Download 8.7 Mutations

8.7 Mutations
KEY CONCEPT
Mutations are changes in DNA that may or may not
affect phenotype.
 May occur in somatic cells (aren‘t passed to offspring)
 May occur in gametes (eggs & sperm) and be passed to
offspring
 Mutations lead to genetic variation because they randomly
change an organisms genetic code.
https://www.youtube.com/watch?v=GieZ3pk9YVo
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Types of Mutations
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Some mutations affect a single gene, while others affect
an entire chromosome.
A gene mutation affects a single gene.
• Many kinds of mutations can occur, especially during
replication.
Types of Gene Mutations:
• A point mutation substitutes one nucleotide for
another. Ex: Sickle Cell Anemia
mutated
base
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Nonsense Mutation
• Type of point mutation
• Results in a premature stop codon and
usually a nonfunctional protein
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• A frame-shift mutation inserts or deletes a nucleotide in
the DNA sequence. Throws off the reading frame.
• THE CAT ATE THE RAT
• THC ATA TET HER AT
• What would an insertion look like? (draw in notes)
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Gene Mutation
Animation
Draw & explain an inversion mutation.
Skip to silent mutations
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• Chromosomal mutations affect many genes and an entire
chromosome. Chromosomal mutations may occur during
crossing over/meiosis.
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• Crossing over is a normal process (NOT A MUTATION)
– Exchange of chromosome segments between homologous
chromosomes
– Increases genetic diversity
– Occurs during Prophase I of Meiosis I
– Results in new combinations of genes (chromosomes have
a combination of genes from each parent)
http://www.youtube.com/watch?v=5x_Rp1mwotQ
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Chromosome
Mutations
 Five types exist:
 Deletion
 Inversion
 Translocation
 Nondisjunction
 Duplication
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Deletion
 Due to breakage
 A piece of a chromosome is lost
 Example: cri du chat –
chromosome 5


http://www.youtube.com/watch?v=hJcPr3MyBww
http://learn.genetics.utah.edu/content/chromosomes/diagnose/
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Inversion
 Chromosome segment breaks off
 Segment flips around backwards
 Segment reattaches
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• Translocation results from the exchange of DNA (piece of
one chromosome) segments between non-homologous
chromosomes. Sometimes chromosomes can fuse onto
another chromosome. http://learn.genetics.utah.edu/content/chromosomes/diagnose/
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Nondisjunction
 Failure of chromosomes to
separate during meiosis
 Causes gamete to have too many
or too few chromosomes (Ex: 3
chromosomes instead of 2 for
any given set, or 1 instead of 2)
 Examples: Klinefelter’s
Syndrome (XXY), Turner
Syndrome (X_)
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Nondisjunction
 Can cause “Trisomy” (three copies of the
same chromosome in an egg or sperm)
 Trisomy 21 (Down syndrome)
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• Gene duplication results from unequal exchange of
segments crossing over. Results in one chromosome
having 2 copies of some genes and the other
chromosomes having no copies of those genes.
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Chromosome Mutation
Animation
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Mutations may or may not affect phenotype.
1. Chromosomal mutations tend to have a big effect.
2. Some gene mutations change phenotype by:
– causing a premature stop codon.
– causing a change in protein shape or the active site.
– causing a change in gene regulation.
blockage
no blockage
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3. Some gene mutations do not affect phenotype.
A mutation may be silent because….
– It occurs in a non-coding region.
– It may not affect protein folding or the active site.
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Silent Mutations
• Some mutations have no effect and are
called “silent”
– Example:
GUC changed to
GUG
Both code for the
amino acid
valine
This would not
affect the protein
being made in
any way
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Mutations can be caused by several factors.
1. Replication errors can cause mutations.
2. Mutagens, such as UV rays and
chemicals, can cause mutations.
3. Some cancer drugs use mutagenic
properties to kill cancer cells.
4. Mutations happen regularly
5. Almost all mutations are neutral
6. Many mutations are repaired by enzymes.
7. Some types of skin cancers and
leukemia result from somatic mutations
8. Some mutations may improve an
organism’s survival (beneficial)
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Several methods help map human chromosomes.
• A karyotype is a picture of all chromosomes in a cell.
XY
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• Karyotypes can show changes in chromosomes.
– deletion of part of a chromosome or loss of a
chromosome
– large changes in chromosomes
– extra chromosomes or duplication of part of a
chromosome