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1

mutational hot spots

short repetitive sequences
palindromes

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short repetitive sequences (mutational hot spot)

pairing of repeats may interfere with replication of repair enzymes

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palindromes (mutational hot spot)

often associated with insertions or deletions

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Does DNA symmetry increase or decrease the likelihood of mutation

increases

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How are repeated genes prone to mutation?

by mispairing during meiosis

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Induced mutation

Caused by mutages, many are also carcinogens and cause cancer

-ex: acridine dyes = add or remove base
-ex: radiation- breaks chromsome

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Dimer mutation

An example of induced mutations because UV radiation creates thymine dimers, which is the mainc ause of melanoma skin cancers

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Ames test

An in vitro test of the mutagenicity of a substance
One version uses Salmonella bacteria with mutation in gene for histidine
- Bacteria are exposed to test substance
- Growth on media without histidine is recorded
- Bacteria only grow if mutations have occurred
- Substance can be mixed with mammalian liver tissue prior to testing to mimic toxin processing in humans

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Mutagen exposure

workplace
industrial accidents - chernobyl
medical treatments
weapons
natural sources (cosmic rays, sunlight, earth's crust)

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Point mutations

A change of a single nucleotide
-transition - purine replaces purine or pyrimidine replaces pyrimidine
-transversion - purine replaces pyrimidine or pyrimidine replaces purine

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consequences of point mutations

-missense mutation - replaces one amino acid with another
-nonsense mutation - changes a codon for an amino acid into a stop codon
a stop codon that is changed to a coding codon lengthens the protein

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splice site mutations

alters a site where an intron is normally removed from mRNA

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How can splice site mutations affect the phenotype?

1. intron is translated or exon skipped (ex: CF mutation)
2. Exon is skipped (ex: familial dysautomia)

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Deletions and Insertions

Nucleotide changes in multiples of 3:
-does not cause a frameshift mutation (disrupts the reading frame and alter amino acids after mutation)
-may alter the phenotype

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Can different types of mutation cause the same single-gene disorder?

yes

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pseudogenes

a DNA sequence similar to a gene but which is not translated
-may not even be transcribed into RNA
-may have evolved form original gene by duplication and acquired mutation
-crossing over between a psuedogene and function gene can disrupt gene expression

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expanding repeats

insertion of triplet repeats leads to extra amino acids
-the longer proteins shut down the cells
-number of repeats correlates with earlier onset and more severe phenotype

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Anticipation

the expansion of the triplet repeat with an increase in severity of phenotype with subsequent generations

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An example of a triplet repeat disease

myotonic dystrophy

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copy number variants (CNV)

-sequences that vary in number from person to person
-range in size from a few bases to millions
-account for about 25% of our genome
-CNVs may have no effect on the phenotype or they can disrupt a gene's function and harm health

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silent mutations

mutations that do not alter the encoded amino acid

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synonymous codons

a mutation that alters the DNA but the protein sequence remains unchanged. for example CAA and CAG both code for glutamine. and therefore are synonymous codons.

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Nonsynonymous codon

created by a missense mutation which alters the encoded amino acid to another amino acid

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conditional mutation

produces a phenotype under particular condition or environments

ex: glucose 6-phosphate dehydrogenase enzyme, which responsds to oxidants, chemicals that strip electrons form other molecules

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DNA repair

errors in DNA replication or damage to DNA create mutations and may result in cancer
-most errors and damage are repaired

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The 3 types of DNA repair in many species

1) photoreactivation repair
2) excision repair
3) mismatch repair

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Excision repair

pyrimidine dimers and surrounding bases are removed and replaced
1) nucleotide excision repair replaces up to 30 bases and corrects mutations caused by different insults
2) base excision repair replaces 1-5 bases specific to oxidative damage

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mismatch repair

enzymes detect bucleotides that do not base pair in newly replicated DNA
-the incorrect base is excised and replaced

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proofreading

the detection of mismatches

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what happens if both copies of a repair gene are mutant

a disorder can result

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failture of DNA repair

if damage of DNA is too severe, the pp53 protein, which motiors repair of DNA, promotes programmed cell death or apoptosis

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Repair disorders: inherited colon cancer

-hereditary nonpolyposis colon cancer
-affects 1/200 individuals
-defect in mismatch repair
-HNPCC gene is on chromosome 2

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statistics of cancer

1/3 will be affected

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cancer

genetic, but not usually inhertied. cancer is a group of diseases caused by loss of cell cycle control.

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carcinogens

substances that cause cancer. most are mutagens (damage DNA)

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tumor

growth is formed if a cell escapes normal control over its division rate; tumor can cause cancer
-a tumor is benign if it does not spear or "invade surrouding tissue
-a tumor is cancerous or malignant if it infiltrates nearby tissues

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metastasis

the tumor spreads to other parts of the body via the blood or lymph vessels

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oncogenes (cancer-causing gene)

cause cancer if inappropriately activated

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tumor suppressor genes

-deletion or inactivation causes cancer
-cell cycle control/checkpoints

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cancer causing genes

oncogenes
tumor suppresor genes
changes in gene expression

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what controls the cell cycle

the checkpoints, which ensure that mitotic events occur in the correct sequence

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what does timing, rate and number of cell divisions depend on

-protein growth factors
-signaling molecules form outside the cell
-transcription factors within

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cancer cells

-result form faulty check points
-goes down a pathway of unrestricted cell dividion
-either lose specializations or never specialize

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telomeres

loss of control of telomere length may also contribute to cancer

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telomerase

the enzyme (complex of RNA and protein) that adds telomere sequences to the ends of chromosomes.
-normal, speciaized cells have telomerase turned off, limits cell division
-cance cells have to express telomerase to be able to divide indefinitely

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inherited vs sporadic cancer

inherited: germline mutations
sporadic: somatic mutations

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somatic mutations

-occur sporadically in nonsex cells
-result from a single dominant mutation or two recessive mutation in the same gene
-cancer susceptibility not passed on to offspring

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germline mutations

-cancer susceptibility passed on to offsrping
-usually requires second somatic mutation
-rarer but strike earlier than sporadic cancers

49

which cancer (inherited or sporadic) strikes earlier?

inherited cancer - germline mutations