Chromosome Classification

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Questions and Answers

What is the primary cause of trisomy 18?

Presence of three copies of chromosome #18 (correct)

Inheritance from parents

DNA damage

DNA repair deficiency

What is the primary function of DNA repair mechanisms?

To regulate gene expression

To introduce mutations into the genome

To remove or tolerate DNA damage (correct)

To initiate apoptosis

What is the consequence of mutagenesis in living organisms?

Enhancing DNA repair mechanisms

Reducing genetic diversity

Preservation of genomic sequence information

Contributing to cancer, human diseases, and aging (correct)

What is the role of DNA polymerases in DNA replication and repair?

To burden cells with potentially disadvantageous mutations Signup and view all the answers

What is the response of cells to DNA damage?

Induction of DNA damage response pathways Signup and view all the answers

What are the two active DNA repair pathways?

Homologous Recombination and Non-Homologous End Joining Signup and view all the answers

Why is preserving genomic sequence information important in living organisms?

To ensure the perpetuation of life Signup and view all the answers

What is the inherent property of DNA that makes it susceptible to chemical modifications?

Its highly reactive nature Signup and view all the answers

What is the primary mechanism of UV-A radiation in causing DNA damage?

Excitation of endogenous and exogenous photosensitizers Signup and view all the answers

Which of the following is NOT a type of UV lesion repair?

Spontaneous deamination Signup and view all the answers

What is the result of pyrimidine dimer formation on the DNA helix?

It distorts the helix, requiring TLS polymerases for replication Signup and view all the answers

Which type of UV radiation is most efficient in causing the formation of pyrimidine dimers?

UV-C Signup and view all the answers

What is the primary function of TLS polymerases?

To replicate past UV lesions during DNA replication Signup and view all the answers

Which of the following is a characteristic mutation resulting from pyrimidine dimers?

CC→TT tandem transition Signup and view all the answers

What is the result of UV-B radiation on DNA?

It causes the formation of pyrimidine dimers, but less efficiently than UV-C Signup and view all the answers

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) Signup and view all the answers

What is the significance of the constant number of chromosomes in a particular species?

It determines the phylogeny and taxonomy of the species Signup and view all the answers

What is the function of chromosomes in the process of mitosis?

To ensure successful division of cells Signup and view all the answers

Which of the following chromosomes have a severely off-centre centromere?

Acrocentric chromosomes Signup and view all the answers

What is the role of chromosomes in the storage of genetic material?

They store genetic material in the form of DNA Signup and view all the answers

Which of the following chromosomes is not found in humans?

Telocentric chromosomes Signup and view all the answers

What determines the sex of a child?

The chromosome passed down by the male Signup and view all the answers

Which of the following is a function of the genetic code stored in chromosomes?

To code for specific proteins required by the cell Signup and view all the answers

What is the significance of chromosomes in the formation of proteins?

They direct the sequence of proteins formed Signup and view all the answers

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 Signup and view all the answers

What type of repair is responsible for removing small, non-helix-distorting base lesions from the genome?

Base-excision repair Signup and view all the answers

What is the consequence of not repairing damaged bases in DNA?

Mutations in the DNA Signup and view all the answers

What type of enzymes are involved in the recognition and removal of a single damaged base?

Glycosylases Signup and view all the answers

What is the result of not processing lesions by BER?

Mutations in the DNA Signup and view all the answers

What is an example of a base lesion that can be repaired by BER?

Single-base alkylation Signup and view all the answers

What is the name of the site that is formed after the damaged base is removed by glycosylases?

AP site Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

Which of the following is a common source of polycyclic aromatic hydrocarbons?

All of the above Signup and view all the answers

What is the role of the P-450 system in the activation of PAHs?

It generates reactive intermediates that react with DNA Signup and view all the answers

What is the effect of aflatoxin B1 on the glycosidic bond?

It weakens the glycosidic bond resulting in depurination Signup and view all the answers

What is the reactive intermediate formed after the metabolism of aflatoxin B1 by the P-450 complex?

Aflatoxin B1-8,9-epoxide Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

Which of the following is a characteristic of polycyclic aromatic hydrocarbons?

They are generally inert, nonpolar and widely distributed Signup and view all the answers

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

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Description

Learn about the four different classes of chromosomes based on the position of the centromere, including metacentric, submetacentric, acrocentric, and telocentric chromosomes.