Lecture 23: mutations, biochemical pathways, and repair mechs Flashcards
(44 cards)
1
Q
Somatic Mutations
A
- arise in somatic tissues which do not produce gametes.
- When a somatic cell with a mutation divides through mitosis, the mutation is passed on to the daughter cells-leading to a population of genetically identical cells. (clones)
2
Q
Germline Mutations
A
- arise in cells that produce gametes.
- The mutation can be passed to future generations producing individual organisms that carry the mutation in all somatic and germ line cells
3
Q
Chromosome mutations
A
affect the number or structure of chromosomes
4
Q
Gene Mutations
A
affect a single gene
5
Q
Point mutations
A
- results in substation of one base for another
- Two types of point mutations:
1) transition:
2) Transversion
6
Q
Forward mutation
A
a mutation that alters the wild type phenotype
7
Q
Reverse mutation
A
changes a mutant phenotype back into the wildtype
8
Q
Missense Mutation
A
a point substitution that results in a different amino acid in the protein
-alters protein function or protein is nonfunctional
9
Q
Nonsense mutation
A
changes the codon so it becomes a stop codon
EX: GAG to UAG
10
Q
Silent Mutation
A
codes for the same amino acid
-AGG to CGG both code for arginine
11
Q
Neutral Mutation
A
changes the amino acid but doesn’t affect protein function
-is a missense mutation that doesn’t alter protein function
12
Q
Spontaneous Point Mutations
A
those mutations that occur under normal conditions
13
Q
Induced Mutations
A
those mutations that result from changes caused by environmental chemicals or radiation
14
Q
Tautomerization
A
- the position of protons in the DNA bases change
- alters pairing properties during replication
- C pairs to A and G to T-results in transitions after replication
- each base can undergo tautomeric shift
15
Q
Wobble Base Pairing
A
misfiring often arose in which normal, protonated and other forms of the bases are able to pair because of flexibility in the DNA structure/Helix
- Results in transitions after replication
- Allows T-G pairing which causes replication errors
16
Q
Depurination
A
- results in a purine site which is a nucleotide that lacks its purine base
- results in transitions after replication
- Removal of glycosidic bond between base and sugar
- Normally 10,000 deprivations per 20 hrs @ 37 celsius in a mammalian cell
- Problem is that during replication, the base opposite the AP site is not specified and improper base can be put in causing transitions and transversion
17
Q
Deamination of Cytosine
A
produces URACIL which pairs with adenine in replication. After another round of replication the adenine will pair with Thymine creating a TA pair in place of the original CG resulting in a transition
- Nitrous acid dominates cytosine creating uracil, which in the next round of replication pairs with adenine producing a CA-TA transition
- Nitrous acid changes adenine into hypoxanthine which pairs with cytosine leading to TA-CG transitions
-can be reversed by nitrous acid
18
Q
Base analogs
A
- one class of chemical mutagens
- chemicals with structures similar to that of any of the four standard bases of DNA
- DNA polymerase can’t distinguish these analogs from the standard basers, so if they are present during replication they may be incorporated into newly synthesized DNA molecules
- These bases shift to their tautomeric forms more than normal bases
19
Q
5-bromouracil
A
base analog
-normally pairs with adenine, but pairs with guanine in tautomeric form leading to a transition
20
Q
2-aminopurine
A
normally pairs with THymine, but pairs with cytosine in tautomeric form
21
Q
Mechanism behind base analogs
A
SAME as tautomerizaiton
- requires round of replication to incorporate base analog
- requires two more rounds to obtain complete transition
22
Q
Hydroxylamine
A
(NH2OH)
- Mutation produced by this chemical agent
- reacts with cytosine to allow it to pair with adenine causing CG to TA transitions
- IS NOT REVERSIBLE
23
Q
Alkylating Agents
A
- chemicals that donate alkyl groups such as methyl and ethyl to the bases
- adding methyl and ethyl groups to guanine and thymine results in transitions
24
Q
EMS
A
alkylating agent
-adds a ethyl group to guanine producing O6-ethylguanine which pairs with Thymine. ALso, capable of adding to THymine producing 4-ehthylthymine which then pairs with guanine leading to transitions
25
Aflotoxin
Chemical used to cause apurinic site for guanine (removal of guanine)
- then causes adenine to be put across from the AP site during replication
- In the next round of replication, the A will pair with T
26
Oxidizing agents
can damage DNA and cause mutations
27
8-oxyguanine
oxidizing agent
- causes GC to TA TRANSVERSION
- oxidative reaction converts guanine into 9-oxoguanine
- 8-oxoguanine pairs with adenine instead of cytosine during replication
- the adenine will pair with a normal Thymine during the next round of replication
28
Frameshift mutations
mutation event leading to the insertion or deletion of one or more base pairs in the gene, shifting the reading frame in all codons following the mutation site. Can alter start/or stop sites
29
Unequal Crossing Over
- causes insertion and deletion
- misalignment of homologous chromosomes during crossing over can lead to unequal crossing over with one product have an insertion and the other having a deletion
30
Fragile X syndrome
- disease caused by expanding trinucleotide repeats
- extra copies of the trinucleotide CGG on X chromosome
- symptoms range from learning disabilities to severe mental retardation, behavior and attention problems, and autistic behavior
- Physical: long face and jaw bone and loose joints
- Normal Range: 6-54
- Disease RANGE: 50-1500
31
Huntington's DIsease
- disease caused by expanding trinucleotide repeats
- repeated sequence CAG
- NORMAL: 9-37
- DISEASE: 37-121
32
Frame Shift Mutation Agents
- Intercalatin agents: proflavin, ethidium bromide, acridine orange
- large planar molecule slips between base pairs of DNA distorting the helix and causing TEMPLATE SLIPPAGE during replication
33
X-Rays
- greatly increase mutation rates in all organisms
- leads to chromosome aberrations by breaking phosphodiester bonds
- can also damage bases and cause point mutations
- x-rays cause most damage in dividing cells when chromosomes are condensed into mitosis
- used in cancer treatment when tumor cells divide more rapidly
34
UV
- has less energy when comported to X-ray
- Purine and pyrimidine bases readily absorb UV, resulting in formation of chemical bonds between adjacent pyridmidine molecules on the same strand of DNA creating PYRIMIDINE DIMERS
- Thymine dimers are most frequent form of pyrimidine dimer
- Dimers distort the configuration of DNA/Helix and often inhibit/block replication
35
Xoderma Pigmentism
- associated with defects in DNA repair systems
- UV
- Human disorder due to a faulty DNA repair mech.
- various mutations cause this but the most is the nucleotide excision repair
- very sensitive to sunlight
- high risk of skin cancer since many pyrimidine dimers do not get repaired
- Frecklike spots on skin, sensitivity to sunlight, predisposition to skin cancer
36
Hereditary Non-polyposis Colon Cancer
- associated with defects in DNA repair systems
- Predisposition to colon cancer
- Defects is due to mismatch repair
37
Proofreading during replication
- DNA repair Mech
- if improper base is added in replication, the 3' OH is not in the proper position for the next nucleotide addition
- DNA POLYMERASE STALLS replication
- EXONCULEASE from the DNA polymerase removes the incorrect nucleotide and then DNA polymerase inserts the correct nucleotide
- Errors are about 1 in 10 million after proofreading
38
Mismatch Repair
- just after replication the new strand is not methylated and the old strand is methylated so that a distinction can be made between old and new strands
- MISMATCH REPAIR PROTEINS recognize a abnormal helical structure and identify the incorrect base
- EXONUCLEASE- remove an area of the new strand from the methylated sequence to the mismatch
- DNA polymerase fill in the gap and LIGASE seals the nicks
39
Direct Repair of Mutations
-corrects structure of abnormal nucleotide with out replacing the nucleotide
EX: photoreactivation repair of pyrimidine DIMERS (bacteria)
-enzyme photolyase absorb light and clips dimer
EX: Methyltransferase- restores correct form to methylated guanine base
O6-methylated guanine to guanine
40
Nucleotide excision repair for THymine dimers
enzyme complex UVR endonuclease recognizes distortion in helix
- Strand of DNA are separated and help apart by SSB
- Enzyme cleaves sugar phosphate bonds on both sides of lesion removing several nucleotides including the defective area
- DNA polymerase fills in the gap
- DNA ligase seals nick
41
Base Excision Repair
Deamination of cytosine
- GLYCOLYSASES recognize and remove defective bases resulting in an AP site
- Then AP ENDONUCLEASE cleaves the phosphodiester bond next to the missing bad (causes a nick) and then removes the rest of the nucleotide
- DNA polymerase fills in the gap
- DNA ligase seals the nick
42
SOS replication bypass
- allows replication to proceed so cells avoid death
- mechanism is unclear, but gap is left in replication then random base is filled in.
- Can cause mutations since 1 in 4 chance of proper are being inserted
43
Animal Bioassay
detecting mutations
- problem threshold as it applies to human risk
- no mutagenic response at 5X dose and above
- mutagenic response below 5X dose of chmical
44
Ames test
Mix:
- chemical to test
- salmonella strain that requires histidine (his-)
- homogenized liver extract
- Mix and incubate 2 days at 37 Celsius
- See how many colonies grow with out histidine (his+)
- These are mutants and compare to results using various doses of chemicals
