Molecular Biology and Genomics Practice Questions

Q: If a polymerase chain reaction (PCR) is performed for three cycles on a single DNA molecule, how many copies of the DNA will be produced?

8 copies. ***8 copies*** - Each cycle of **PCR** theoretically doubles the number of DNA molecules present. - Starting with one DNA molecule, after 3 cycles, the number of copies will be 2^3 = **8 copies**. *2 copies* - This would be the result after only **one cycle** of PCR, not three cycles. - PCR involves an exponential amplification process. *3 copies* - This number does not follow the **exponential amplification** pattern characteristic of PCR. - PCR is not a simple additive process. *4 copies* - This would be the result after **two cycles** of PCR (2^2 = 4), not three cycles. - Each subsequent cycle doubles the product from the previous cycle.

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: Xeroderma pigmentosum is caused by a group of closely related abnormalities in.

Nucleotide excision repair mechanism. ***Nucleotide excision repair*** - Xeroderma pigmentosum results from defects in **nucleotide excision repair (NER)**, crucial for repairing DNA damage caused by UV light [1]. - Patients are highly sensitive to **sunlight**, presenting with skin changes like pigmentation and increased risk of skin cancers [1]. - UV radiation causes cross-linking of pyrimidine residues, preventing normal DNA replication, and several proteins are involved in nucleotide excision repair [1]. - With excessive sun exposure, the capacity of the nucleotide excision repair pathway can be overwhelmed, leading to error-prone repair mechanisms that result in genomic mutations [2]. *SOS repair* - SOS repair is a **bacterial response** to severe DNA damage and not related to xeroderma pigmentosum. - It is not involved in the repair processes in human cells, particularly in response to UV damage. *Base excision repair* - Base excision repair primarily addresses minor **DNA base damage**, not the bulky lesions caused by UV exposure seen in xeroderma pigmentosum. - The molecular mechanics differ, and mutations in base excision repair do not lead to the symptoms characteristic of this condition. *Mismatch repair* - Mismatch repair corrects errors that occur during DNA replication, such as base pair mismatches, which are unrelated to UV-induced damage. - Deficiencies in this pathway lead to **Lynch syndrome**, not xeroderma pigmentosum. **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: 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 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: 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: 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.

Question 1

If a polymerase chain reaction (PCR) is performed for three cycles on a single DNA molecule, how many copies of the DNA will be produced?

Accepted Answer

8 copies

Answer

2 copies

Answer

3 copies

Answer

4 copies

Question 2

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 3

Xeroderma pigmentosum is caused by a group of closely related abnormalities in.

Accepted Answer

Nucleotide excision repair mechanism

Answer

SOS repair mechanism

Answer

Mismatch repair mechanism

Answer

Base excision repair mechanism

Question 4

What term describes the external manifestations of the genome?

Accepted Answer

Phenotype

Answer

Genotype

Answer

Allele

Answer

Polymorphism

Question 5

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 6

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 7

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 8

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 9

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 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

On this page

Practice by Chapter

Want unlimited practice?