bio 3201 unit 2b yellow

Causes of Mutations

A substance or event that increases the rate of mutation in an organism is called a mutagen.

Many mutations are caused by molecular interactions that take place naturally within cells.

These mutations are known as spontaneous mutations.

One source of spontaneous mutations is incorrect base pairing by DNA polymerase during the

process of DNA replication. The rate of spontaneous mutations varies among organisms and

even among different genes within a single cell.

Mutations that are caused by agents outside the cell are said to be induced.

Exposure to certain factors in the environment can increase the rate of mutation.

Mutagens fall into two general categories: physical and chemical.

Mutagens that cause physical changes in the structure of DNA are known as physical

mutagens.

High-energy radiation is the most damaging form of mutagen known. X rays are a form of high-

energy radiation. They tear through DNA molecules, causing random changes that range from

point mutations to the loss of large portions of chromosomes. Ultraviolet (UV) radiation, which

is present in sunlight, has a lower range of energy levels than X rays, but it is still a powerful

mutagen.

Damage from UV radiation, as a result of exposure to sunlight, is a known cause of melanoma, a

form of skin cancer. A single sunburn doubles a light-skinned person’s chances of developing

skin cancer.

A chemical mutagen is a molecule that can enter the nucleus of a cell and induce mutations by

reacting chemically with the DNA. A chemical mutagen may act by inserting itself into the DNA

molecule in a manner that causes a nucleotide substitution or a frameshift mutation.

Other chemical mutagens have a structure that is similar to the structure of ordinary

nucleotides but with different base-pairing properties. When these mutagens are incorporated

into a DNA strand, they can cause incorrect nucleotides to be inserted during DNA replication.

Examples of chemical mutagens include nitrites (which are sometimes used as a food

preservative), gasoline fumes, and more than 50 different compounds found in cigarette

smoke.

Most chemical mutagens are carcinogens—that is, they are associated with one or more forms

of cancer.

Note: Through a process called epigenetics or epigenetic tagging, your genes are responding to

changes in their environment. These changes involve diet, exercise, stress, and even your social

behaviour and that of others. Epigenetic tags regulate genes, telling them when to turn on or

off. From the early stages of embryo development to the final days of your life, this process

enables your body to grow and respond to a changing environment, all without a single change

to the nucleotide sequence of your genes.

Genomics

Since the late 1990s, progress in the field of genetics has opened up the field of genomics.

Genomics is the study of entire genomes, including the interactions among multiple genes.

One of the most important scientific achievements in history was made possible by

advancements in techniques for analyzing DNA. This was the determination of the DNA

sequence of the human genome.

DNA sequencing is the process of identifying the precise nucleotide sequence of a DNA

fragment.

The international Human Genome Project (HGP) was completed in 2003. The main goals of the

HGP were to determine, nucleotide by nucleotide, the complete sequence of the human

genome and to identify all of the genes.

Major findings:

• The human genome consists of about 3 billion base pairs of DNA.

• Humans have about 21 000 genes, which is much fewer than scientists had predicted.

• Our genes are only about 1.5% of our DNA. Scientists still do not know what most of the

remainder of our DNA is used for.

The DNA sequences of the genomes of thousands of organisms have also now been

determined.

Together, genomics and proteomics are influencing the research in many fields of biology,

including medicine. For example, rather than targeting only the action of individual genes and

proteins, scientists now study the interactions among genes and regulatory proteins that

contribute to particular disorders. This, in turn, enables scientists to develop new treatments.

Key tools in these fields are computerized databases of the DNA sequences and associated

proteins that are found in different organisms.

Public Benefits of Genetic Research

 Offers the potential for developing drugs that are tailored not only to the expression of

individual genes associated with particular disorders, but also to the unique genome of

a patient.

 Studying the differences in gene expression among individuals can help medical

researchers understand why certain drugs work better in some people than in others,

and why certain people experience side effects from medications.

 The findings support the development of new techniques for predicting risks and

diagnosing medical conditions.

Ownership of Genetic Information

Projects such as this can contribute valuable information to researchers in many fields. Who

owns the genetic information, however?

For example,

Should companies have the right to sell DNA information to other companies without the

permission of the people who provided the samples?

Should companies that use DNA in medical research be required to share the results of their

work with the individuals or communities whose genetic information was used?

Many people argue that genetic information is a natural resource that belongs to everyone.

Other people believe that an individual’s genetic information belongs only to this individual. On

the other hand, if companies cannot earn a profit from their research, there is little incentive

for them to invest in genetic studies.

In the world of genetics, where is the boundary between public and private property?