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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

Biopolymers 237

3 end ----GCAC TAG---- 5 end DNA template contains one molecule of 16s RNA and 21 different pro-

5 end ----CGUGAUC---- 3 end new RNA strand teins, while the larger subunit consists of one molecule of
each of the 5s and 23s RNA and 32 different proteins. The
IX
ribosomes of higher organism cell cytoplasm have larger
subunits, larger RNA molecules, one extra RNA molecule,
A short stretch of RNA-DNA double helix forms be- and more proteins, but carry out the same function as bac-
tween the new RNA and the DNA template, but this terial ribosomes.
quickly dissociates. The “used” template strand then re- The initial step in protein biosynthesis is the bind-
forms a double helix with its original partner strand of the ing of the smaller ribosomal subunit (30s in bacteria) to
DNA molecule. (In a similar way, an RNA polymerase an m-RNA molecule at a specific base sequence on the
makes the short primers for DNA synthesis.) m-RNA. This sequence is close to the start of the “mes-
The enzyme RNA polymerase, often with the aid of sage” coding for protein structure and is recognized by a
other proteins, recognizes sequences on the DNA which complementary base sequence on the 16s RNA of the ribo-
are the “start” and “stop” signals for synthesis of a par- some. In higher organisms, the “cap” at the 5 end of the m-

ticular m-RNA, so that messenger of the correct length is RNA is recognized by the smaller subunit of the ribosome.
made. In higher organisms, the m-RNA usually carries the The base sequence of the m-RNA determines the amino
information specifying the structure of a single protein. acid sequence of the protein to be synthesized; in fact, a
The other types of RNA, both ribosomal and trans- sequence of three bases, a unit called a codon, specifies
fer, are synthesized in the same way by using different one amino acid. From four different bases, A, G, C, and
stretches of the DNA as template. After transcription (i.e., U, 64 different triplet codons can be made, but only 20
RNA biosynthesis) the primary products are processed be- amino acids become incorporated into proteins. Hence
forebeingusedtodetermineproteinstructure.Nucleotides many amino acids have more than one codon. The so-
are often trimmed off the 5 and 3 ends of the RNA, a spe- called genetic code (i.e., the base sequences coding for

cial nucleotide “cap” is added to the 5 -end, and a stretch each amino acid) was first investigated in bacteria but is
of adjacent adenine nucleotides is added to the 3 end of believed to be almost universal and is shown in Table III.

most m-RNA of higher organisms. Segments can be cut Some codons do not commonly specify an amino acid;
out of the interior of m-, r-, and t-RNA molecules. These these are UAA, UAG, and UGA, and are believed to be
stretches, called introns, are excised at specific base se- “stop” signals used to terminate protein synthesis. The
quences, and the remaining sections, called exons in m- codon AUG is the most common “start” signal, although
RNA, are joined together. For some RNA molecules, the GUG and UUG occasionally have this function.
introns are self-splicing, i.e., catalyze their own removal The sequence of bases on m-RNA is read in the 5 to

and joining together of the ends of the remaining stretches 3 direction, and is translated into an amino acid sequence
of RNA. These catalytic RNA segments are known as ri- starting at the N-terminal end of the protein. Amino acids,
bozymes. After intron removal bases can be modified, par- however, cannot interact directly with m-RNA. Instead,
ticularly in r- and t-RNA. each amino acid becomes attached to the 3 end (the ac-

In the case of some m-RNA, different introns may be ceptor end) of a specific t-RNA (see Fig. 20); glycine,
removed from the primary RNA produced by transcrip- for example, becomes attached to glycine-specific t-RNA.
tion (the transcript) to give different m-RNA molecules The enzymes, amino acyl-t-RNA-synthetases, which cat-
and hence different proteins from one transcript. This is alyze this attachment, ensure that an amino acid is linked
one way in which a wide diversity of antibodies can be only to its correct t-RNA. The t-RNA carrying its amino
formed in the human body from a relatively small number acid can interact directly at the ribosome with m-RNA. In
of genes. fact, each t-RNA has a three-base sequence complemen-
For protein synthesis to take place, the completed mes- tary to the codon on m-RNA specifying its amino acid.
senger RNA must now move to the ribosomes. Ribosomes This sequence on the t-RNA, called the anticodon, is lo-
are roughly spherical particles consisting of r-RNA and cated at the lower end of the molecule and the bases are
several proteins, but their size and composition depend on held in such a way that a short stretch of double helix can
the source. Most bacteria contain ribosomes having a par- be formed, with the bases of the anticodon pairing with
6
ticle weight of 2.5 × 10 Da, but the ribosomes of the cell the codon bases of the m-RNA.
cytoplasm of higher organisms are larger. Bacterial ribo- After the m-RNA has become bound to the small sub-
somes are made of two subunits, the 30s and 50s subunits. unit of the ribosome, a specific initiating t-RNA carrying
(Again these numbers refer to the speed of movement of methionine (or in bacteria, a modified methionine) forms
the subunits through a solution spinning in a high-speed a complex with the ribosome subunit and the AUG codon
centrifuge and are related to subunit size.) The 30s subunit which signals the beginning of the amino acid sequence