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Mutations
Types of Mutations
• Gene mutation – a mutation that only affects one gene
• Types of mutations can be determined by
Type
Description
Example
• Step 1 | Write the wild-type allele
…TAGCCAGATA…
above the mutated allele so that the
Insertion
A new nucleotide is added
…TAGCGCAGATA…
bases line up.
…TAGCCAGATA…
• Step 2 | Starting on the left, look
Deletion
A nucleotide is removed
…TAGCAGATA…
along both strands and underline
Substitution
A nucleotide is replaced
…TAGCCAGATA…
the first base that is different in the
(point mutation) with a different nucleotide …TAGCCAGTTA…
mutated allele.
• Step 3 | Based on the difference between the two strands, identify the mutation that occurred.
• Step 4 | If possible, use the overall lengths of the two strands to check your answer.
• Chromosomal mutation – a mutation that affects multiple genes
• Deletion
• Amplification
• Inversion
• Insertion
• Translocation
• Some mutations happen during meiosis
• Non-disjunction – an event in which the homologous chromosomes do not separate and instead both go to the
same daughter cell
Biology 6.3 – Mutations
Mutations
Effects of Mutations
• Gene mutations can have different effects
• Chromosomal mutations affect multiple genes
• Have large impact on organism
• Non-disjunction creates one gamete with extra
copy and one with no information
• If either fuses with normal gamete,
individual formed has abnormal
number of chromosomes
• Ex) Down syndrome – a human genetic
disorder caused by having an extra
chromosome #21
• Physical and mental retardation
• Overexpression – the creation of more protein
than normal from a gene
• If chromosome segment is deleted or
inversed, organism might not be able to
express any of the genes
• Beneficial mutations – a mutation that increases an
organism’s fitness
• Ex) Fur color may allow organism to better blend
into the environment, avoiding predators
• Harmful mutation – a mutation that decreases an
organism’s fitness
• Ex) May make protein used to carry oxygen less
efficient
• Neutral mutation – a mutation that does not affect an
organism’s fitness
• All silent mutations are neutral mutations
Biology 6.3 – Mutations
Mutations
Causes of Mutations
• Mistakes that happen during DNA replication
• Mutagen – an environmental agent that can damage DNA
• Ultraviolet radiation and chemical toxins
• Spontaneous damage by free radicals – reactive forms of molecules made
by metabolism
• Cause damage directly to nitrogenous bases of DNA
• Antioxidants thought to neutralize free radicals
• Checkpoint procedures during cell cycle prevent cell division if mistakes are
found in DNA
• Cells that do not pass checkpoint are marked for destruction
• Mutations can accumulate if checkpoint system breaks down
• If mutation removes checkpoint, cells with damaged DNA can
replicate
• Cells can repair own damage
• UV radiation can be repaired when the damaged section of the
strand is removed
• Enzymes identify abnormal nitrogenous bases and chemically cut
them out
• Sometimes cannot repair damage
• Ionizing radiation (from X-ray machines) causes physical breaks in one or both strands of DNA
• Pieces of chromosome are lost or become attached to other chromosomes
• Very dangerous and protection should be used when exposed to it
• Babies and young children should take extra precautions to prevent mutations
• Rapidly dividing cells are more susceptible to radiation, UV light, and free radicals
• Extra precautions should be taken during pregnancy
• Damage from mutagens accumulates as we age
Biology 6.3 – Mutations