12-4 Mutations
Mutation: A Change in DNA
•
Mutation – any change in the DNA sequence that can also
change the protein it codes for
•
Mutations in Reproductive Cells
•
If mutation occurs in egg or sperm the altered gene would
become part of the genetic makeup of the offspring
•
Sometimes the mutation is so severe that the embryo
does not survive
•
In rare cases a gene mutation may have positive effects
• Mutations in Body Cells
• If the cells DNA is changed this mutation
would not be passed on to offspring
• But the mutations can cause harm to the
individual
2 Types of Mutations in DNA
1. Point Mutation
•
Is a change in a single base pair in DNA
•
A change in a single letter changes the meaning of this
sentence
2. Frameshift Mutation
•
A mutation in which a single base is added or deleted
from DNA
•
This mutation would cause nearly every amino acid in
the protein after the deletion to be changed.
mutated
base
Chromosomal Mutations
•
Chromosomal Mutations – changes in chromosomes during
replication. Parts can be broken or lost.
•
They occur in all living organisms, but they are especially
common in plants
•
Although rare, changes in an organisms chromosome
structure do occur.
•
Chromosomal mutations are rarely passed on to the next
generation because:
•
The zygote usually dies
•
The mature organism is usually sterile
•
4 Types of Chromosomal Mutations
1. Deletion – a fragment of a chromosome breaks off, it
can be lost when a cell divides
2. Duplication – the chromosome fragment attaches to its
homologous chromosome, which will then carry two
copies of a certain set of genes
3. Inversion – fragment reattaches to the original
chromosome in the reverse orientation
4. Translocation – a fragment may join a nonhomologous
chromosome
Causes of Mutations
•
Spontaneous Mutations – a mistake in base pairing during
DNA replication. It occurs at random or at any given moment
•
Mutagen – any agent that can cause a change in DNA
•
Ex. Chemicals, radiation, high temperatures
Repairing DNA
•
When mistakes do occur repair mechanisms fix mutations
•
Proofreading Enzymes – reads the DNA strand and checks it
for mistakes
•
Repair Enzymes – fixes any mistakes in the DNA strand
Mistakes in Meiosis
•
Sometimes accidents occur during meiosis and chromosomes fail
to separate correctly
•
Nondisjunction – failure of homologous chromosomes to
separate
•
During meiosis I one chromosome from each pair is supposed to
move to opposite poles but occasionally both chromosomes of a
pair move to the same pole
•
Trisomy – 1 extra chromosome (47)
•
•
Ex: extra chromosome on pair number 21 – down syndrome
Monosomy – missing 1 chromosome (45)
•
Ex: missing chromosome on pair number 23 – turner syndrome
•
Tetraploid – 2 extra chromosomes (48)
•
Polyploids – organisms with more than the usual number of
chromosome sets
•
Is rare in animals and almost always results in death.
What is abnormal about this person’s chromosomes?
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation
Deletion
Inversion
Duplication
Translocation