Gene mutation and DNA repair Flashcards
Variation by mutation
A change in composition of bases in the DNA
(genetic variation)
- Beneficial mutations are usually passed to future generations and become distributed in the population.
- Detrimental mutations are usually not passed on as they result in abnormal/infertile/dead organism.
This form the basis of evolution.
Gene mutation
Failure to store genetic information faithfully
- The genetic code consists of triplet codons (three nucleotides specifies a single amino acid in the corresponding polypeptide).
- Any change that disrupts these sequences or the coded information provides sufficient basis for a mutation.
2 types of targets
Somatic mutation and gametic mutations
Somatic mutations
Affects one individual and are not transmitted to future
generations
Gametic mutations
Part of germline (i.e. sperm or egg) so transmitted to
offspring and enters the gene pool (i.e. effects subsequent generations)
List the 2 types of mutations
Point mutations and frameshift mutations
Point mutations
- Involve the substitution / replacement of a base
- Protein length remains the same but the changed base may or may not lead to a change in the amino acid due to the redundancy of the genetic code
Frameshift mutations
Result in a change of the transcript and translated protein due to a change in the reading frame
Missense mutation
Point mutation that induces single base substitution and change the corresponding amino acid.
Nonsense mutation
Point mutation that induces stop codon in the open reading frame.
Silent mutation
Point mutation that induces single base substitution but it does not change the corresponding amino acid.
Insert mutation
Addition of a base in the open reading frame that causes change in the triplet codon or reading frame.
Deletion mutation
Deletion of a base in the open reading frame that causes change in the triplet codon or reading frame
List the 2 classifications of mutations
- spontaneous mutation
2. induced mutation
Spontaneous mutation
- Caused by random changes in the nucleotide sequences of genes.
- These rare mutations arise as a result of errors during DNA replication.
- Once an error is present in the genetic code, it is reflected in the amino acid composition of the specified protein.
- If the changed amino acid is in a part of the protein that is critical to its structure or biochemical activity, then the protein becomes non-functional.
3 characteristics of spontaneous mutation rates
- Is extremely low
- Varies considerably in different organisms
- Even within the same species, spontaneous mutation
rate varies from gene to gene - can be induced by mutagens
Induced mutations
Caused by artificial factors which increases chemical
reactivity in cells leading to mutations.
List 3 mutagens that can cause induced mutations
(a) High energy radiation
(b) Base analogues
(c) Other chemical mutagens
UV light
- UV radiation from the sun produces thymine dimers
- The dimers distort DNA conformation and inhibit
normal replication
Ionization radiation from cosmic or mineral sources
- Gamma rays, X rays, cosmic rays etc
- Ionizing radiation is more energetic than UV radiation since it has a shorter wavelength
- Ionizing radiation produces DNA strand breaks
Base analogues
- Molecules that substitute purines or pyrimidine during
DNA replication - Changes to the base analogue leads a new base pair to appear, this leads to point mutations
- Examples: 5-bromouracil (5-BU), 2-aminopurine
B-bromouracil (keto form) is just thymine with CH3 substituted with Br
5-bromouracil
- > Acts as a thymine analogue
- > If 5-BU (keto form) is incorporated into DNA in place of thymine, it base-pairs with adenine
- > 5-BU then changes to the enol form which base-pairs with guanine during DNA replication
- After one round of replication, an AT pair is changed to GC pair
- 5-BU can cause a transition from AT pair to GC pair
Acridine dyes
- 3 ringed molecules
- cause frameshift mutations
- addition or deletion of one or more base pairs in the sequence of the gene
Examples of acridine dyes
Acridine orange, proflavin, acriflavine
Mechanism of acridine dyes
-> Acridine dyes have roughly the same dimension as
nitrogenous bases
- > They intercalates between purines and pyrimidines of DNA
- > Intercalation of acridine dyes induces contortions in the DNA helix, causing deletions and additions
UV induced damage
- UV radiation is mutagenic and causes formation of
pyrimidine dimers in DNA, mainly between two adjacent thymine residues (i.e. thymine dimers). - The thymine dimers distort the DNA conformation and block replication.
• This replication block is responsible for the UV killing
the cells.
2 types of UV specific repair mechanisms
Light dependent repair (photoreactivation repair)
Light independent repair (Nucleotide excision repair)
Photoreactivation repair
- Showed that UV-induced damage in E. coli could be partially reversed by exposure to visible light after irradiation
- Only blue light was necessary to correct radiation damage
- Further studies showed that this blue light effect is temperature sensitive (implying the involvement of a protein)
- This photoreactivation repair depended on a protein called photoreactivation (PR) enzyme
3 steps in photoactivation repair
1) The PR enzyme binds specifically to a thymine dimer. Although the enzyme will associate with a dimer in the dark, it must absorb a photon of light to cleave the dimer
2) Once blue light is absorbed, the PR enzyme cleaves the bonds between thymine dimers to reverse the effect of UV radiation on DNA
3) The PR enzyme then dissociates from the DNA
Nucleotide excision repair
-> Damaged DNA is removed and resynthesized using the opposite strand as template
- Isolated several E. coli mutants which showed increased sensitivity to UV light (i.e. mutants died faster than wild type when exposed to UV light)
- Four E. coli genes designated UVr (ultraviolet repair) were involved
- UvrA, UvrB, UvrC & UvrD are required for the excision of dimer
3 steps in nucleotide excision repair
The thymine dimer is recognized and enzymatically cut out by UvrABCD.
This excision which includes several nucleotides adjacent to the dimer, leaving a gap in the DNA strand.
2) DNA polymerase I fills this gap by inserting nucleotides complementary to those on the intact strand. The enzyme adds these bases to the 3’ OH end
of the cut DNA.
3) The enzyme DNA ligase seals the final nick that remains at the 3’ OH end of the last base inserted, closing the gap.
Nucleotide excision repair in humans
-> This type of mutation is detected by a complex of the XP-C (Xeroderma pigmentosum C) protein and 23B proteins.
-> The complex will recruit transcription factor TFIIH, whose helicase subunit (powered by ATP) will unwind and stabilize the helix until a bubble of 25
bases is formed.
- > Two endonucleases, XP-G and XP-F, will cut the unwanted bases, releasing it from the helix .
- > The gap is filled by DNA Polymerase d / e by inserting nucleotides complementary to those on the intact strand.
- > DNA ligase closes the nick.
Xeroderma Pigmentosum (XP)
(Failure of Nucleotide Excision Repair)
Xeroderma Pigmentosum (XP) cause
- > Rare autosomal recessive disorder in humans (very severe and may be lethal).
- > Predisposes individuals to epidermal pigment abnormalities.
- People with XP develop skin lesions when exposed to UV radiation (present in sunlight) and commonly develop skin cancer.
- Human XP cells are
- killed by smaller ultraviolet doses than cells from a normal person
- ability to remove thymine dimers from their DNA is very much reduced
• XP individuals lack nucleotide excision repair system which causes them to be susceptible to UV-induced skin damage.
