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Lecture 1 Flashcards

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1
Q

Which bases are purines?

A

Adenine

Guanine

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2
Q

Which bases are pyrimidines

A

Thymine
Cytosine
Uracil

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3
Q

What are transversions

A

Purine to pyrimidine etc

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4
Q

Define mutation

A

Change in the DNA sequence of a gene

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5
Q

In unicellular organisms what can a mutation effect

A

Mutation in ANY cell can affect future generations

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6
Q

Missense

A

Codon specifies DIFFERENT amino acid

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7
Q

Nonsense

A

Codon signals termination codon

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8
Q

Synonymous

A

Codon specifies different amino acid but one that is functionally equivalent for example basic arginine swapped for basic lysine

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9
Q

How can mutagens work 3 points with examples

A

By replacing base (5BU)
By altering base (base mid pairing HN02 and alkyl aging agent EMS and NMG)
By damaging base (so can no longer base pair)

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10
Q

What are transitions?

A

Purine to Purine

Pyrimidines to pyrimidines

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11
Q

Reverse mutation

A

Exact reversion

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12
Q

How can point mutations arise ? (2)

A

Spontaneously

If they are induced after dna is damaged by MUTAGENs

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13
Q

What are base analogues?

A

Chemical analogies of natural bases incorporated into dab but can cause replication errors due to high rate of tautomeric shifting

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14
Q

Define tautomer

A

Isomers that differ in positions of atoms and bonds between them

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15
Q

What form of bases is in normal base pairing?

A

Keto form

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16
Q

What base forms are rare?

A

Imino and Enol forms

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17
Q

C*

A

Rare imino form of cytosine

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18
Q

Explain how rare imino form and Enol form of C and T respectively cause dna base mispairing

A

Instead of usual AT and GC pairs
C-A
T-G

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19
Q

How does imino A* and Enol G* cause base mispairing?

A

Instead of AT and GC
A-C
G-T

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20
Q

What are the rare Enol forms of bases?

A

G

T

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21
Q

What are the rare imino forms of bases

A

c

A

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22
Q

What form does 5 bromo uracil causes bad base mispairing?

A

Replacement with Enol/ionisised 5 bromo uracil causes pairing with G as oppose to N

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23
Q

What is 5BU an analogue of?

A

Thymine

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24
Q

How is 5bu an analogue of thymine

A

Same but has bromine at carbon 5 position instead of methyl group

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25
What type of mutation does the Enol 5BU cause?
Promotes transitions during replication
26
How many forms of 5Bu are there
3 Enol Ion Keto
27
What base do the Enol and ionised forms of 5BU pair with?
Guanine
28
what types of chemicals can cause base alterations? (2) | what types of dna can they be active in?
alkylating agents nitrous acid HNO2 both replicating and non replicating DNA
29
how does nitrous acid NHO2 work and what does it work better in?
promotes oxidative deamination works better in prokaryotes converts NH2 groups to keto groups -NH2 --> =O
30
what does HNO3 convert cytosine,guanine and adenine to?
cytosine-->uracil guanine--> xanthine adenine--> hypoxanthine
31
what do uracil, xanthine and hypoxanthine pair with?
uracil--> a xanthine--> c hypoxanthine --> c
32
what are alkylating agents? what do they work best in? give 2 examples of alkylating agents:
chemicals tha tare NOT incorporated into the DNA but add methyl and ethyl groups to positions in all bases (bu particularly Guanine and Thymine) They are effective in both eukaryotes and prokaryotes. Therefore are often used in labs. they are both mutagenic and carcinogenic 1) ethyl methan sulfonate EMS 2) Nitrosoguanidine NMG (NG)
33
How does ethyl methane sulfonate (EMS) work?
alkyl group is added to oxygen at position 6 to create 0-6-alkylguanine-guanine. This makes it a guanine analogue sto pair with thymine G-T instead of GC
34
what happens when you add ems to guanine?
it becomes 0-6 ethyl guanine and pairs with thymine
35
what happens when you add EMS to thymine?
it becomes 0-4 ethylthymine and pairs with guanine
36
EMS
ethylmethane sulfonate
37
NG
nitrosoguanidine
38
what 3 types of things can cause base damage?
1) UV light 2) Radiation 3) intercalating agents
39
Uv light properties wrt mutagenesis
``` non ionizing low penetration 254nm= most mutagenic wavelength can be used on bacteria and yeast in humans can cause cancer ```
40
How does UV radiation damage bases?
produced pyrmidine dimers between adjacent pyrimidines
41
what are pyrimidine dimers?
molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions
42
How do dimers cause missense or nonsense mutations?
they distort the DNA helix and interfere wit hreplication as wrong base can be inserted opposite them causing mutations. There induction gives a premutational lesion
43
What does UV light induce and where?
ltraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds
44
what are two common photoproducts?
6-4 photoproducts | cyclobutane pyrimidine photodimer (CPDs)
45
which photoproduct is more common?which more mutagenic?
CPDs are more 3 x more common | BUT 6-4 photoproducts are more mutagenic
46
what can unrepaired purimidine dimers lead to?
melanoma
47
what is the name of the disease caused by a lack of the nucleotide excision repair system? (autosomal recessive)
Xeroderma pigmentosum
48
What is Xeroderma pigmentosum ?
genetic disease in humans in which the nucleotide excision repair process of pyrimidine dimers is lacking, resulting in skin discolouration and multiple tumours on exposure to UV light. Unrepaired pyrimidine dimers in humans may lead to melanoma
49
What allows recognition lesions by repair enzymes?
The fact pyrimidine dimers introduce local conformational changes in the DNA structure
50
what are the systems of repair in a)prokaryotes b)eukaryotes ?
pro- SOS system | euk- Translesion synthesis using translesion polymerases
51
explain excision repair
Nucleotide excision repair is a more general mechanism for repair of lesions. repair enzymes recognise DNA distortion and then excises the CPD and synthesizes new DNA using DNAP to resynthesize length of remaining single strand. ligase joins the ends.
52
how many bases are excised in base excision repair in a) prok b)euk
pro- for each wrong base pair e.g G=T 13 base pairs excised | euk- for each wrong base pair 29 bp excised
53
how does the sos system work?
damage= ssdna regions recA means negative repressor lexA cleaves therefore reducing REPRESSION of sos genes first system NER THEN umuC and umuD expressed form UMUDC complex umuDC complec alters DNAP3 allowing it to become capable of replicating DNA containing miscoding and noncoding lesions.
54
what is recA?
protein that forms filaments around ssDNA after damage that causes lexA to self cleave off
55
what is lexA ?
negative repressor of sos genes (binds to SOS boxes )
56
what does the umuDC complex allow?
the alteration of DNAP3 to enable it to proceed past lesions
57
How do eukaryotes cope with UV light?
Translesion DNA synthesis
58
what is transletion dna synthesis TLS ?
DNA damage tolerance process allows DNA replication machinery to replicate past DNA lesions (ie thymine dimers or AP sites). involves specialized translesion polymerases with larger active sites that can facilitate the insertion of bases opposite damaged nucleotides.
59
what types of ionizing radiation can cause DNA base damage?
gamma rays x rays cobalt 60 rays
60
radiation properties
number of mutations induced directly proportional to dose received shorter the wavelength the higher energy ionising radiation causes trial of ions giving rise to free radicals in tissue
61
why can sex linked recessive lethal alleles be detected in drosophila through use of radiation?
radiation kills hemizygous (1 copy only) sons but NOT heterozygous daughters
62
what do free radicals do?
cause single and double strand breaks in DNA
63
most common radiation mutation products (2)
thymidine glycol | 8-oxo hydrodeoxyguanosine 8-oxodG
64
which breaks can be repaired? what do double stranded breaks cause?
single stranded | deletions,inversions,duplications and translocations
65
8-oxo dG
8-oxo hydrodeoxyguanosine
66
what is intercalation?
the insertion of molecules between the planar bases of DNA. This process is used as a method for analyzing DNA and it is also the basis of certain kinds of poisoning.
67
examples of intercalating agents (acridine dyes)
proflavin | acridine orange
68
what are intercalating agents?
flat planar molecules that mimic base pairs and slip in between bases in DNA
69
where do intercalating agents insert and what do they do?
insert between 2 adjacent purines. | increase spacing by 3.6- 6A
70
what is the result of intercalting agents being inserted?
they cause insertions or deletion of a base pair at replication.This induces frameshift mutations leading to the incorporation of additional base on the opposite strand
71
can you revert mutations caused by a)intercalating agent b) base substitutions
a) yes | b) no
72
what is the ames test?
test using strains of salmonella typhimurium his mutants to detect mutagens

Genetics (6 decks)

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