matter as the code for protein synthesis. mRNA is synthesized as required and,
once its message has been delivered, it is eventually decomposed.
Transfer RNA (tRNA)transports the required amino acids from the cell’s
amino acid pool to the ribosome. Each type of amino acid can only be trans-
ported by its own specific tRNA molecule. The tRNA, together with its amino
acid residue, binds to the mRNA already bound to the ribosome. It recognizes
the point on the mRNA where it has to deliver its amino acid through the use of
a consecutive sequence of three bases known as ananticodon, which is found on
one of the loops of the tRNA (Figure 1.33(b) ). This anticodon binds to the
complementary codon of the mRNA. Consequently, the amino acids can only
be delivered to specific points on the mRNA, which controls the order in which
amino acid residues are added to the growing protein. This growth occurs from
the N-terminal end of the protein.
Ribosomal RNA (rRNA)is involved in the protein synthesis. It is found in the
ribosomes which occur in the cytoplasm. Ribosomes contain about 35%protein
and 65%rRNA. Experimental evidence suggests that rRNA molecules have
structures that consist of a single strand of nucleotides whose sequence varies
considerably from species to species. The strand is folded and twisted to form a
series of single stranded loops separated by sections of double helix, which is
believed to be formed by hydrogen bonding between complementary base pairs.
The general pattern of loops and helixes is very similar between species even
though the sequences of nucleotides are different. However, little is known
about the three dimensional structures of rRNA molecules and their inter-
actions with the proteins found in the ribosome.
1.7 Questions
(1) Draw the general structural formula of each of the following classes of
compound: (a) ab-amino acid, (b) a nucleotide, (c) a plasmalogen, (d) an
a-amino acid, (e) a tripeptide and (f) a diacylglycerol (diglyceride).(2) Describe, using a suitable example, each of the following: (a) a prosthetic
group, (b) a peptide link, (c) a S–S bridge and (d) the tertiary structure of a
protein.(3) Define, by reference to glucose, the meaning of each of the following terms:
(a) a monosaccharide, (b) mutarotation, (c) a pyranose ring system, (d)
anomers and (e)a- andb-glucosides.34 BIOLOGICAL MOLECULES