Biology 12

292 MHR • Unit 3 Molecular Genetics

Excision repair involves three basic steps. First, a

repair enzyme recognizes an incorrectly paired base

or a distortion in the double helix and cuts out

the damaged section. Second, DNA polymerase

synthesizes a new section, using the undamaged

DNA strand as a template. In the final step, DNA

ligase seals the new section in place along the

molecule.

Recombination Repair

When a mutation causes damage to both strands of

a DNA molecule, there is no template from which

to build a replacement section of DNA. When this

type of damage occurs in a somatic cell, enzymes

in the cell may use the homologous portion of a

sister chromatid as a template to construct new

DNA — a process called a recombination repair.

While a repair made this way is likely to contain

some errors, the extent of damage to the cell will,

nevertheless, be much less than if no repair took

place at all.

In cases where DNA damage is severe, the

mutation may trigger the action of so-called

suicide genes rather than DNA repair mechanisms.

These suicide genes cause the cell to die, thereby

preventing the mutation from being passed on to

daughter cells.

In the natural world, mutations generally occur

randomly. Even though mutagens do not always

produce mutations, the probability that certain

types of mutations will occur increases in the

presence of specific mutagens. In the laboratory,

however, scientists have developed a number of

techniques by which they can alter the genetic

make-up of living organisms in very specific and

predictable ways. You will learn about these

processes in the next section.

Your Electronic Learning Partner has an interactive

exploration on mutations.

ELECTRONIC LEARNING PARTNER

SECTION REVIEW

1. Write one sentence that explains the relationship
   between each of the following pairs of terms:
   (a)point mutation and substitution mutation
   (b)nucleotide insertion and mis-sense mutation
   (c)physical mutagen and thymine dimer
   (d)chemical mutagen and insertion mutation


2. Describe the difference between a substitution
   mutation and a frameshift mutation. Which is likely to
   cause the greatest damage to a cell?


3. Write a short paragraph explaining how a base
   analogue can act as a mutagen.


4. Explain how the action of a transposon can
   affect the expression of two separate genes at once,
   even when these genes are located on different
   chromosomes.


5. A new food colouring is being tested in a lab.
   Using the Ames test, researchers find that a
   compound in the dye is associated with a rate of
   reverse mutation twice as high as that in the control
   sample.
   (a)What conclusions can the researchers draw from
   this information?
   (b)Many substances known to be mutagenic are,
   nevertheless, used in manufacturing and in food

processing. In a small group, brainstorm some of

the social and ethical issues that are associated

with the deliberate use of mutagens. Write a short

report that explains the circumstances under

which you believe the risks of exposure to

mutagens can be outweighed by the benefits of

using these substances.

6. Older people are at a higher risk of developing
   most cancers than young people. Why is this?


7. Draw a diagram or a flowchart that could be used
   to teach the main steps involved in excision repair.


8. One mutation results in the replacement of a
   G nucleotide with a T nucleotide in the sense strand
   of a DNA molecule. Under what circumstances will
   this substitution result in
   (a)a silent mutation?
   (b)a mis-sense mutation?
   (c)a nonsense mutation?

Identify any mutagens that may be associated with the

cancer you have chosen to study for your Unit Project.

Explain how these mutagens work. Why can DNA repair

mechanisms not correct the damage?

UNIT PROJECT PREP

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