Mutations are
alterations
of genetic material. They occur frequently
during DNA
duplication in cell
division. This should not be surprising considering the fact that
mitosis and
meiosis are essentially mechanical processes with
many complex
operations that must be precisely completed in order for duplicate DNA
molecules to be created. There are four common categories of
mutations:
| 1. |
DNA
base substitution, insertion, and deletion |
| 2. |
unequal crossing-over
and related
structural modifications of chromosomes |
| 3. |
partial or complete gene
inversion and duplication |
| 4. |
irregular numbers of chromosomes |
Substitutions, insertions, and deletions of single bases
are common. For example, an
adenine can be accidentally substituted for a guanine in a sequence of bases. Such small errors in
copying DNA are referred to as point mutations. There is a self-correcting mechanism in DNA replication that repairs these small errors, but it
does not always find every one of them.
The Making of the Fittest: The Birth and Death of Genes--a
mutations selected for fish that
are able to survive Subarctic water.
This link takes you to a video at an
external website.
To return here, you must click the
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(length = 13 mins,
10 secs) |
What Causes Mutations to
Occur?
Structural
modifications of chromosomes generally occur as a consequence of the
crossing-over process during cell division. Normally, there is an
equal exchange of end sections of
homologous chromosomes.
Occasionally, there is a reunion of an end section onto a chromosome that is
not homologous. Likewise, there can be an orphaned end
section that does not reattach to any chromosome. The genes on such orphans
are functionally lost.
Sometimes, extra copies of one or
more genes are produced when a DNA molecule is replicated.
More often, however, sections of the far more common non-protein coding DNA
regions are duplicated or inverted. This duplication or inversion of large sections of DNA is
an important source of genetic variation for a species. Spare
copies of genes or inactive genes can mutate and change their function over time thereby producing a new
variation that natural selection can favor or reject. Large-scale
evolutionary changes in a species line generally occur in this way.
Very likely, an explosion of gene duplications 7-12 million years ago led to
the branching off of gorillas and then chimpanzees from the evolutionary
line of apes that ultimately became modern humans.
Irregular numbers of chromosomes can occur as a
consequence of errors in meiosis and the combining of parental chromosomes
at the time of conception. Such is the case when there are three
instead of two autosomes for pair 21
(i.e., trisomy 21). This specific error is
characteristic of Down syndrome.
Types of
mutations |
|
 |
In
order for a mutation to be inherited, it must occur in the genetic material of
a sex cell. It
is likely that most sex cells contain gene mutations
of some sort. It is
now thought that the
frequency of new mutations in
humans is about 1 for every 10,000 genes per generation. If this
number is correct, every individual would be expected to have 2-3 mutations
on average. Complicating the picture is the fact that mutation rates
for different genes and chromosomes apparently vary.
Mutations are common occurrences even in healthy people.
The majority of them probably do not confer a
significant advantage or disadvantage because they are
point mutations that occur in
non-gene coding regions of DNA molecules. They are
relatively neutral in their effect. However, some
mutations are extremely
serious and can result in death before birth, when an individual is still in the
embryonic or early fetal stages of development.
Mutations can occur naturally as a result of occasional
errors in DNA replication. They
also can
be caused by exposure to radiation, alcohol, lead, lithium, organic mercury,
and some other chemicals. Viruses and other microorganisms may also be
responsible for them. Even some commonly prescribed drugs are thought to
be potential mutagens
. In this group are
| 1. |
androgens
(steroid hormones
that control the development
and maintenance of masculine characteristics) |
| 2. |
ACE inhibitors
(a class of blood pressure medication) |
| 3. |
streptomycin and
tetracycline (two
classes of antibiotics) |
| 4. |
vitamin A |
Mutations appear to be
spontaneous in most
instances. That does not mean that they occur without cause but, rather, that the
specific cause is almost always unknown.
Subsequently, it is usually very difficult for lawyers to prove in a court
of law that a particular mutagen is
responsible for causing a specific mutation in people. With the aid of
expert scientific testimony, they can often demonstrate that the mutagen
can cause a particular kind of mutation. However, that is not the same
thing as proving that a plaintiff's mutation was caused by that mutagen
instead of some others.
In order for
a mutation to be subject to natural selection, it must be expressed in the
phenotypes of individuals. Selection favors mutations that result in adaptive
phenotypes and eliminates nonadaptive ones. Even when mutations produce
recessive
alleles that are seldom expressed in phenotypes, they become part of a vast reservoir of
hidden variability that can show up in future generations.
Such potentially harmful recessive alleles add to the
genetic load of a population.
Even mutations that have a neutral effect can become advantageous or harmful
if the environment changes to select for or against them.
The great
diversity of life forms that have been identified in the fossil record is evidence that
there has been an accumulation of mutations producing a more or less constant supply of
both small and large variations upon which natural selection has operated for billions of
years. Mutation has been the essential prerequisite for the evolution of
life.
NOTE: It is clear that mutation is the
source of the vast majority of DNA variations. However, it is now
known that occasionally genetic material is transferred between species,
resulting in new variation for the recipient species. This is
particularly true at the level of primitive single-cell organisms such as
bacteria. Recent research has shown that small amounts of DNA also
have been transferred by viruses between more complex organisms such as
birds and mammals, including humans. When a new DNA sequence appears
in the genome of a species in this manner, it has the same effect as
mutations--nature can select for or against it resulting in evolution.