of bases that act as the code for the production of one specific peptide or protein
molecule is known as a gene.
Genes can normally contain from several hundred to two thousand bases. In
simple organisms, such as bacteria, genetic information is usually stored in a
continuous sequence of DNA bases. However, in higher organisms the bases
forming a particular gene may occur in a number of separate sections known as
exonsseparated by sections of DNA that do not appear to be a code for any
process. These noncoding sections are referred to asintrons(Figure 1.32). A
number of medical conditions have been attributed to either the absence of a
gene or the presence of a degenerate or faulty gene in which one or more of the
bases in the sequence have been changed.
Exon
240
basesExon
500
basesExon
250
basesIntron
120
basesIntron
240
basesFigure 1.32 A schematic representation of the gene responsible for the control of the production
of theb-subunit of haemoglobin
The complete set of genes that contain all the hereditary information of a
particular species is called agenome. The Human Genome Project, initiated in
1990, has identified all the genes that occur in humans and also the sequence of
bases in these genes.
1.6.4 RNA, structure and transcription
Ribonucleic acids are found in both the nucleus and the cytoplasm. In the
cytoplasm RNA is located mainly in small spherical organelles known as
ribosomes, which consist of about 65%RNA and 35%protein.
The structures of RNA molecules consist of a single polymer chain of nucle-
otides with the same bases as DNA, with the exception of thymine, which is
replaced by uracil, which forms a complementary base pair with adenine (Figure
1.33(a) ). These chains often form single strandedhairpin loopsseparated by
short sections of a distorted double helix formed by hydrogen bonded comple-
mentary base pairs (Figure 1.33(b) ).
All types of RNA are formed from DNA by a process known astranscription,
which occurs in the nucleus. It is thought that the DNA unwinds and the RNA
molecule is formed in the 5’to 3’direction. It proceeds smoothly with the 3’end
of the new strand bonding to the 5’end of the next nucleotide (Fig. 1.34). This
NUCLEIC ACIDS 31