Download Gene and Chromosomal Mutations

Gene and Chromosomal
Mutations
What is a mutation?
• Mutations are changes made to an organism’s
genetic material.
• These changes may be due to errors in replication,
errors during transcription , radiation, and many
other things.
• Mutations can occur within a specific gene (small
scale), as well as to the chromosome as a whole
(large scale).
Are mutations always bad?
• Favorable mutations present organisms with an
advantage over others and ensure their survival.
These mutations will accumulate in a population.
• Less favorable mutations are removed from the gene
pool through natural selection.
– Organisms with these mutations will not survive.
Mutations
A. Small Scale Mutations
B. Large Scale Mutations
Small Scale Mutations:
Gene Mutations
• There are three main types of gene mutations
according to Mechanism:
– Point Mutations
– Deletions (Frame shift mutations)
– Insertions (Frame shift mutations)
Point Mutations
 Point mutations occur when a single nucleotide in a
gene is exchanged for another nucleotide.
 For example, an A could be exchanged for a T or a C
could be exchanged for a G.
 These mutations have an effect on protein synthesis.
One DNA triplet is altered, therefore one mRNA
codon is altered, therefore one amino acid is affected
Examples of Point Mutations
 Sickle cell anemia
 Lactose intolerance
Insertions
• Insertions occur when one
or more new nucleotides
are inserted into the DNA
sequence.
• When this occurs, there is a
shift in the reading
sequence of DNA during
protein synthesis and a
completely different protein
strand may be formed.
Deletions
 Deletions are when one or
more nucleotides are
removed from the DNA
sequence.
 As with insertions, this
causes a shift in the reading
sequence of DNA and can
produce a completely
different protein strand than
the original.
 Deletions are irreversible
mutations.
Types of Mutations according to
Cause:
1.Spontaneous Mutation
2.Induced Mutation
Spontaneous Mutation
• The cause of a spontaneous mutation is
unknown.
• Spontaneous mutations result from DNA
replication errors
– The rate is approximately 1in106 replicated genes.
Induced Mutation
• An induced mutation is brought about by a
mutagen.
• A mutagen is a physical agent or a chemical
agent that causes an alteration of the base
sequence of the DNA molecule.
Induced Mutations
Types of Mutagens :
• Physical mutagens
– heat
– ultraviolet radiation
• Chemical mutagens
–
–
–
–
nitrous acid
benzpyrenes
aflatoxin
base analogues
Ultraviolet Radiation
• Ultraviolet radiation causes the formation of
thymine dimers.
• Thymine dimers block DNA replication.
• Cell excises (cuts out) the thymine dimers
using enzymes and repairs the damage.
• A mutation occurs when the repair is faulty.
Nitrous Acid
• Nitrous acid converts the adenine molecule so
that it will pair with cytosine.
• When the DNA replicates, the newly
synthesized stand will have a cytosine
opposite the adenine on the conserved strand
(instead of a thymine).
Benzpyrene and Aflatoxin
• Benzpyrene and Aflatoxin both produce
frameshift mutations so that one or more
base pairs are inserted or deleted from the
DNA when it replicates.
Large Scale Mutations:
Chromosomal Mutations
• There are three major
chromosomal
mutations that can
occur:
– Duplications
– Inversions
– Deletions
Duplication
• Duplication occurs when a
segment of a chromosome is
doubled.
• The segment of DNA that is
doubled may contain many
genes.
Inversions
 Inversions occur when a piece of DNA is removed
from the chromosome, inverted, and then placed
back into the chromosome.
 When it is re-inserted,
the genes are in reverse
order.
 There is no loss of genetic material, however gene
interactions may be altered and phenotypes may
change.
Deletions
• Deletions occur when a whole chunk
of the chromosome goes missing.
• Breaks in chromosomes can be caused by radiation,
heat, viruses, chemicals, and other environmental
causes.
• Consequences of these mutations will depend on
what part of the chromosome has been deleted.
Deletions
• Examples of disorders caused by deletions:
– Prader-Willi Syndrome
• Deletion of the long arm on chromosome 15
• Characterized by compulsive eating