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MBG 2040 > mutations > Flashcards

mutations Flashcards

1
Q

What happenes when DNA damage is not corrected

A
  • there is an inherited change in genetic information (a mutation)
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2
Q

true or false - all mutations are created equal

A

false

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3
Q

mutation

A

a heritable change in the sequence of an organisms genetic material
- may alter the phenotype
- the process by which genetic change occurs

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4
Q

mutant

A

an organism that carries one or more mutations in its genetic material

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5
Q

mutation and evolution

A
  • mutations are the source of all genetic variation
  • natural selection preserves combinations best adapted to the existing environment
  • recombination between homologous chromosomes in meiosis rearranges genetic variability into new gene combinations
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6
Q

somatic mutations

A

occur in somatic cells
- passed to new cells through mitosis
- will not be transmitted to the progeny

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7
Q

germinal mutations

A

occur in germ line cells
- passed to new cells through meiosis
- will be passed to about 1/2 progeny who will carry the mutation in all their cells

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8
Q

bacterial and phage mutants

A
  • useful in genetic studies because they reproduce fast
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9
Q

point mutations

A

occur at localized sites in DNA
3 main types…
- base substitution
- frameshift mutation
- tautomeric shifts

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10
Q

dynamic mutation

A

when the nucleotide repeat copy can expand or contract dramaticaly

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11
Q

gross chromosomal rearrangement

A

a change in chromosome number or structure

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12
Q

base substitution

A

change in one base with a different base
- transition: replaces a pyrimidine with another pyrimidine (or purine with purine)
- transversion: replaces a pyrimidine with a purine (or visa versa)

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13
Q

frame shift

A

insertion or deletion of one or two base pairs alter the reading frame of the gene distal to the site of the mutation
- protein sequences change dramatically

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14
Q

tautomer

A

different arrangements of the same molecule - usually with H atoms

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15
Q

tautomeric shift

A

movement of H atoms from one position in a purine or pyrimidine base to another
- rare, can occur spontaneously during DNA replication where they alter DNA base pairing

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16
Q

rare A:C and G:T base pairing

A
  • occurs due to tautomeric shifts
  • when the bases are in their enrol or imino states
  • A and C create 2 H-bonds
  • G and T create 3 H-bonds
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17
Q

expanding nucleotide repeats

A
  • during DNA replication a hairpin forms on the newly synthesized strand, part of the template is replicated twice
  • the 2 strands of the new DNA molecule separate and the strand with the extra codon copies serves as a template for replication
  • expansion of triplet repeats is the cause of numerous human diseases
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18
Q

forward mutation

A

genetic alteration that changes the wild-type phenotype to mutant

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19
Q

reverse mutation

A

changes the mutated site back to normal

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20
Q

missense mutation

A

a base substitution that results in an amino acid change in the protein
- change in a codon

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21
Q

nonsense mutation

A

a base substitution at the 3rd codon position that changes a sense codon to one of the three stop codons
- stops translation
- position of the mutation determines length of the protien

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22
Q

silent mutation

A

a base substitution at the 3rd codon position that changes the codon to one still specifying the same amino acid
- doesn’t affect the overall protein

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23
Q

neutral mutation

A
  • missense mutation where the amino acid is changed to one of a similar chemical type
  • little to no effect on protein function
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24
Q

a loss-of-function mutation

A
  • cause complete or partial loss of normal protein function
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25
Q

gain-of-function mutation

A

causes the cell to produce a protein or gene product whose function is not normally present

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26
Q

conditional mutation

A

expressed only under certain conditions (ex. temp sensitive)

27
Q

lethal mutation

A

causes premature cell death

28
Q

suppressor mutation

A

a second site mutation that hides or suppresses the effect of a first mutation (reverse it)

29
Q

intragenic suppressor

A
  • suppressor mutation that occurs in the same gene
  • when a missense mutation alters a single codon a second mutation at a different site in the same gene may restore the original amino acid
30
Q

intergenic suppressor

A
  • suppressor mutation present within a different gene
31
Q

internal factors of mutations

A
  • spontaneous
  • something has gone wrong in the cell itself
  • most DNA damage is caused by internal factors generated by normal metabolic processes
32
Q

external factors of mutation

A
  • induced
  • such as chemicals and the environment
33
Q

what can damage DNA inside the cell

A
  • water (hydrolysis)
  • oxygen (oxidation)
  • alkylating agents (alkylation)
34
Q

what can cause spontaneous DNA damage

A
  • DNA replication errors
  • DNA replication pausing
  • Endogenous chemical reactions
35
Q

DNA replication error: tautomeric shift

A
  • causes spontaneous DNA damage
  • movement of H atoms in a base results in non-standard base pairing
36
Q

DNA replication error: wobble-induced base mispairing

A
  • flexibility in base-pairing (wobble) can result in non standard GT and AC base pairs
37
Q

DNA replication error: strand slippage during replication

A
  • occurs in repeated DNA sequences and misalignments during recombination
38
Q

DNA replication pausing

A
  • replication stalling at a DNA nick
  • generated by ROS or enzymes like topoisomerase
  • double-stranded break (DSB) is a lethal or mutagenic lesion unless properly repaired by recombination mechanisms
39
Q

endogenous chemical reactions: depurination

A
  • spontaneous loss of a purine base through hydrolysis of a glycosidic bond (A is lost more often than G)
  • loss of a pyrimidine base is less common
  • leave an AP site
  • during replication, single-stranded DNA containing an AP site is susceptible to replacement with A or C generating transition or transversion mutations
40
Q

endogenous chemical reactions: deamination

A
  • ## spontaneous loss of -NH2 group on DNA bases, causes transition mutations
41
Q

endogenous chemical reactions: oxidation

A
  • ROS damage DNA
  • can produce oxidized bases which frequently mispairs with C or A to produce transversion mutations (G:C to A:T)
42
Q

endogenous chemical reactions: alkylation

A

endogenous alkylating agents can add methyl groups to DNA bases

43
Q

Induced DNA damage

A
  • results from exposure to known mutagens
  • such as chemical agents and radiation
44
Q

categories of chemical-induced mutations

A
  • chemicals that are mutagenic to both replicating and non-replicating DNA
  • chemicals that are mutagenic only to replicating DNA
45
Q

chemical mutagens affect replicating and non-replicating DNA

A
  • alkylating agents
  • nitrous acid
  • hydroxylamine
  • base analogs
  • acridines
46
Q

alkylating agents

A
  • mutagens that react with DNA bases and add methyl or ethyl group
  • directly or indirectly induce transitions, trans versions or frameshifts
47
Q

nitrous acid

A
  • delaminating agent
  • removes amino NH2 from DNA bases A, C and G, cause transition mutations
48
Q

hydroxylamine

A
  • hydroxylates the NH2 group of cytosine causing the modified base to pair with adenine after replication
49
Q

base analogs

A
  • two common base analogs are 5-bromouracil and 2-aminopurine
  • incorporated into DNA during replication
  • don’t change bases themselves but they look like alternate bases
50
Q

acridines

A
  • intercalation of an acridine dye causes a frameshift mutation during DNA replication
  • long aromatic compounds that slip between bases and cause DNA backbone to become buckled
  • it is no longer straight causing polymerase to slip and mess up
51
Q

mutations induced by radiation

A
  • uv light induces mutations through excitation
  • x-rays induce mutation through ionization
52
Q

mutagenesis by ultraviolet irradiation

A
  • forms thymine dimers which block DNA replication
  • can cause DNA breaks
  • can lead to skin cancer
  • damages vitamin D
53
Q

mutagenesis by X-rays

A
  • ionizing radiation can cause DNA breaks
  • result in changes in chromosome structure
  • causes nicks in DSB in chromosomes
  • faulty repair can cause gross chromosomal rearrangements such as deletions, duplications, inversions and translocations
54
Q

DNA repair mechanisms

A
  • direct reversal of DNA damage
  • excision repair
  • mismatch repair
  • recombination
  • translation DNA polymerases
55
Q

direct reversal of DNA damage

A
  1. light-dependent repair: direct repair of thymine dimers by the enzyme photolyase (only in prokaryotes)
  2. enzymatic removal of alkyl groups from DNA bases
  3. ligation of single-stranded nicks in DNA
56
Q

excision repair

A
  • DNA repair endonuclease recognize, bind to and excises the damaged bases
  • a DNA polymerase fills in the gap using undamaged complementary strand of DNA as a template
  • DNA ligase seals the break (nick) left by DNA polymerase
    2 kinds…
    1) base excision repair
    2) nucleotide excision repair
57
Q

base excision repair

A
  • recognizes and repairs DNA bases damaged by deamination, alkylation or oxidation
58
Q

nucleotide excision repair

A

-removes thymine dimers and other bulky forms of DNA damage
- enzyme complex recognizes distortion, strands are separated and damaged part is removed, short tract DNA synthesis takes place

59
Q

mismatch repair (MMR)

A
  • recognizes a mismatched base in the newly synthesized DNA through hemimethylated GATC sequence
  • an exonuclease removes a portion of the new DNA that includes the incorrect base
  • DNA poly !!! fills in the longer gap and ligase seals the nick
  • mismatch is always associated with unmethylated
60
Q

Recombination DNA repair mechanism

A
  • have important roles in repairing spontaneous or induced DNA double-stranded breaks (DSB)
    2 types…
  • homologous (HR)
  • non-homologous (NHEJ)
61
Q

homologous recombination repair

A
  • occurs during or after DNA replication
  • if one sister chromatid suffers DSB it can be repaired using an identical sister chromatid
  • used to ensure proper chromosome separation during meiosis
  • can result in accurate repair or potential loss of homozygosity
62
Q

non-homologous recombination repair

A
  • available throughout eukaryotic cell cycle
  • provides a mechanism for T:T to be by-passed so that replication can continue
  • non-homologous end joining repairs double-stranded breaks and can result in loss of homozygosity due to lost sequences
63
Q

translesion synthesis DNA polymerases

A
  • recruited to replicate through the DNA damage, bypassing the lesion, normal DNA replication is to follow
  • TLS DNA polymerases replicate inaccurately, get error-prone DNA synthesis at site of the original blocking lesion
64
Q

SOS response of TLS DNA polymerases

A
  • DNA is heavily damaged
  • involves the activation of a host DNA recombination, DNA repair and DNA replication proteins
  • increases chance of cell survival but with an increased frequency of replication errors

MBG 2040 (23 decks)

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