Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 243

non-polypeptide products, such as the various
types of RNA molecules that play a role in protein
synthesis and other cellular processes. In addition to
genes, chromosomes contain regulatory sequences,
which are strands of DNA that help determine
when various genetic processes are activated.
You will examine some of these processes in
more detail in Chapter 8.

Arrangement of the Genome

Genes are not spaced regularly along chromosomes.
In any eukaryotic organism, the density of genes
varies from one chromosome to another. In
humans, for example, chromosome 4 is close to
1300 million bases long and has about 200 genes,
while chromosome 19 is only 72 million bases long
and has about 1450 genes. This makes chromosome
19 approximately three times richer in genes than
chromosome 4. There is, in fact, no set relationship
between the number of genes on a chromosome
and the total length of the chromosome.
The same variation holds true in the relationship
between the number of genes in any organism and
the overall size of its genome. The single-celled
protozoan Amoeba dubiahas an enormous genome
of over 650 billion base pairs, but fewer than
7000 genes. The human genome, in contrast,
contains about three billion base pairs and an
estimated 35 000 genes. A roundworm has 30 times
less DNA than a human, but over half as many
genes. This means that the genomes of different
organisms contain varying quantities of DNA that
do not serve as genes or regulatory sequences.
These non-coding sequences may be interspersed
with coding sequences in a variety of ways.

Exons and Introns

In general, prokaryotes have only one copy of DNA,

which in turn contains only one copy of any given

gene. Structural genes and regulatory sequences

make up most or all of the bacterial genome. This

means that very little of the DNA is not actively

used in metabolic processes over the course of the

cell’s life cycle. Figure 7.31 shows some of the

genes found on the E. colichromosome.

Figure 7.31This reference map of the E. coligenome

shows only a few of the several thousand genes on its

circular DNA strand. In Chapter 8, you will learn more

about how the lacand trypgenes govern the metabolism

of lactose by an E. colicell.

In contrast, eukaryote genes have a much more

complex structure. Each gene is typically

composed of one or more coding regions, known as

expressed regions or exons. These coding regions

are interspersed with a number of intervening non-

coding sequences, or introns. As shown in Figure

7.32 on the following page, introns may make up

well over half of the total length of any given gene.

Table 7.2 on the next page shows the average

number of introns in relation to the average length

of genes for several different types of organisms. In

general, the frequency and length of introns is

loosely related to the developmental complexity of

the organism. Only five percent of yeast genes have

introns, and it is rare for any one yeast gene to

contain more than one intron. Vertebrates, on the

other hand, have introns in about 95 percent of

their genes.

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http://www.mcgrawhill.ca/links/biology12
Comparative genomics is the study of the similarities and
differences among the genomes of different organisms. Use the
Internet to compare human DNA with that of another organism.
Go to the web site above, and click on Web Links. What
percentage of the genes in the human genome are found in the
genome of your comparison organism, and vice versa? What
do these numbers indicate? What are some of the practical
applications of studying the DNA of the comparison organism?

WEB LINK

In the human genome, genes make up just over one percent

of the total length of DNA.

BIO FACT