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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
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• 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
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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.
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mutatedbase
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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
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• 4 Types of Chromosomal Mutations
1. Deletion – a fragment of a chromosome breaks off, it can be lost when a cell divides
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2. Duplication – the chromosome fragment attaches to its homologous chromosome, which will then carry two copies of a certain set of genes
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3. Inversion – fragment reattaches to the original chromosome in the reverse orientation
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4. Translocation – a fragment may join a nonhomologous chromosome
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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
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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
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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
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• 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.
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What is abnormal about this person’s chromosomes?
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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Deletion
Inversion
Duplication
Translocation
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