MECHANISMS OF GENETIC
CHANGE
GENETIC MUTATION
•Mutations occur all the time in our cells and usually do not cause us any
harm, particularly if the mutation takes place in a single somatic cell.
•If the mutation takes place in a germ line cell then we have the chance of
passing it on to our offspring, who will then have the mutation in all their
somatic cells. Depending on the mutation, the results could be problematic.
•Mutations can take place in whole chromosomes. Cells can have a different
number of chromosomes due to non-disjunction or fertilization from 2
sperm - these can be seen easily under a light microscope.
•Chromosomes can also exchange parts of themselves. In this case the
same amount of genetic material is still present in the cell. Usually, when
large enough sections of chromosomes exchange, this can also be seen
under a light microscope.
•Chromosome can also gain or loose sections.
•DNA mutations in the base pairs cannot be seen with a microscope. The
gene must be sequenced in order to identify the specific change.
CHROMOSOME MUTATION
Change in number
•Aneuploidy occurs when cells can contain the wrong number of
chromosomes. They can have more or less chromosomes than their usual
number. Trisomy 21 is an example of this.
•Aneuploidy is quite common in somatic cells and does not have a
negative effect on the person.
•Aneuploidy in gametes can be devastating for the embryo that in most
cases would not survive to term.
•Polyploidy is when a cell contains one or more complete extra sets of
chromosomes. Humans would therefore be 3N=69 or 4N=92.
•When a change in number of chromosomes occurs, the number of copies
of genes decreases or increases, but the genes themselves remain intact.
CHROMOSOME MUTATION
Change in number
Triploidy – 3 copies of every
chromosome.
Trisomy - 3 copies
of one
chromosome.
http://www.tokyo-med.ac.jp/genet/kry/tripk.jpg
CHROMOSOME MUTATION
Rearrangement
•Other than the number of chromosomes being altered, the
actual chromosomes themselves can be rearranged.
•Parts of chromosomes can be deleted, duplicated or inverted.
•Chromosomes can even exchange parts of their chromosome
with other chromosomes. This is called a translocation.
•In a translocation there is usually no addition or deletion of
genetic material.
•During a chromosome rearrangement a gene maybe
interrupted.
CHROMOSOME MUTATION
Rearrangement
- deletion
http://www.uvp5.univ-paris5.fr/UV_MED/AC/Icono.asp?Cours=251&Spe=CYTO
CHROMOSOME MUTATION
Rearrangement
- duplicated
http://www.uvp5.univ-paris5.fr/UV_MED/AC/Icono.asp?Cours=251&Spe=CYTO
CHROMOSOME MUTATION
Rearrangement
- inversion
http://www.uvp5.univ-paris5.fr/UV_MED/AC/Icono.asp?Cours=251&Spe=CYTO
CHROMOSOME MUTATION
Rearrangement - translocation
Normal
22
Normal
9
•Here is a case study of a translocation
between chromosome 9 and
chromosome 22.
t(9;22)
22q11
Normal t(9;22)
22
9q34
Normal
9
t(9;22)
•When part of one chromosome is
translocated onto another chromosome
there is a chance the function of a gene
can be disrupted.
t(9;22)
•The breaks in this translocation
happens to be in the 2% coding region.
•A new gene is formed on chromosome
22 that causes a type of leukaemia.
•The gene formed on chromosome 9 has
no apparent harmful affect.
CHROMOSOME MUTATION
Rearrangement - translocation
•The abl gene on chromosome 9
has been translocated (moved)
onto chromosome 22, right next
to the bcr gene.
•This makes a fusion of two
genes that would not normally be
together. It is called the bcr-abl
fusion gene and it sits on
chromosome 22.
•100% of patients with this type
of leukaemia have this gene
rearrangement including the 5%
who don’t have the chromosomal
translocation.
CHROMOSOME MUTATION
Rearrangement - translocation
abl gene – Chr 9
bcr gene – Chr 22
BCR-ABL fusion gene
BCR-ABL protein
CHROMOSOME MUTATION
Rearrangement - translocation
CHROMOSOME MUTATION
Rearrangement - translocation
Red
probe
shows
abl
gene
Green
probe
shows bcr
gene
Larger
orange spot
shows
fused bcrabl probes
GENE MUTATION
•Mutations can also take place in genes where the base sequence is
changed due to substitution, addition or deletion of single bases.
•Particular codons can also repeat themselves a number of times.
•Base substitution affects only the triplet it takes place in. This may
then affect one amino acid in a chain.
•Base addition or deletion not only affects the triplet that it takes place
in but every codon that follows. These are known as frameshift
mutations and can be far more problematic than a substitution
mutation.
•Trinucleotide repeats include the addition or deletion of a large
number of repeats. Huntingtons disease (a neurological disorder)
becomes more severe the more repeats of CAG a person has.
GENE MUTATION
Substitution mutation
THE CAT SAT ON THE MAT
THE RAT SAT ON THE MAT
Frameshift mutation
THE CAT SAT ON THE MAT
THE ATS ATO NT HEM AT
THE CAT SAT ON THE MAT
THE SCA TSA TO NT HEM AT
Trinucleotide repeats
THE CAT SAT ON THE MAT
THE CAT SAT SAT SAT SAT
SAT SAT SAT ON THE MAT
DNA REPAIR
•Even though DNA has the ability to repair itself it is
always better to prevent mutations in cells.
•Avoiding pollutants (avoid smoke filled areas), UV
radiation (slip slop slap) and x-rays (lead apron) where
ever possible.
•When something goes wrong and the base sequence is
changed DNA does have a repair kit that can come to the
rescue.
•Certain enzymes can recognize a change in the DNA
sequence and set a whole range of other enzymes into
action.
•Restriction enzymes can cut the DNA, DNA polymerase
can fill in any gap and DNA lygase rejoins the DNA strands.
•Mutations occuring in chromosomes cannot be repaired by
the DNA.
DNA REPAIR
http://microbiology.ucdavis.edu/sklab/genetic%20recomb.htm
TRANSPOSABLE ELEMENTS
•Transposable elements or transposons for short are sections of DNA that
have the ability to move from one section of a chromosome to another.
•Due to this wandering nature, they can insert themselves into genes and
disrupt their function.
•Here the gene for kernel
•They are also nicknamed ‘jumping genes’. colour (light yellow) has been
disrupted by a transposon so
that a mutant red colour has
been produced.
•The stripped effect occurs due
to the transposon moving away
and allowing the gene to
function normally again.
•Many thin stripes occur when
the transposon jumps in and
out of the gene.
WHERE WILL IT ALL END?
http://waynesword.palomar.edu/lmexer3.htm
References
• [no author] [n.d.] Iconographie [Internet] retrieved form site http://www.uvp5.univparis5.fr/UV_MED/AC/Icono.asp?Cours=251&Spe=CYTO August 2004.