Biology 12

244 MHR • Unit 3 Molecular Genetics

Where similar genes are found in different

species, the order of introns and exons tends to be

the same from one species to another. The length

of the introns, however, can be quite variable.

Human introns typically contain about 100 base

pairs, although some can be tens of thousands of

Organism

Average number of

introns per gene

Average gene

length (1000 bases)

bacteria

fungi

roundworm

fruit fly

chicken

mouse

0 1 3 3 8 7

1.0

1.5

4.0

11.3

14.0

16.5

Table 7.2

Number of introns compared to gene length

Figure 7.32The gene for

ovalbumin, the protein found in

egg whites, is divided into

seven exons. The total length

of the exons (the expressed

portions of the gene) is much

smaller than the total length of

the six introns. Some eukaryote

genes have many more introns.

The human gene for the muscle

protein titin, for example,

contains almost 180 introns.

i = introns

e = exons

e 1

i 1

i 2

i 3

i 4

i 5

i 6

e 2

e 3

e 4

e 5

e 6

e 7

Canadians in Biology

Chromosomal Abnormalities

in Sperm

We know that women’s eggs deteriorate as women age,

but what about men’s sperm? Do they also deteriorate

with age? If men who are diagnosed with testicular

cancer receive chemotherapy, will their sperm be affected?

Do sperm have a greater-than-normal proportion of

chromosomal abnormalities (in either their structure or

number) after being stored in sperm banks at very low

temperatures? Do men who smoke have more defective

sperm than men who do not? Do infertile men, who now

have the option of using reproductive technologies, run a

higher risk of having unhealthy offspring?

Dr. Renee Martin, a professor with the department of

Medical Genetics at the University of Calgary, has

researched these questions. When Dr. Martin began

her medical career 20 years ago, fertility studies focussed

almost entirely on women even though half of the genetic

material in human embryos is contributed by men.

Consequently, Dr. Martin decided to study the causes of

chromosomal abnormalities in humans with an emphasis

on the abnormalities that could be traced to sperm. After

all, if we are ever to prevent the profound problems

people sometimes inherit as a result of chromosomal

abnormalities, we need to determine what causes those

abnormalities.

New Molecular Technologies at Work

Dr. Martin’s lab, which uses new molecular technologies

to study sperm chromosomes and the genes on them,

was the first in the world to demonstrate that a large

proportion of some men’s sperm can have abnormal

chromosomes. For instance, men exposed to

radiotherapy and chemotherapy during cancer treatment

have more chromosomal abnormalities in their sperm.

Some of these chromosomal abnormalities result in

children who are born with abnormal chromosomes.

Elevated levels of sperm with chromosomal abnormalities

are also associated with infertile men, who can now take

advantage of a new reproductive technology technique

called Intracytoplasmic Sperm Injection (ICSI). ICSI is

used when natural fertilization cannot take place due

to, for example, low sperm count or poor motility

(locomotion). During ICSI, sperm is injected directly

into an egg in a test tube.

Among the new molecular techniques at the disposal of

Dr. Martin’s lab are those involving Fluorescence in situ

Dr. Renee

Martin