Mutations Flashcards
0
Q
What are mutations?
A
- any change to the nucleotide sequence of DNA has the potential to alter the amino acid sequence of a protein
- also, a mutation in DNA regulatory sequences can alter the expression of the gene
- mutations in RNA processing signals can alter mRNA sequence or stability
- in any of these cases, the normal cellular expression of the protein may be compromised
**only mutations in germ cells will be inherited
1
Q
Not all mutations are bad
A
- some are the basis for evolution and for refinement of protein function
- some provide medical benefit
3
Q
What is the major cause of some large scale mutations?
A
radiation in our environment
4
Q
What are the four major forms of large scale mutations?
A
- deletions
- inversions
- insertions
- translocations
5
Q
Deletions
A
- generally caused by uneven crossing over during meiosis or DNA breaks from radiation - natural radiation or medical X-rays
- a deletion can include many different genes
- often occurs when DNA is physically broken (possibly by above methods) and then rejoins (and that middle section is degraded
6
Q
Human disease associated with chromosomal deletion:
Chromosome 22q11 deletion syndrome
A
- aka Catch-22, di George, or Velocardiofacial syndrome
- symptoms: heart defects, cleft palate, and autism
- ~1 in 4000 births
- most commonly involves loss of about 3 million base pairs (and about 30 genes) from one copy of chromosome 22 (i.e. heterozygous)
- 93% of cases are de novo (new mutation not inherited from parents) deletions
7
Q
Inversions
A
- sometimes DNA breaks or recombination events may result in inversions
- a rejoin event in the middle of a gene can be disastrous
- however, if entire gene is contained in a large enough inversion, the gene may continue to function normally
- basically, all DNA sequences are still present in chromosome, but they are in the wrong orientation
*inversions are quite common and can be completely harmless
Ex: A-B-/-C-D-/-E-F (/ = break)
inverted: A-B-D-C-E-F
8
Q
Inversions may not have any consequences; remember the lysozyme gene
A
- it is surrounded by very long stretches of non-essential DNA
- it could easily be contained within an inversion without any problems
9
Q
Human disease associated with chromosomal inversion:
Hunter syndrome
A
- loss of function of an enzyme, iduronate-2-sulfatase (I2S), that degrades glycosaminoglycans (GAGs), which are long polymers of sugar
- build-up of GAGs leads to heart disease, restricted airways, enlargement of liver and spleen
- most common in males because gene is on X-chromosome
- mutation results from inversion recombination event between the functional I2S gene and its inactive pseudogene
10
Q
Insertions
A
- insertions of sequences up to several thousands of bases long may be caused by DNA breaks or by transposons and retroviruses
- insertions into coding or regulatory regions may completely destroy gene function
- we have previously discussed insertion of LINEs into the factor VIII and factor IX genes resulting in hemophilia
11
Q
Insertion mutation disease
Hemophilia
A
insertion of LINEs into the factor VIII and factor IX genes
12
Q
Translocations
A
- genetic material is interchanged between two chromosomes
- huge amounts of DNA can be involved, but major disruption occurs at the join point
- if translocation occurs in the coding or regulatory region of a gene it may disrupt normal gene function
looks like: green sister chromatid chromosome, Red chromosome –translocation–> tip of one green exchanges with tip of red
13
Q
Burkitt’s Lymphoma is caused by what mutation?
A
Translocation
14
Q
Human disease associated with translocation:
What is Burkitt’s Lymphoma?
A
- cancer of lymphocyte cells
- the c-myc proto-oncogene on chromosome 8 is translocated adjacent to the immunoglobulin heavy chain region on chromosome 14
- because the Ig genes are extremely active in lymphocytes, this activates the myc gene in these cells and contributes to malignancy
15
Q
What are the different types of point mutations?
A
- mis-sense
- non-sense
- frame shift
16
Q
Mis-sense mutations
A
- a single base change can alter a codon so that it encodes a different amino acid
- mutations that cause a single amino acid change are called mis-sense mutations
- depending on the amino acid changed, and the substituted base/amino acid, these mutations may have no detectable effects or positive or negative effects on the activity of the resulting protein (effects can range from undetectable to complete loss of function of resulting mutant protein)
17
Q
Conservative substitutions
A
- e.g. a basic aa may be mutated to a different basic aa, say lysine to arginine
- similarly a hydrophobic aa can be substituted for another hydrophobic aa - leucine or isoleucine
- in these cases, protein function is very likely to be retained
18
Q
Conservative vs Non-conservative
A
- non-sense mutations (e.g. basic aa to an acidic aa, hydrophobic aa to hydrophilic aa) may lead to problems with protein folding within the protein, or interactions with other proteins may be compromised
- conservative mutations are likely to retain protein function
19
Q
Non-sense mutations
A
- there is no codon for this sequence, so termination
- mutation of a codon into a termination codon
- protein synthesis will stop at the new termination codon, resulting in a truncated polypeptide
- anything after will be lost
- can result in total degradation of protein
20
Q
Frame shift mutations
A
- addition or deletion of a single base (or any number that is not a multiple of 3)
- results in the alteration of the coding information of all codons beyond the mutation point
- usually a stop codon is encountered
- can result in complete destruction of protein
- usually results in shortened polypeptide where all amino acids after the mutation are incorrect sequence
21
Q
Ehlers-Danlos Syndrome (EDS)
A
- mutations in collagen 5 (other forms can have mutations in other collagen)
- remember, collagen forms triplets (three collagens wound together to form functional unit)
- remember: (ex collagen) having one damaged protein in a protein complex can damage entire protein complex
22
Q
What mutation occurs in EDS?
A
- missense in RNA
- deletion (causing frameshift after deletion)
- missense, nonsense, frameshift, and splice site mutations
- all can lead to EDS (different severities, symptoms, etc)
- all result in poor or absent expression of collagen
23
Q
When do mutations become permanent?
A
- a mutation only becomes permanent after DNA replication
- until replication occurs, there is still a chance that DNA repair mechanisms in the cell will correct the error
24
Q
What are different causes of point mutations?
A
- normal DNA replication
- spontaneous chemical changes (deamination, depurination)
- chemical mutagens
- UV radiation
25
Normal DNA replication
- during DNA replication, point mutations arise due to unavoidable errors in reading the base sequence or finding the correct nucleotide
- avg error rate about 10^-9 per base per round of replication
- DNA polymerase has proofreading activity that constantly checks for correct base pairing during replication; if it detects an error, it degrades a short segment of the new DNA strand using an intrinsic 3'-5' exonuclease activity; it then repolymerizes the sequence correctly
26
Spontaneous chemical changes
- these are due to normal chemical reactions that occur in cells and to equilibria between isomeric forms of molecules
- depurination or deamination
27
depuriniation (spontaneous chemical changes)
- cleavage of a purine base from the deoxyribose sugar
- it leaves a gap in base sequence
- about 5000 depurinations per genome per day
- opposite the gap, DNA polymerase tends to insert an A (another purine) and so will always result in a mutation
28
deamination (spontaneous chemical changes)
- occurs when a cytosine (which pairs with G) is chemically altered to a uracil (which pairs with A)
- rate estimated about 100 per genome per day
29
Chemical mutagens
- hundreds of chemical mutagens are known and they all have the effect of increasing the number of mutations
- very often the mutagen has a direct effect on the chemical structure of the base, leading to altered base pairing
- therefore, perfect base pairing with a modified base during DNA replication may result in a mutation
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dont need to know hcemical changes
above
31
UV radiation
- UV radiation is absorbed strongly by bases and may result in covalent bonds between adjacent pyrimidines, especially thymidines
- the most common mutant structure is thymidine dimer
- dimerized thymidines are unable to base pair correctly
- removal of the dimer can also result in errors during the DNA repair process
- UV causes covalent bonds to form between adjacent thymidine bases
- the process for repairing thymidine dimers is extremely efficient; dimers are removed by one of the cells repair mechanisms but these processes are much more prone to error than DNA replication
32
What human disease is caused by failure to repair thymidine dimers?
Xeroderma pigmentosum
| xeroderma means dry skin
33
Xeroderma
- caused by defects in the DNA repair mechnaism
| - disease leads to an abnormally high rate of skin cell death and a very high rate of skin cancers
34
DNA repair systems
- all cells contain a number of enzymes whose function is to repair damaged bases and thereby to avoid mutations
- most of these proteins function as tumor suppressors, ie proteins that impede the cellular events leading to cancer
- loss of function of components of the DNA repair system is amongst the common events contributing to oncogenic transformation of a cell
ex: BRCA1 gene encodes a protein which is part of a complex involved in repair of damaged DNA; loss of function of BRCA1 protein is commonly associated with breast cancer
35
What genes encode proteins that are involved in repair of damaged DNA (especially breaks)?
BRCA1 and BRCA2 genes
36
Loss of function of either the BRCA1 or BRCA2 proteins is commonly associated with what? Describe.
- breast cancer
- women with mutated BRCA1 or BRCA2 have a 50-85% risk of developing breast cancer by age 70
- in the Ashkenazi Jewish population, defects in BRCA1 gene are responsible for a very high incidence of familial breast cancer (approx 1 in 50 women)
37
How do we get exposed to radiation in our lives?
- chest CT (7 mSv)
- hour on grounds at Chernobyl plant in 2010 (6mSv)
- two weeks in Fukushima Exclusion zone (1 mSv)
- Living in a stone, brick, or concrete building for a year (70 microSv)
- chest x-ray (20 microSv)
