40 Questions
What is the primary cause of trisomy 18?
Presence of three copies of chromosome #18
What is the primary function of DNA repair mechanisms?
To remove or tolerate DNA damage
What is the consequence of mutagenesis in living organisms?
Contributing to cancer, human diseases, and aging
What is the role of DNA polymerases in DNA replication and repair?
To burden cells with potentially disadvantageous mutations
What is the response of cells to DNA damage?
Induction of DNA damage response pathways
What are the two active DNA repair pathways?
Homologous Recombination and Non-Homologous End Joining
Why is preserving genomic sequence information important in living organisms?
To ensure the perpetuation of life
What is the inherent property of DNA that makes it susceptible to chemical modifications?
Its highly reactive nature
What is the primary mechanism of UV-A radiation in causing DNA damage?
Excitation of endogenous and exogenous photosensitizers
Which of the following is NOT a type of UV lesion repair?
Spontaneous deamination
What is the result of pyrimidine dimer formation on the DNA helix?
It distorts the helix, requiring TLS polymerases for replication
Which type of UV radiation is most efficient in causing the formation of pyrimidine dimers?
UV-C
What is the primary function of TLS polymerases?
To replicate past UV lesions during DNA replication
Which of the following is a characteristic mutation resulting from pyrimidine dimers?
CC→TT tandem transition
What is the result of UV-B radiation on DNA?
It causes the formation of pyrimidine dimers, but less efficiently than UV-C
Which of the following is the pathway of choice for removing bulky lesions such as CPDs and (6 – 4)PP from UV radiation?
Nucleotide excision repair (NER)
What is the significance of the constant number of chromosomes in a particular species?
It determines the phylogeny and taxonomy of the species
What is the function of chromosomes in the process of mitosis?
To ensure successful division of cells
Which of the following chromosomes have a severely off-centre centromere?
Acrocentric chromosomes
What is the role of chromosomes in the storage of genetic material?
They store genetic material in the form of DNA
Which of the following chromosomes is not found in humans?
Telocentric chromosomes
What determines the sex of a child?
The chromosome passed down by the male
Which of the following is a function of the genetic code stored in chromosomes?
To code for specific proteins required by the cell
What is the significance of chromosomes in the formation of proteins?
They direct the sequence of proteins formed
What is the primary mechanism by which DNA repair occurs when one of the double helix strands is damaged?
Removing the damaged DNA on one strand and replacing it with the synthesis of new nucleotides using the non-defective strand as a template
What type of repair is responsible for removing small, non-helix-distorting base lesions from the genome?
Base-excision repair
What is the consequence of not repairing damaged bases in DNA?
Mutations in the DNA
What type of enzymes are involved in the recognition and removal of a single damaged base?
Glycosylases
What is the result of not processing lesions by BER?
Mutations in the DNA
What is an example of a base lesion that can be repaired by BER?
Single-base alkylation
What is the name of the site that is formed after the damaged base is removed by glycosylases?
AP site
What are the two types of processing that can occur after the single-strand break is formed in BER?
Short-patch and long-patch BER
What is the effect of C8-guanine lesion on DNA when it is in the internal conformation?
It completely alters the geometry and acts as a mutagenic substrate
Which of the following is a common source of polycyclic aromatic hydrocarbons?
All of the above
What is the role of the P-450 system in the activation of PAHs?
It generates reactive intermediates that react with DNA
What is the effect of aflatoxin B1 on the glycosidic bond?
It weakens the glycosidic bond resulting in depurination
What is the reactive intermediate formed after the metabolism of aflatoxin B1 by the P-450 complex?
Aflatoxin B1-8,9-epoxide
What is the product formed after the reaction of aflatoxin B1-8,9-epoxide with N7 of guanine?
8,9-dihydro-8-(N7-guanyl)-9-hydroaflatoxin B1
What is the primary mechanism by which TLS polymerases bypass C8-guanine lesions in the external conformation?
They effectively bypass the lesions due to minimal disturbance to Watson-Crick base pairing
Which of the following is a characteristic of polycyclic aromatic hydrocarbons?
They are generally inert, nonpolar and widely distributed
Study Notes
Chromosome Structure and Classification
- Chromosomes can be classified into four types based on centromere position:
- Metacentric: centromere is in the middle, with p and q arms of comparable length (e.g., chromosomes 1, 3, 16, 19, 20)
- Submetacentric: centromere is off-center, with a shorter p arm relative to the q arm (e.g., chromosomes 2, 4-12, 17, 18, X)
- Acrocentric: centromere is severely off-center, with a much shorter p arm (e.g., chromosomes 13-15, 21, 22, Y)
- Telocentric: centromere is at the end of the chromosome, with no p arm (not found in humans)
Function and Significance of Chromosomes
- Chromosomes contain the genetic material required for an organism to develop and grow
- The number of chromosomes is constant for a particular species, making them important for determining phylogeny and taxonomy
- Chromosomes play a role in:
- Genetic code storage: DNA molecules contain genes that code for specific proteins
- Sex determination: humans have 23 pairs of chromosomes, including sex chromosomes (XX in females, XY in males)
- Control of cell division: chromosomes ensure correct information is passed on to daughter cells during mitosis
- Formation of proteins and storage: chromosomes direct protein sequences and maintain DNA order
Chromosomal Abnormalities
- Trisomy: having three copies of a chromosome instead of the usual two (e.g., Trisomy 18, Trisomy 21, Trisomy 13)
- Trisomy can result in birth anomalies, delayed development, and intellectual disabilities
DNA Damage, Repair, and Mutagenesis
- DNA is susceptible to damage from various agents, including UV radiation, chemicals, and biological agents
- DNA repair mechanisms include:
- Direct reversal of UV-damaged bases
- Nucleotide excision repair (NER)
- Interstrand crosslink (ICL) repair
- Trans lesion synthesis
- Homologous recombination (HR)
- Mutagenesis plays a role in maintaining and evolving genomic sequence information, but can also contribute to cancer, human diseases, and aging
DNA Damage Response
- Cells respond to DNA damage by instigating DNA damage response (DDR) pathways, which allow for repair or tolerance of damage
- Homologous recombination (HR) and non-homologous end joining (NHEJ) are active throughout the cell cycle, allowing for repair of DNA damage
Photoproducts and UV Radiation
- UV radiation can cause DNA damage, leading to the formation of photoproducts (e.g., cyclobutane pyrimidine dimers, pyrimidine hydrate, thymine glycols)
- UV-C is the most effective at producing photoproducts, while UV-A and UV-B also cause DNA damage
- Photoproducts can be repaired by various mechanisms, including NER and trans lesion synthesis
Polycyclic Aromatic Hydrocarbons (PAHs)
- PAHs are carbon compounds with two or more aromatic rings, known to be carcinogenic and widely distributed in the environment
- PAHs depend on the P-450 system to generate reactive intermediates that react with DNA
Toxins
- Natural toxins, such as aflatoxins, are genotoxic and carcinogenic compounds produced by microorganisms or fungi
- Aflatoxins can cause DNA damage by forming adducts with guanine, leading to depurination
DNA Repair by Excision
- Excision is the general mechanism by which repairs are made when one of the double helix strands is damaged
- There are three types of excision repair:
- Base-excision repair (BER)
- Nucleotide excision repair (NER)
- Mismatch repair
- Base-excision repair involves the recognition and removal of a single damaged base, with the non-defective strand serving as a template for repair
Learn about the four different classes of chromosomes based on the position of the centromere, including metacentric, submetacentric, acrocentric, and telocentric chromosomes.
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