42 Questions
What percentage of mutations accumulate as permanent mutations in the human DNA genome?
Less than 0.02%
What is the consequence of deamination of cytosine?
It leads to the removal of an amino group from the nucleotide, causing a G to A mutation.
What is the primary reason behind the importance of genetic stability?
To ensure the accurate replication of DNA
What is the name of the process by which purine bases such as adenine and guanine are lost from the DNA molecule?
Depurination
What is the term for agents that cause mutations?
Mutagens
What is the consequence of defective DNA repair in humans?
Increased risk of diseases such as breast and ovarian cancer
What is the term for the process by which an amino group is removed from a nucleotide?
Deamination
What is the frequency of mutations repaired every day in the human body?
Tens of thousands
What type of radiations are classified as physical mutagens?
X- and gamma-rays
What is the result of ionizing radiations interacting with DNA?
Production of reactive ions
What is the consequence of diminished capacity for DNA repair in humans?
Increased chances of cancer
What is the function of DNA repair?
To correct errors in DNA
What is the foundation for DNA repair?
DNA double helix
What is stored in each strand of the DNA double helix?
A copy of the genetic information
What is the effect of high-dose ionizing radiations on DNA?
Both single-strand and double-strand breaks
What is the characteristic of DNA repair?
High efficiency
What is the primary purpose of homologous recombination?
To repair damaged DNA
During which stage of the cell cycle does homologous recombination typically occur?
S phase
What is the result of nuclease digestion during homologous recombination?
Release of single-strand 3' ends
What is the function of the invading strand during strand exchange/invasion?
To anneal to the complementary region
What is the outcome of strand displacement during homologous recombination?
Release of the invading strand
Why does homologous recombination facilitate chromosomal crossover?
Because it increases genetic diversity
What type of chromosomes can result from aberrant homologous recombination?
Chromosomes with two centromeres or no centromeres
What is the role of DNA polymerase during homologous recombination?
To synthesize new DNA using the template duplex
What is the primary mechanism by which 5-bromouracil (5-BrU) induces mutations in DNA replication?
By replacing thymine and pairing with guanine or adenine
What is the result of nitrous acid (HNO2) reacting with cytosine and adenine?
Conversion of cytosine to uracil and adenine to hypoxanthine
What type of radiation is responsible for the formation of TT dimers?
Electromagnetic radiations, such as UV radiation
What is the result of an unrepaired TT dimer?
It will be read normally, with no mutations
What is the primary effect of ethidium bromide (EB) on DNA?
It inserts between the bases, causing additions or deletions
What is the result of a CC dimer being misread?
It shows up in the p53 tumor suppressor gene, leading to skin cancers
What type of mutation is caused by 5-bromouracil (5-BrU)?
Transition mutation, leading to a change from A:T to G:C
What is the primary effect of physical mutagens, such as UV radiation, on DNA?
It forms TT dimers that initiate nucleotide excision repair
What is the primary mechanism by which nitrous acid (HNO2) induces mutations?
By converting cytosine to uracil and adenine to hypoxanthine
What is the result of an unrepaired CC dimer?
It will be read as a TT dimer, leading to mutations
What is the primary function of Base-Excision Repair?
To replace chemically modified bases
What is the outcome of the four-step process in Base-Excision Repair?
The replacement of the altered base with a new one
What type of DNA damage is caused by UV radiation?
Thymine dimers
What is the role of AP endonuclease and Phosphodiesterase in Base-Excision Repair?
To remove the remainder of the nucleotide
What is the characteristic of the type of DNA damage recognized by Nucleotide-Excision Repair?
It is a general distortion caused by bulky lesions
What is the outcome of the process in Nucleotide-Excision Repair?
The restoration of the original DNA sequence
What is the number of enzymes involved in Nucleotide-Excision Repair in eukaryotes?
25+
What is the result of the action of Glycosylase in Base-Excision Repair?
The removal of the altered base
Study Notes
DNA Damage and Repair Mechanisms
- Maintaining genetic stability is crucial for life, and it involves extremely accurate mechanisms for DNA replication and highly efficient DNA repair.
- Tens of thousands of mutations occur every day, but less than 0.02% of them accumulate as permanent mutations in the human DNA genome.
Sources of DNA Damage
- DNA damage can be endogenous/spontaneous, arising from natural errors during DNA replication, or exogenous/induced, resulting from exposure to mutagens.
- Endogenous DNA damage can lead to depurination, deamination, and oxidative damage, which can cause mutations if not repaired.
Exogenous DNA Damage
- Exogenous DNA damage can be caused by chemical or physical agents that alter the primary sequence of DNA or promote errors in replication or repair.
- There are three classes of chemical mutagens: nucleotide-base analogs, frameshift mutagens, and DNA-reactive chemicals.
Chemical Mutagens
- Nucleotide-base analogs, such as 5-bromouracil (5-BrU), can replace thymine and cause A:T to G:C mutations.
- Frameshift mutagens, such as ethidium bromide (EB), can insert between DNA bases and cause additions or deletions.
- DNA-reactive chemicals, such as nitrous acid (HNO2), can change the chemical structure of base pairs and cause mutations.
Physical Mutagens
- Radiation was the first known mutagen, reported in the 1920s.
- There are two types of radiation: electromagnetic radiation (UV) and ionizing radiation (X- and gamma-rays).
- UV radiation can cause pyrimidine dimers, while ionizing radiation can produce reactive ions that damage DNA.
DNA Repair
- DNA repair is the process of correcting errors in DNA, and it involves the interplay between various substrates, enzymes, and co-factors.
- DNA repair has a remarkable efficiency, and diminished capacity for DNA repair has been linked to many human diseases.
Base Excision Repair
- Base excision repair is a four-step process that replaces chemically modified bases.
- The process involves glycosylase, AP endonuclease, phosphodiesterase, polymerization, and ligation.
Nucleotide Excision Repair
- Nucleotide excision repair recognizes general distortions caused by bulky chemical adducts.
- The process involves the recognition of bulky lesions, excision of the defect, and restoration of the original DNA sequence.
- In eukaryotes, this process involves more than 25 enzymes.
Homologous Recombination
- Homologous recombination is a process that exchanges DNA strands between two homologous duplex DNA molecules.
- It occurs during or right after DNA replication, facilitating chromosomal crossover and accurately repairing double-strand breaks.
- The process involves nuclease digestion, strand exchange, polymerase synthesis, and strand displacement.
This quiz covers the sources and types of DNA damage, consequences of different forms of DNA damage, and the basic steps of different DNA repair systems. It also identifies diseases associated with DNA repair mechanisms.
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