
according to the rules of base 
pairing (A with U, and G with 
C). The molecule’s other end 
carries an amino acid. As the 
mRNA passes through the 
ribosome, tRNA brings the 
correct amino acids in and 
they are linked together by 
chemical bonds to form a long 
chain. When all the amino 
acids for a protein are joined, 
the chain is released. 
During translation, a strand 
of mRNA moves between the 
two parts of a ribosome like a 
piece of thread being pulled 
through the eye of a needle. 
The ribosome reads the 
Each strand of mRNA can be 
read many thousands of 
times. Indeed, at any one 
moment a strand of mRNA 
containing the instructions 
for a protein may be attached 
to as many as 30 ribosomes. 
Moreover, ribosomes work 
very quickly to connect the 
Complementary l)l\A strands 
RNA strand 
