Mechanisms of Gene Mutation Flashcards
Mutagens are characterised by: … for ….
distinct mutational specificity
for both the type of
MUTATION & for the SITE of the mutation
What are “HOT SPOTS” for mutations?
“Hot spots” are sites in genes where MUTATIONS OCCUR MORE FREQUENTLY than other sites in the
gene;
may be spontaneous or induced mutation
Mechanisms of POINT MUTATION Induction
—Mutagens can act through at least three different
mechanisms:
- can REPLACE a base in the DNA
- can ALTER a base & cause it to MISPAIR
- can DAMAGE a base so it CAN’T PAIR
Mechanisms of Mutation Induction: 2 AND EXPLAIN
- Base mispairing
-TAUTOMERIC forms of bases
- IONISED bases
- PURINE - PURINE & PYRIMIDINE-PYRIMIDINE (less common)
2.Frameshift
- slipped mispairing (slippage during DNA rep)
- unequal crossing over (during recombination or
meiosis)
EXPLAIN BASE REPLACEMENT
Base replacement - BASE ANALOGUES are STRUCTURALLY SIMILAR to bases & may be inserted
into DNA.
However, base analogues often DO NOT PAIR CORRECTLY - hence can cause CAUSE MUTATIONS by insertion of incorrect nucleotide during replication
Explain BASE ALTERATIONS
Base alterations - bases can have different forms
= TAUTOMERS
Base tautomers are in equilibrium.
Keto (>C=O) form normally present in DNA.
Imino (>C=NH) & enol (>C-OH) forms rare.
Imino & enol base tautomers may pair with the wrong base = mispair,
Imino & enol base tautomers may pair with the wrong base = mispair,
Explain WOBBLE base Pairing…
- DNA strands separate for replication.
- Thymine on the original template strand base pairs with guanine through wobble, leading to an incorporated error.
- At next round of replication, the guanine nucleotide pairs with CYTOSINE, leading to a TRANSITION MUTATION.
What is INCORPORATED ERROR?
Thought to be responsible for many of the mispairings in DNA replication
Incorporated error = mispair in NEW strand… in the next
round of replication becomes a fixed/permanent replicated error (TA -> CG).
What type of mutation is this?
Mutations Caused by Base Mispairing:
- Typically transitions:
- purine to purine
- pyrimidine to pyrimidine
Due to wrong partners in purine - pyrimidine pair
G with T: GC to AT
A with C: AT to GC
2.Rarely transversions:
Purine to Pyrimidine
Pyrimidine to Purine
Due to purine-purine pairing or pyrimidine- pyrimidine pairing
G with A: GC to TA
T with C: TA to GC
Energetically unfavourable (distort dimensions of double helix)
Understanding Base Ionisation:
Mispairs can also be formed when base analogues
are ionised 5-bromouracil a thymidine analogue, Br
replaces CH3
- in replication, 5-bromouracil may become incorporated into DNA in place of thymine, producing an incorporated error.
- 5-Bromouracil may mispair with guanine in the next round of replication.
- in the next replication, this guanine nucleotide pairs with cytosine, leading to a permanent mutation.
- If 5-bromouracil pairs with adenine, no replication error occurs.
CONCLUSION: incorporation of bromouracil followed by mispairing leads to a TA –> CG transition mutation.
Strand separation, replication, Incorporated error, replication, strand separation, replication, replicated error.
Base Ionisation: Understaning what 5-BU ionisation is = 4
- 5-BU ionisation thus causes mispairings of altered
“T” to G … - So 5-BU causes GC (G⋅C) to AT or AT to GC
transitions in the course of replication.
(Recall transitions are purine to purine, or pyrimidine to pyrimidine replacements.) - Which occurs depends on whether the 5-BU was ionised within the DNA, or as an incoming base
- 5-BU’s action as a mutagen is caused by it being in the ionised form.
5-BU transitions - how it works…
GC in DNA, during replication 5-BU(ionised) mispairs with G… now have G5-BU. During replication if 5-BU reverts to common form, then have A5-BU pair.
In next round will have AT … so have GC to AT transition!
Similarly if have A5-BU in the DNA, and 5-BU is ionised, will get G5-BU* after replication, and after next
round GC in DNA … so have an AT to GC transition!
What is 2-AP mutagen?
- 2-AP is a Commonly Used Mutagen
- 2-AP (2-amino purine) is an adenine analogue, which pairs with T.
- Protonated 2-AP pairs with C
2- AP ->T
Protonated 2-AP -> C
Why/ how is 2-AP a commonly used mutagen?
If 2-AP is incorporated into the DNA by pairing with T, it can cause AT to GC transitions, by mispairing with C in subsequent replications.
If 2-AP is incorporated into the DNA by pairing with C, it can cause GC to AT transitions when 2-AP pairs with T in subsequent replications
SO SPECIFIC FOR TRANSITION MUTATIONS
