6.1 Gene Structure
The primary function of polymeric nucleic acids in all living organisms is the storage and transmission of
genetic information. Every living thing on Earth is constructed from a genetic blueprint encoded by its
nucleic acid genome. For all independently living organisms, this blueprint is comprised of DNA. Less
complex entities, such as viruses, which rely on hosts to live and reproduce themselves, may use RNA
instead. This chapter will describe how this genetic information is stored, replicated, repaired and copied
into the functional products on which life depends.
The basic unit of genetic information is the gene. Genes were described originally in 1865 by Mendel
as apparently indestructible factors, which specify traits of an organism such as colour or shape. The pion-
eering work of Avery and co-workers^1 showed that genes are in fact comprised of nucleic acid and a
‘Golden Age’ of molecular biology in the 1950s and 1960s laid the foundation for our present day under-
standing of gene structure and expression.^2 The modern definition of a gene is a discrete nucleic acid that
encodes an RNA or protein that has biological function. It is important to note here that not all genes encode
proteins. Many genes encode functional RNAs, such as transfer RNAs, ribosomal RNAs (rRNAs) or spliceo-
somal RNAs (Sections 2.4 and 7.3).
Gene structure is remarkably diverse. The only property shared by all genes is the presence of a nucleic
acid region that encodes a functional component. There are three dominant types of gene structure seen in
living cells (Figure 6.1). The first and simplest (Figure 6.1a) consists of a single uninterrupted coding
region flanked by signals necessary for starting and stopping the transcription of the gene into RNA. The
former signal is known as a transcriptional promoter and the latter as a transcriptional terminator. The second
type of gene structure (Figure 6.1b) commonly found in prokaryotes, such as in the bacterium Escherichia
coli, dispenses with individual promoters and terminators and pools genes together into a cluster called an
operon^3 under the control of a single promoter. The third major type of gene structure found (Figure 6.1c)
is the interrupted gene,4–6where the internal region is split into segments, which either are present in the
mature functional RNA gene product (exons^6 ) or removed during RNA splicing (Section 7.2.2) and
destroyed (introns^6 ). This seemingly bizarre organisation points back to the origin of genes in that small
segments of DNA, representing discrete units of function, are thought to have gradually become assem-
bled into the exons of more complex genes that now code for multi-domain proteins.^7
210 Chapter 6
Figure 6.1 Basic gene structures. (a) A gene with its promoter and terminator. (b) An operon containing several
genes under the control of a single promoter. (c) An interrupted gene containing exons (red shaded
boxes) and introns (uncoloured smaller boxes). Red shaded regions are protein coding