RIBOSOMES, THE 
"PROTEIN FACTORIES" 
Ribosomes, which were discovered in 
the mid- 1 950'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. Each 
ribosome is made of two unequally 
sized subunits, which are composed 
of at least 40 different proteins and a 
form of RNA called ribosomal RNA. 
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 
message of the mRNA not one nucleo- 
tide at a time, but rather in groups of 
three. These groups, called codons, 
are like words. Each word specifies 
one of the 20 different amino acids 
(the chemical subunits that form pro- 
teins) 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 1 
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 peptide 
bonds to form a polypeptide chain. 
When all the amino acids for a pro- 
tein are joined, the chain is released. 
Each strand of mRNA can be read 
many thousands of times. Indeed, at 
any one moment a strand of mRNA 
containing the transcript for a protein 
may be attached to 30 separate ribo- 
somes. Moreover, ribosomes work 
very quickly to connect the required 
amino acids into a protein. Each 
ribosome in a single E. coli, for ex- 
ample, 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 
example, in each cell of a silk worm'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! 
Ribosomes fall into two categories: 
those that are free in the cytoplasm 
and those that are bound to mem- 
branes. The two kinds of ribosomes 
play similar roles in the manufacture 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. 
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