What is a transition mutation?
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A type of point mutation where T changes to C or A changes to G.
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What is a transition mutation?
A type of point mutation where T changes to C or A changes to G.
What are the three excision repair systems?
Base excision repair, nonhomologous end joining, and homologous recombination.
Which specialized DNA polymerase is associated with Apurinic lyase?
DNA polymerase β.
What is the role of the MSH2-MSH6 protein complex?
It binds to a mispaired segment of DNA and distinguishes between the template and the newly synthesized daughter strand.
What does the DNA-protein complex bind to during mismatch repair?
An endonuclease that cuts the newly synthesized daughter strand.
How many depurination events occur per day?
18,000 depurination events.
What are spontaneous point mutations?
Mutations that change a single base or base pair.
What is the role of DNA glycosylase specific for G·T mismatches?
It flips the thymine base out of the helix and cuts it off the sugar-phosphate DNA backbone.
What role does DNA helicase play in mismatch repair?
It unwinds the DNA helix.
In which type of cells does mismatch excision repair occur?
Human cells.
What is associated with defects in DNA repair?
Cancers.
What is the product of the oxidation of guanine (G)?
8-oxoG.
Which proteins bind to the unwound DNA during nucleotide excision repair?
XP-G and RPA proteins.
What happens to the damaged DNA fragment after it is cut out?
It is degraded to mononucleotides.
How many cytosine deamination events occur per day?
100 cytosine deamination events.
What is the three-dimensional structure of DNA polymerases compared to?
A half-opened right hand.
What is deamination?
A process that can lead to DNA mutations.
What is the role of Apurinic endonuclease I (APE1) in DNA repair?
APE1 cuts the DNA backbone at the abasic site.
What is the function of DNA ligase in base excision repair?
DNA ligase links the inserted base to the backbone.
What type of DNA damage does the XP-C protein complex recognize?
Double-helix distortion caused by T-T dimers.
What does nucleotide excision repair fix?
DNA regions containing chemically modified bases (chemical adducts) that distort the normal shape of DNA.
What causes sensitivity to UV in relation to nucleotide excision repair?
Mutations in any of at least seven different genes, called XP-A through XP-G.
What is formed as a result of the action of XP-G and RPA proteins?
A bubble of about 25 bases in the DNA.
What is the function of the exonuclease in mismatch repair?
It removes several nucleotides from the cut end of the daughter strand, including the mismatched base.
What type of mismatch is repaired in the described base excision repair process?
A T·G mismatch.
What is formed at the apurinic site after depurination?
An apurinic site.
What do high-fidelity DNA excision-repair systems do?
They recognize and repair damage in DNA.
What is left after the DNA glycosylase cuts the thymine base from the G·T mismatch?
Just the deoxyribose phosphate.
What role does DNA Pol β play in base excision repair?
DNA Pol β inserts the single base.
What is the role of the XP-C protein complex in nucleotide excision repair?
It recruits the transcription factor TFIIH.
What can genetic instability lead to?
Increased risk of cancer and other diseases.
What causes a T·G mismatch?
Deamination of C to U or 5-methyl C to T.
How does genetic instability affect cell behavior?
It can lead to uncontrolled cell growth and tumor formation.
What type of DNA repair mechanism is being described?
Nucleotide excision repair.
What role does DNA polymerase play in proofreading?
It repairs damage and copying errors.
What is a frameshift mutation?
Mutations caused by insertions or deletions that alter the reading frame of the gene.
What are nonsense mutations?
Mutations that create a premature stop codon in the protein sequence.
What does the mismatch excision repair pathway correct?
Errors introduced during replication.
What is a common cause of spontaneous DNA damage?
Replication errors.
How is an incorrect base usually corrected during DNA replication?
By the 3’ to 5’ exonuclease activity (proofreading) of DNA Polymerase.
What triggers the binding of MLH1 and PMS2 in mismatch repair?
The binding of MSH2 - MSH6.
What is the role of XP-F and XP-G in nucleotide excision repair?
They act as endonucleases to cut out the damaged strand.
What is the purpose of the repair system regarding T·G mismatches?
To remove the mismatched T and replace it with a C.
What type of repair is used for a T·G mismatch?
Base excision repair.
What type of mismatch does the DNA glycosylase specifically mentioned repair?
T·G mismatches.
What is the consequence of pyrimidine dimers on cell replication?
They halt cell replication, potentially leading to cell death.
What type of bonds are formed between adjacent thymines due to UV irradiation?
C – C bonds, resulting in thymine-thymine dimers.
What are genotoxic chemical byproducts?
Byproducts of metabolism that can cause DNA mutations.
How many proteins are involved in the nucleotide excision repair process?
Approximately 30 proteins.
What are the causes of DNA sequence changes?
Copying errors and the effects of various physical and chemical agents.
What is the role of Apurinic lyase (AP lyase) in DNA repair?
It removes the deoxyribose phosphate.
What type of bond is hydrolyzed during depurination?
N-glycosidic bond.
What function do the helicase subunits of TFIIH perform?
They partially unwind the double helix using ATP.
What type of DNA damage can lead to deletions?
Spontaneous DNA damage.
What type of mutations are transversions?
Mutations where T <-> G/A and A <-> C/T.
How many different XP genes are associated with nucleotide excision repair?
At least seven different genes (XP-A through XP-G).
What can cause DNA mutations?
DNA damage, which is a change in the normal DNA sequence.
What happens when an incorrect base is added at the 3ʹ end?
It causes melting of the newly formed end and polymerase pausing.
What are exogenous causes of mutations?
External factors like carcinogens and radiation.
What type of mismatch does base excision repair address in the context provided?
A T·G mismatch.
What is the role of the XP-C protein complex?
It recognizes double-helix distortion, such as that caused by a T-T dimer.
What is the mechanism involved in repairing T-T dimers in human cells?
Nucleotide excision repair.
What is unclear about strand distinction in eukaryotes?
It is unclear how the MSH2-MSH6 complex distinguishes between strands, possibly related to the status of the 3’ end of the daughter strand.
What is a missense mutation?
A point mutation that changes the protein amino acid sequence.
What is a common cause of spontaneous DNA damage mentioned?
Oxidation.
What type of mutations does deamination lead to?
Point mutations.
What part of DNA polymerase binds the single-stranded segment of the template strand?
The 'fingers'.
What is depurination?
A type of DNA damage that can result in mutations.
What fills the gap during nucleotide excision repair?
DNA polymerase.
What happens when an incorrect base is added during DNA replication?
It affects base pairing during the next round of DNA replication.
What is genetic instability?
A condition where the genetic information of a cell is prone to mutations and alterations.
Which bases are primarily affected by depurination?
Adenine and guanine.
Which base can 8-oxoG base pair with?
Adenine (A).
What is the role of the XP genes in nucleotide excision repair?
They are components of the nucleotide excision-repair system that, when mutated, cause sensitivity to UV.
What often happens to dimers involving Cytosine?
They often lead to deamination.
What are synonymous mutations?
Mutations that do not change the amino acid sequence of a protein.
What type of repair mechanism is involved in correcting mismatches in DNA?
Mismatch excision repair.
What type of mismatch does base excision repair address in this context?
A T·G mismatch.
How does the MSH2-MSH6 complex distinguish between the template and daughter strand in prokaryotes?
The daughter strand is transiently uniquely methylated.
What is a nonsense mutation?
A point mutation that introduces a stop codon.
What is a specific example of spontaneous DNA damage?
Depurination.
What happens if deaminated cytosine (C) is not restored?
The normal C·G base pair will become a T·A mutation during replication.
Which type of dimers usually get corrected?
T - T dimers.
What is the difference between coding and non-coding regions of the genome?
Coding regions contain sequences that are translated into proteins, while non-coding regions do not.
What can lead to copy errors in DNA?
Mistakes during DNA replication.
What environmental factor increases cancer risk in individuals with xeroderma pigmentosum?
UV rays from sunlight.
What are common causes of genetic instability?
Errors in DNA replication, damage from environmental factors, and defects in DNA repair mechanisms.
What is a frequent cause of point mutations?
Deamination of 5-Methylcytosine (C) to thymine (T).
Are there other types of glycosylases?
Yes, other glycosylases are specific for different base modifications.
What effect do pyrimidine dimers have on the DNA helix?
They generate a kink in the helix.
What is one endogenous cause of mutations?
Spontaneous cleavage of bonds.
What is the function of DNA ligase in DNA repair?
It connects the new section of DNA to the backbone.
What enzyme seals the remaining nick after DNA polymerase fills the gap?
DNA ligase.
How far apart do XP-F and XP-G cut the damaged strand?
24 – 32 bases apart on each side of the lesion.
What role do DNA repair mechanisms play in genetic stability?
They correct errors in DNA to maintain genetic integrity.
What are point mutations?
Mutations that involve a change in a single nucleotide.
Where does the polymerase catalytic activity lie?
In the junction between the fingers and the palm.
What happens to the 3ʹ end after the mispaired base is removed?
It flips back into the polymerase site and elongation resumes.
What hereditary disease is associated with defects in DNA repair?
Xeroderma pigmentosum.
What is a silent mutation?
A point mutation that may not change the amino acid sequence, often occurring in the third codon position.
How can a gene regulatory region point mutation affect a gene?
It can change gene expression.
What type of DNA damage is caused by UV radiation?
Pyrimidine dimers.
What are transversion mutations?
Point mutations where T changes to G/A, A changes to C/T, or C changes to A/G, and G changes to C/T.
What occurs at the 3ʹ → 5ʹ exonuclease site?
The mispaired base and probably other bases are removed.
What role do carcinogens play in mutations?
They are substances that can cause DNA mutations.
What role does DNA polymerase (Pol δ) play in DNA repair?
It fills the gap in the DNA strand.
What mutations predispose individuals to nonpolyposis colorectal cancer?
Inheritable loss-of-function mutations in MSH2 or MLH1.
What are missense mutations?
Mutations that result in a change in one amino acid in the protein sequence.
How do ultraviolet and ionizing radiation contribute to mutations?
They can cause DNA damage leading to mutations.
What types of skin cancers are individuals with xeroderma pigmentosum prone to develop?
Melanomas and squamous cell carcinomas.
