MUTATIONS
Chromosomal Mutations – affect a chromosome
and therefore many genes. Example: Down’s
syndrome.
Gene Mutation – affect a single gene. Example
Hemophilia, sickle cell anemia.
** Further Classified as to where they occur:
Germinal Mutation – (inherited)—a change or
mutation of a sex cell. (egg or sperm), so all body
cells inherit that defective DNA. (Germinal
mutations can be chromosomal or gene
mutations).
Somatic Mutation – a mutation after development
is underway. Only some cells are affected.
Example: Birth marks, Cancer.
Causes of Mutation :
Can arise spontaneously (due to an unknown
cause) or they may be caused by an environmental
factor.
A factor that increases the chance of a mutation is
called a mutagen.
Environmental Mutagens :
1) Chemical – food additives
- hallucinogenic drugs (LSD)
- pesticides/fertilizers
- industrial chemicals
2) Radiation –(non visible short wavelength
from electromagnetic spectrum)
- The greater the exposure the greater the risk.
- Suspected mutagens are tested on bacteria such
as Fruit flies, and then mammals (rats, mice). If
mutations (such as cancer) develop, the
government restricts its use.
- Mutagens that lead to an increased chance of
cancer are called carcinogens.
Types of Defects (examples):
1. GENE MUTATIONS – affect single genes.
Result from 1 or more nucleotide changes.
* Categorized as being either:
A) Frameshift or
B) Point (change to only one codon)
Example: Normal strand
CAT/ATT/TAG
ADDITION
CAA/TAT/TTA/G (added an extra A)
DELETION
CTA/TTT/AG (lost the first A)
SUBSTITUTION
CAT/TAT/TAG (the A and T switched)
2.
CHROMOSOMAL MUTATIONS
NORMAL HUMAN KARYOTYPE BELOW:
- During the process of meiosis, homologous
chromosomes separate out into different gametes.
Example: One from the pair goes to sperm #1,
then its homologous pair must go to sperm #2.
- Also during meiosis, before separating out from
one another homologous chromosomes often
undergo crossover:
*** Things can go wrong during meiosis !
a. Translocation—exchange of chromosome pieces
between 2 different (non-homologous)
chromosomes.
b. Deletion – loss of a piece of chromosome.
c. Duplication – more than 1 copy of the same
gene.
Deletion off of Chromosome #7 = Williams
Syndrome:
Deletion off of Chromosome #5 = Cri Du Chat
Syndrome.
d. Inversion – Portion of a chromosome breaks
loose and rejoins backwards (upside down)
EXAMPLES OF MUTATIONS :
1. DOWN SYNDROME – A chromosomal
translocation.
The majority of DS is caused by having three
chromosome #21s.
- but it may also occur when part of chromosome
#21 becomes attached to another chromosome
(#’s 12, 14, 15, or 22)
2. SICKLE-CELL ANEMIA – substitution of a
base to produce a different amino acid.
-results in abnormal hemoglobin molecules.
-prevalent in African Americans (10% carry the
trait)
-causes decreased ability for the blood to carry
oxygen.
-2 chromosomes: 1 normal and 1 sickle actually
protects from Malaria
-If both chromosomes are sickle, the condition is
fatal.
3. CANCER – a somatic mutation (affects an
individual’s body cells)
-results in uncontrolled growth of abnormal body
cells that fail to stay in the organ where they
arose.
4. HEMOPHILIA – a germinal mutation ( a
mutation of the original sex cells)
-passed on to the individual from a previous
generation.