Molecular Biology and Genomics Practice Questions

Q: Which of the following is the origin of the mitochondrial DNA of all human adult cells?

Maternal only. ***Maternal only*** - **Mitochondria** and their DNA are almost exclusively inherited from the **mother's ovum**. - During fertilization, the sperm contributes only its nucleus to the zygote, while the ovum provides the bulk of the cytoplasm, including mitochondria. *Paternal only* - The **sperm's mitochondria** are typically found in its tail and are usually **destroyed** or degrade after entering the egg, or are actively excluded. - This prevents paternal mitochondrial DNA from being passed on to the offspring. *A combination of paternal and maternal DNA is inherited* - While nuclear DNA is a combination of paternal and maternal contributions, **mitochondrial DNA** inheritance is **uniparental** (from one parent only). - This is a fundamental aspect of human genetics and ancestry tracking. *Either paternal or maternal DNA is inherited* - This option incorrectly suggests variability, as **maternal inheritance** of mitochondrial DNA is the virtually universal rule in humans. - Rare exceptions of paternal mitochondrial DNA inheritance have been reported but are considered **anomalous** and not typical.

Q: What term describes the external manifestations of the genome?

Phenotype. ***Correct: Phenotype*** - The **phenotype** refers to the **observable characteristics** or traits of an organism, which result from the interaction of its genotype with the environment. - These external manifestations can include physical appearances (e.g., eye color), physiological traits (e.g., blood pressure), and even measurable behaviors. - This is the classic definition of phenotype - the external/outward expression of the genome. *Incorrect: Genotype* - The **genotype** describes the **genetic makeup** of an individual, specifically the set of alleles possessed for a particular gene or genes. - It represents the internal genetic code, not its outward expression. - Genotype is the "blueprint," while phenotype is the "building." *Incorrect: Allele* - An **allele** is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. - It is a specific variation of a gene, not the observable manifestation of the entire genome. - Alleles are components of the genotype. *Incorrect: Polymorphism* - A **polymorphism** is a common variation in the DNA sequence among individuals in a population. - It refers to differences in the genetic code itself, not the resulting physical or functional traits. - Polymorphisms contribute to genetic diversity at the DNA level.

Q: Xeroderma pigmentosum is due to:

Nucleotide excision defect. ***Nucleotide excision defect*** - **Xeroderma pigmentosum (XP)** is a genetic disorder characterized by a defect in the **nucleotide excision repair (NER)** pathway. - This pathway is crucial for repairing **DNA damage**, particularly **pyrimidine dimers** caused by **ultraviolet (UV) radiation**. *Base excision repair defect* - **Base excision repair (BER)** is a distinct DNA repair pathway that primarily deals with **small, non-helix-distorting base lesions** and single-strand breaks. - Defects in BER are associated with different conditions, such as some forms of **cancer susceptibility** but not XP. *Cross-linking repair defect* - **Cross-linking repair** mechanisms address formations where two strands of DNA are covalently linked, or DNA is linked to proteins, often by agents like **alkylating agents**. - Disorders like **Fanconi anemia** are associated with defects in **DNA interstrand crosslink repair**, which is different from the UV-induced damage seen in XP. *SOS repair mechanism defect* - The **SOS response** is a global bacterial mechanism dealing with extensive DNA damage, primarily involving the induction of various DNA repair enzymes. - There isn't a direct human disease called "SOS repair mechanism defect" that is analogous to XP; human repair mechanisms are more complex and distinct from the bacterial SOS response.

Q: Which of the following techniques can be used to detect single base pair substitutions?

PCR. ***PCR (with sequencing or allele-specific methods)*** - **PCR-based techniques** are the most versatile methods for detecting single base pair substitutions (point mutations) - **Allele-specific PCR** can directly detect known point mutations by using primers specific to mutant or wild-type alleles - **PCR followed by Sanger sequencing** is the gold standard for identifying any single base pair substitution - **High-resolution melting (HRM) analysis** after PCR can detect mutations based on melting curve differences - PCR amplification is the foundation that enables these detection methods *FISH (Fluorescence in situ hybridization)* - FISH detects **large chromosomal abnormalities** such as aneuploidy, translocations, large deletions, and duplications - It visualizes chromosomal-level changes using fluorescent probes - **Not sensitive enough** to detect single base pair changes, as these are too small to visualize cytogenetically *Southern blot* - Southern blot detects **large DNA rearrangements**, insertions, deletions, or copy number variations - Analyzes restriction enzyme fragments separated by gel electrophoresis - **Generally cannot detect** single base pair substitutions unless they create or abolish a restriction enzyme recognition site - Even when applicable, PCR-based methods are more efficient and sensitive *Restriction Fragment Length Polymorphism (RFLP)* - RFLP can detect single base pair substitutions **only if** they create or abolish a **restriction enzyme recognition site** - Classic example: **Sickle cell mutation** (GAG→GTG in β-globin gene) abolishes an MstII restriction site - **Limited applicability** - can only detect the subset of point mutations that affect restriction sites - PCR-based methods are preferred as they can detect **any** single base pair substitution, not just those affecting restriction sites

Q: Which of the following statements about polymerase chain reaction (PCR) is true?

It is a method for enzymatic amplification of DNA.. ***It is a method for enzymatic amplification of DNA.*** - PCR is a laboratory technique used to make **millions to billions of copies** of a specific DNA segment. - This amplification is carried out enzymatically by **DNA polymerase**, which synthesizes new DNA strands. *It is a method for recombinant DNA synthesis.* - **Recombinant DNA synthesis** involves combining DNA from different sources, often for genetic engineering, which is a broader application than the core function of PCR. - While PCR can be a *tool* used within recombinant DNA technology (e.g., to amplify a gene for cloning), its primary definition is not synthesizing recombinant DNA itself. *It is used for the separation of protein fragments.* - The separation of protein fragments is typically achieved through techniques like **electrophoresis** (e.g., SDS-PAGE), which separates proteins based on size and charge. - PCR is specifically designed for manipulating and amplifying **DNA**, not proteins. *None of the options.* - This option is incorrect because the statement "It is a method for enzymatic amplification of DNA" accurately describes the fundamental function of PCR.

Q: What is the technique for accurate quantification of gene expression?

Real-Time Reverse Transcriptase PCR. ***Real-Time Reverse Transcriptase PCR*** - This technique allows for the **quantification of gene expression** by concurrently reverse-transcribing RNA to cDNA and amplifying it while monitoring the accumulation of DNA in real-time using fluorescent reporters. - The ** threshold cycle (Ct) value** is inversely proportional to the initial amount of target mRNA, enabling precise quantification. *Northern blot* - This method is used to detect **RNA sequences** and can provide semi-quantitative data about gene expression levels based on band intensity. - However, it is generally **less sensitive** and provides less precise quantification compared to real-time PCR. *PCR* - **Standard PCR** amplifies DNA, but it is not directly used for gene expression quantification as it starts with DNA templates. - While it can be used to detect the presence of a gene, it does not quantify its expression without further modifications or additional steps like reverse transcription and real-time monitoring. *Reverse Transcriptase PCR* - This technique involves **reverse transcribing RNA into cDNA** and then performing standard PCR to amplify the cDNA. - While it confirms the presence of mRNA and allows for cDNA amplification, it is a **qualitative or semi-quantitative** method for expression, as the endpoint detection does not accurately reflect initial mRNA concentration due to plateau effects.

Q: Which of the following conditions is associated with a defect in DNA repair mechanisms?

Xeroderma pigmentosum. ***Xeroderma pigmentosum*** - It is a genetic disorder caused by defects in **DNA repair mechanisms**, specifically affecting the body's ability to repair UV-induced DNA damage [1]. - Patients are highly sensitive to **sun exposure**, leading to skin lesions, and have an increased risk of skin cancers due to the accumulation of unrepaired DNA damage [1,2]. *DiGeorge's syndrome* - Primarily characterized by **thymic hypoplasia** leading to immune deficiency and **hypoparathyroidism**, and does not directly involve DNA repair defects. - Associated with **chromosomal deletion (22q11.2)** but not a defect in DNA repair itself. *Icthyosis* - Refers to a group of skin disorders characterized by **thickened, dry skin**, which is not primarily related to defects in DNA repair mechanisms. - It can be inherited but lacks the association with **UV sensitivity** and skin cancer risk observed in xeroderma pigmentosum. *Angelman syndrome* - A neurodevelopmental disorder usually caused by a deletion of the **UBE3A gene** on chromosome 15, not related to DNA repair processes. - Characterized by **severe developmental delays and ataxia**, but does not involve defects in DNA repair mechanisms. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 322-323. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 332-333.

Q: Which of the following statements is true regarding DNA polymerases?

DNA polymerase I has significant repair activity.. ***DNA polymerase I has significant repair activity*** - DNA polymerase I plays a crucial role in **DNA repair**, including **excising RNA primers** and filling in the resulting gaps during replication. - Its **5' to 3' exonuclease activity** allows it to remove nucleotides ahead of its synthetic work, which is essential for correcting damage and removing primers. - This unique combination of activities makes DNA Pol I particularly important in **DNA repair mechanisms**. *Leading strand is synthesized by DNA polymerase I* - The **leading strand** in prokaryotes is primarily synthesized by **DNA polymerase III**, not DNA polymerase I. - DNA polymerase I is mainly involved in **primer removal** and filling gaps, not the bulk synthesis of new DNA strands. *Okazaki fragments are synthesized by DNA polymerase I* - **Okazaki fragments** are synthesized by **DNA polymerase III** in prokaryotes. - DNA polymerase I then replaces the RNA primers with DNA nucleotides within these fragments. *Only DNA polymerase III has proofreading activity* - This is **incorrect** - **DNA polymerases I, II, and III** all have **3' to 5' exonuclease proofreading activity**. - This proofreading function allows these enzymes to correct errors during DNA synthesis by removing incorrectly paired nucleotides.

Q: Which of the following statements about gene therapy is false?

Gene therapy is only used for genetic disorders.. ***Gene therapy is only used for genetic disorders.*** - This statement is **false** because gene therapy has applications beyond just genetic disorders. It is also being explored and used in the treatment of acquired diseases such as **cancer** and **infectious diseases**. - While it's a prominent approach for correcting genetic defects, its scope is much broader, involving the introduction or modification of genes to achieve a therapeutic effect in various conditions. *Gene also considered as drug* - This statement is **true**. Gene therapy products are often regulated as **drugs** or **biological products** by regulatory bodies like the FDA. - This is because they involve the delivery of genetic material that acts to modify gene expression or cell function to produce a therapeutic effect, similar to how traditional drugs work. *Has been tried in cystic fibrosis* - This statement is **true**. Gene therapy has been extensively investigated as a potential treatment for **cystic fibrosis (CF)**. - CF is caused by mutations in the **CFTR gene**, and researchers have attempted to deliver functional copies of this gene to the affected cells, particularly in the lungs, to correct the underlying defect. *Gene therapy can be used to treat some cancers.* - This statement is **true**. Gene therapy is an active area of research and treatment for various **cancers** [1]. - Approaches include introducing genes that make cancer cells more susceptible to chemotherapy, enhancing the immune system's ability to fight cancer, or directly killing cancer cells through gene delivery [1].

Question 1

Which of the following is the origin of the mitochondrial DNA of all human adult cells?

Accepted Answer

Maternal only

Answer

Paternal only

Answer

A combination of paternal and maternal DNA is inherited

Answer

Either paternal or maternal DNA is inherited

Question 2

What term describes the external manifestations of the genome?

Accepted Answer

Phenotype

Answer

Genotype

Answer

Allele

Answer

Polymorphism

Question 3

Xeroderma pigmentosum is due to:

Accepted Answer

Nucleotide excision defect

Answer

Base excision repair defect

Answer

Cross-linking repair defect

Answer

SOS repair mechanism defect

Question 4

Which of the following techniques can be used to detect single base pair substitutions?

Accepted Answer

PCR

Answer

FISH

Answer

Southern blot

Answer

Restriction Fragment Length Polymorphism (RFLP)

Question 5

Which of the following best defines genomic imprinting?

Accepted Answer

Different expression of a gene depending on the parent of origin.

Answer

Expression of genes silenced on one chromosome through random X-inactivation.

Answer

Parent-specific gene expression due to chromosomal deletions only.

Answer

Uniparental disomy is a condition where both alleles come from one parent.

Question 6

Which of the following statements about polymerase chain reaction (PCR) is true?

Accepted Answer

It is a method for enzymatic amplification of DNA.

Answer

It is used for the separation of protein fragments.

Answer

It is a method for recombinant DNA synthesis.

Answer

None of the options.

Question 7

What is the technique for accurate quantification of gene expression?

Accepted Answer

Real-Time Reverse Transcriptase PCR

Answer

PCR

Answer

Reverse Transcriptase PCR

Answer

Northern blot

Question 8

Which of the following conditions is associated with a defect in DNA repair mechanisms?

Accepted Answer

Xeroderma pigmentosum

Answer

Angelman syndrome

Answer

DiGeorge syndrome

Answer

Ichthyosis

Question 9

Which of the following statements is true regarding DNA polymerases?

Accepted Answer

DNA polymerase I has significant repair activity.

Answer

Leading strand is synthesized by DNA polymerase I.

Answer

Okazaki fragments are synthesized by DNA polymerase I.

Answer

Only DNA polymerase III has proofreading activity.

Question 10

Which of the following statements about gene therapy is false?

Accepted Answer

Gene therapy is only used for genetic disorders.

Answer

Gene also considered as drug

Answer

Gene therapy can be used to treat some cancers.

Answer

Has been tried in cystic fibrosis

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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