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Ch. 13 Notes: Mutations and Regulation of Gene Expression
Gene Mutations
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Mutation = permanent change in the sequence or number of nucleotides in DNA
o
if in gametes, passes on from generation to generation
o
if in somatic cells, passes on from cell to daughter cells
Mutagenesis = creation of mutations
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May be spontaneous = cellular accidents
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Ex. Transposons = pieces of DNA that “jump” from one location to another,
disrupting gene expression
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o
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Replication errors happen but are rare (1 in 1 billion base pairs)
May be induced = triggered by environmental effects
Mutagens = physical or chemical agents that interact with genetic material to cause mutations
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Ex. Radiation (like X-rays) and chemicals (like radon)
Point Mutations = mutations that involves a single nucleotide
Substitution = changing one nucleotide with a different one
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May be:
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Silent mutation = has no effect on the amino acid sequence  codes for same
amino acid
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Missense mutation = codes for and inserts a different amino acid
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May affect protein structure and function
o
Ex. Hemoglobin  one nucleotide difference between normal
and sickle cell hemoglobins (T changed to A  Glu changed to
Valine)
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conservative = doesn’t make difference in protein’s tertiary structure
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nonsense mutation = changes a sense codon to a STOP codon
Frameshift Mutations = a deletion or addition of nucleotides may cause a shift in the reading frame of
mRNA
1) Deletion = one or more nucleotides are omitted from the sequence
2) Insertion = one or more extra nucleotides are added to the sequence
o
changes the codon triplets read from that point on
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Ex. THE RED CAT ATE THE FAT RAT
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almost always bad
All mutations have the potential to have huge impacts on the phenotype, especially if they create
nonfunctional proteins
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Most critical if they are part of a metabolic pathway
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Ex. PKU, albinism, and androgen insensitivity
CANCER
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Occurs when a cell escapes from the normal controls on growth and division (the cell cycle)
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Caused by accumulations of mutations that alter normal gene expression in somatic cells that are
random and spontaneous
o
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This is why cancer becomes more common with aging
Carcinogens = physical agents such as X-rays and chemical agents that cause cancer by mutating
DNA
o
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May activate oncogenes = cancer-causing genes
Proto-oncogenes = gene that normally codes for regulatory proteins controlling cell growth,
division and adhesion, and that can be transformed by mutation into an oncogene
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Three types of mutations can convert proto-oncogenes to oncogenes:
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Movement of DNA within the genome
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Often have broken and rejoined pieces of DNA
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Gene amplification
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Point mutations
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Changes in tumor-suppressor genes that normally inhibit growth can also promote cancer.
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Oncogene proteins and faulty tumor-suppressor proteins interfere with normal signaling pathways
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“Guardian Angel” proteins exist that prevent a cell from passing on mutations due to DNA
damage
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Ex. suicide genes make products that cause cell death (apoptosis)
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Multiple mutations underlie the development of cancer
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Viruses play a role in about 15% of human cancer cases worldwide
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Genetics also plays a role in many cancers