Module 8 - Causes of DNA mutation and repair Flashcards
Mutation
An alteration in the nucleotide sequence of the DNA
Two types of mutation
Spontaneous mutations - occur during DNA replication due to base tautomerism
Mutagens - chemical or environmental agents causing changes in DNA through base analogs or direct structure changes
Tautomers: what are they, what are the examples, and what effect does this have on daughter DNA molecules?
Isomers of bases with slightly different structures, usually due to the movement of a double bond and one hydrogen
- Amino-adenine bonds with thymine, imine-adenine bonds with cytosine
- Keto-thymine bonds with adenine, enol-thymine binds with guanine
- Keto-guanine bonds with cytosine, enol-guanine bonds with thymine
- Amino-cytosine bonds with guanine, imino-cytosine bonds with guanine
One out of four granddaughter DNA molecules will be a mutated DNA molecule
Difference between amino-adenine and imino-adenine?
Amino-adenine (R-(H₂N\C=N)-R) bonds with thymine, imine-adenine (R-(HN\C-NH)-R) bonds with cytosine
Difference between keto-thymine and enol-thymine?
Keto-thymine (R-(N=C/OH)-R) bonds with adenine, enol-thymine (R-(HN-C//O)-R) binds with guanine
Difference between keto-guanine and enol-guanine?
Keto-guanine (R-(N=C/OH)-R) bonds with cytosine, enol-guanine (R-(HN-C//O)-R) bonds with thymine
Difference between amino-cytosine and imino-cytosine?
Amino-cytosine (R-(H₂N\C=N)-R) bonds with guanine, imino-cytosine (R-(HN\\C-NH)-R) binds with guanine
Base analogues: what are they and what are the examples?
Molecules that can act as substitutes for bases in nucleic acids
The most popular example is 5-bromouracil (5bU) which can act as an analogue for thymine and be incorporated into DNA in place of thymine
When is the DNA incorporation of 5bU, the analogue of thymine, not an issue and an issue and what effect does this have on daughter DNA molecules?
If keto-5bU is incorporated, it bonds with adenine as thymine would so there is no issue
The enol-5bU tautomer, which is very common, binds with guanine and this causes a mutation to occur
One out of four granddaughter DNA molecules will be a mutated DNA molecule
Deaminating agents: what are they, what are the examples, what are the examples of deaminated bases, and what is the effect on DNA molecules?
Changes the structure of bases from R-(H₂N\C=N)-R to R-(O\C-NH)-R:
Replaces NH₂ with a =O and a =N becomes a -NH
Nitrous acid (HNO₂) and sodium bisulfite (NaHSO₃)
- Adenine may be deaminated into hypoxanthine which bonds with cytosine
- Cytosine may be deaminated into uracil which bonds with adenine
- Guanine may be deaminated into xanthine which inhibits DNA replication
- Thymine has no NH₂ so deamination does not occur
Two out of four granddaughter DNA molecules will be a mutated DNA molecule
Difference between adenine and hypoxanthine
Adenine (R-(H₂N\C=N)-R) may be deaminated into hypoxanthine (R-(O\\C-NH)-R) which binds with cytosine
Difference between cytosine and uracil
Cytosine (R-(H₂N\C=N)-R) may be deaminated into uracil (R-(O\C-NH)-R) which binds to adenine
Difference between guanine and xanthine
Guanine (R-(H₂N\C=N)-R) may be deaminated into xanthine (R-(O\C-NH)-R)
Alkylating agents: what are they, what are the examples, and what are the examples of alkylated bases?
Molecules that add alkyl groups to the =O of a base, causing that oxygen to be unable to form hydrogen bonds
Ethyl methane sulphonate (EMS)
O⁶ethylguanine can only form two hydrogen bonds so it bonds with thymine instead of cytosine
Causes transition mutations
Intercalated agents: what are they and what are the examples?
A molecule that inserts itself between base pairs
Ethidium bromide
Can make insertion mutations
Ultraviolet radiation: what does it do, what are the examples of its effect, and how difficult is the damage to repair?
Ultraviolet rays may hit DNA and cause base dimerisation of DNA molecules
Two adjacent thymine bases may bind to each other instead of forming their base pairs with adenine and this inhibits DNA replication which may be fatal
This is very difficult to repair, which explains why UV radiation is so dangerous
Heat: what does it do, what does it cause, and how difficult is the damage to repair?
Heat may cause bases to detach from DNA and give rise to AP (apurinic/apyrimidinic) sites
AP sites will lack a purine or a pyrimidine
This happens very often (~1,000 times a day) and is easy to repair
The four types of DNA repair
Direct repair -
Excision repair -
Mismatch repair -
Non-homologous end-joining -
Direct repair: what is it, how does it occur, what does it, and how frequent is this method?
Corrects a nucleotide alteration
E.Coli - the ADA enzyme removes alkyl groups from position 4 of T and/or position 6 of G
Humans - MGMT enzyme removes alkyl groups from position 6 of G
Uncommon method
Why is UV radiation less dangerous to some species than others?
Some species, for example, E.Coli, have enzymes such as DNA photolyse which directly repair base dimers formed by UV radiation
Other species, humans for example, do not have an enzyme capable of repairing UV damage so suffer from UV radiation
Excision repair: what is it and what are the two examples?
The damaged nucleotide is removed and repaired by DNA synthesis
- Base excision repair - the base is removed
- Nucleotide excision repair - A longer piece of DNA containing the altered base is removed
How does base excision repair work?
In E.Coli a DNA glycosylase enzyme removes the damaged base, then an AP endonuclease (ie phosphodiesterase) removes the sugar-phosphate backbone that has no base, and then DNA polymerase adds a new nucleotide which then forms phosphodiester bonds due to DNA ligase
How does nucleotide excision repair work?
In E.Coli, the UvrABC endonuclease does this
Firstly, the UvrAB trimer attaches to the damaged nucleotide, then the UvrA dimer detaches and the UvrC monomer attaches
Then the UvrBC dimer cuts the segment which is then fully excised by DNA helicase II
UvrB bridges the gap and then the DNA is synthesised by DNA polymerase I and DNA ligase makes the nucleotides form phosphodiester bonds
Mismatch error: what is it and how is it recognised?
Correcting errors in DNA replication
The two stands are distinguished by methylation on the strands: (in E.Coli) the parent strands of the new DNA molecules have methyl groups attached to nucleotides and the new daughter strand does not so the correct strand is used as a template
