Molecular Biology and Genomics Practice Questions

Q: A patient presents with skin cancer and hyperpigmentation that worsens with sunlight exposure. Which of the following DNA repair mechanisms is most likely defective in this condition?

Nucleotide excision repair. ***Nucleotide excision repair*** - This mechanism is responsible for repairing large, bulky DNA lesions, most notably **pyrimidine dimers** caused by **UV radiation** (sunlight). - A defect in NER is the underlying cause of **Xeroderma Pigmentosum**, which presents with severe photosensitivity, hyperpigmentation, and a significantly increased risk of **skin cancer** (melanoma and non-melanoma). *Base excision repair* - Primarily repairs **small, non-helix-distorting lesions** caused by spontaneous factors (like deamination) or oxidative damage, such as single base modifications. - Defective BER is implicated in hereditary cancers like MUTYH-associated polyposis (MAP), which is not associated with **UV-induced skin pathology**. *Non-homologous end joining (NHEJ)* - Responsible for repairing **double-strand breaks (DSBs)** in DNA, usually in the G0/G1 phase of the cell cycle. - While critical for genome stability, defects in NHEJ lead to severe immunodeficiency (due to impaired V(D)J recombination) and are not the primary cause of susceptibility to **solar-induced skin cancer**. *Mismatch repair* - Corrects errors (mismatched bases) incorporated during **DNA replication** that escape proofreading. - Defective MMR leads to a high frequency of microsatellite instability, characteristic of conditions like **Hereditary Non-polyposis Colorectal Cancer (HNPCC)** or Lynch syndrome, but not the primary cause of photosensitivity and UV-related skin cancer.

Q: A frameshift mutation occurs due to the insertion of a new nucleotide at the 4th position of an mRNA sequence with 900 nucleotides. What is the most likely outcome of this mutation?

Partial protein production. ***Partial protein production*** - The insertion of a single nucleotide at position 4 causes a **frameshift mutation**, which fundamentally alters the reading frame starting from the second codon. - Frameshift mutations typically lead to the introduction of a **Premature Termination Codon (PTC)** shortly downstream, resulting in the synthesis of a **truncated** (partial) and non-functional protein. *No change in the final protein* - A change from position 4 onward affects almost the entire coding sequence; thus, there cannot be **no change** in the protein structure. - Only an insertion/deletion of a multiple of three nucleotides (in-frame mutation) or a mutation late in the sequence might result in a preserved protein function. *Complete change in protein production* - While the subsequent mRNA sequence is entirely changed, the outcome on the protein level is usually **truncation** (partial protein), not the complete synthesis of a full-length, completely altered protein. - The term "complete change" is less accurate than "partial production," as the protein synthesis is typically **aborted prematurely**. *No protein production* - The **start codon** (positions 1-3) is upstream of the mutation site (position 4) and remains intact, allowing for **translation initiation** to occur normally. - Protein production starts; it is only terminated prematurely when a stop codon is encountered in the shifted reading frame.

Q: Which of the following correctly describes gene therapy ?

Introduction of gene sequence into a cell. ***Introduction of gene sequence into a cell*** - **Gene therapy** fundamentally involves the **delivery of genetic material** (DNA or RNA) into a patient's cells to treat or prevent disease. - The goal is to correct a **genetic defect**, provide a new therapeutic function, or alter gene expression for a desired outcome. *Mapping and isolation of gene sequence* - While essential steps in understanding and preparing for gene therapy, **mapping and isolation** alone do not constitute the therapy itself. - These processes are preliminary research activities to identify the target gene and its location. *Synthesis of DNA probes with specific sequence* - **DNA probes** are short, single-stranded DNA or RNA sequences used to detect the presence of complementary nucleic acid sequences. - Their synthesis is a technique used in **molecular diagnostics** and research, not gene therapy, which aims to introduce functional genes. *Use of polymerase chain reaction technique* - **PCR** is a molecular biology technique used to amplify a specific segment of DNA, creating many copies. - It is a tool for **diagnosis**, research, and sometimes in the preparation of genetic material for therapy, but it is not gene therapy itself.

Q: Which PCR technique is best suited for identifying a syndrome with multiple causative agents?

Multiplex PCR. ***Multiplex PCR*** - **Multiplex PCR** allows for the simultaneous amplification of **multiple DNA targets** in a single reaction, making it ideal for identifying syndromes with numerous potential causative agents. - This method uses **multiple primer pairs** in one reaction tube, each designed to amplify a specific target sequence, thus efficiently detecting various pathogens or genetic markers. *RT-PCR* - **Reverse Transcription PCR (RT-PCR)** is used to detect **RNA targets** by first converting RNA into cDNA, which is then amplified. - While useful for RNA viruses or gene expression studies, it is not primarily designed for simultaneous detection of multiple diverse causative agents in the same way as multiplex PCR. *Nested PCR* - **Nested PCR** uses two sets of primers in sequential reactions to **increase sensitivity and specificity** by reducing non-specific binding. - This technique is generally employed to detect very low copies of a specific target or to overcome issues with non-specific amplification, rather than for identifying multiple different agents concurrently. *Conventional PCR* - **Conventional PCR** amplifies a **single specific DNA target** using one pair of primers per reaction. [1] - It requires separate reactions for each potential causative agent, making it inefficient and labor-intensive when testing for a syndrome with multiple etiologies. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 56-57.

Q: A patient presents with a skin rash that is exaggerated on sun exposure. What is the repair mechanism involved in this condition?

Nucleotide excision repair. ***Nucleotide excision repair*** - This mechanism is responsible for repairing **bulky lesions** in DNA, such as **pyrimidine dimers** caused by **UV radiation** from sun exposure. - Patients with defects in nucleotide excision repair (e.g., **xeroderma pigmentosum**) are highly sensitive to sunlight and develop skin rashes, pigment changes, and skin cancers. *Base excision repair* - This pathway primarily corrects **small damaged bases** that do not cause significant distortion of the DNA helix, such as deaminated, oxidized, or alkylated bases. - It does not primarily address the bulky lesions induced by UV light that cause exaggerated sun sensitivity. *Mismatch repair* - This system corrects errors, like **mismatched base pairs**, that are incorporated during DNA replication. - It is not directly involved in repairing DNA damage caused by environmental factors like UV radiation. *Double stranded DNA break repair* - This mechanism repairs **double-strand breaks** in DNA, which are highly deleterious lesions caused by ionizing radiation or oxidative stress. - While critical for genome stability, it is not the primary repair pathway for UV-induced DNA lesions or the direct cause of sun sensitivity.

Q: Which of the following is used to detect abnormal gene sequences EXCEPT?

Flow cytometry. ***Flow cytometry*** - **Flow cytometry** is primarily used to analyze **cell populations** based on their physical and biochemical characteristics (e.g., size, granularity, and protein expression) by passing them single file through a laser beam, not for direct gene sequencing. - It detects and quantifies cells labeled with **fluorescent antibodies**, making it useful for immunophenotyping, cell sorting, and DNA content analysis, but not for identifying specific gene sequences or mutations. *RFLP analysis* - **Restriction fragment length polymorphism (RFLP) analysis** detects variations in **DNA sequences** by using **restriction enzymes** to cut DNA at specific sites. - Differences in fragment lengths indicate **polymorphisms** or **mutations** within the recognition sites, thereby identifying abnormal gene sequences. *Pyrosequencing* - **Pyrosequencing** is a method of **DNA sequencing** that determines the sequence of nucleotides by detecting the release of pyrophosphate during DNA synthesis. - It is used to identify **single nucleotide polymorphisms (SNPs)** and **short genetic variations**, making it suitable for detecting abnormal gene sequences. *FISH* - **Fluorescence in situ hybridization (FISH)** uses **fluorescently labeled DNA probes** that bind to specific complementary **DNA sequences** on chromosomes. - It is a powerful cytogenetic technique for detecting **chromosomal abnormalities**, such as deletions, translocations, and amplifications, thereby identifying abnormal gene sequences.

Q: DNA sequence is determined by?

Sanger sequencing. ***Correct: Sanger sequencing*** - **Sanger sequencing** (chain-termination method) is the gold standard technique used to determine the exact order of nucleotides within a DNA molecule - It uses dideoxynucleotides (ddNTPs) to terminate DNA strand elongation at specific bases, producing fragments of varying lengths - These fragments are separated by capillary electrophoresis and the sequence is read based on the terminal fluorescent label - Directly determines DNA sequence with high accuracy *Incorrect: PCR* - **Polymerase Chain Reaction (PCR)** amplifies specific DNA segments to create millions of copies - It does NOT determine the sequence itself - it only makes copies of DNA - PCR-amplified DNA can be used as a template for subsequent sequencing, but PCR itself doesn't reveal sequence information *Incorrect: FISH* - **Fluorescence in situ hybridization (FISH)** detects and localizes specific DNA sequences on chromosomes - Used for chromosomal mapping and detecting chromosomal abnormalities - Does not determine the nucleotide sequence *Incorrect: Gel electrophoresis* - Separates DNA fragments based on size and charge - Used to analyze DNA but cannot determine the specific nucleotide sequence - Useful for visualizing DNA after amplification or restriction digestion

Q: miRNA binds to which part of the mRNA to inhibit translation?

3'UTR. ***3'UTR*** - MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression. - They primarily bind to the **3' untranslated region (3'UTR)** of messenger RNA (mRNA) molecules, leading to translational repression or mRNA degradation. *Gene promoter* - The **gene promoter** is a region of DNA located upstream of a gene, where regulatory proteins bind to initiate transcription. - miRNAs do not directly bind to gene promoters to inhibit translation. *Gene body* - The **gene body** refers to the entire transcribed region of a gene, including exons and introns. - While some regulatory elements can be found within the gene body, the primary binding site for miRNAs to exert translational control is the 3'UTR. *5'UTR* - The **5' untranslated region (5'UTR)** is located at the 5' end of an mRNA molecule, upstream of the start codon. - While the 5'UTR can play a role in regulating translation initiation, it is not the primary target for miRNA binding to inhibit translation.

Q: What is the repair mechanism associated with CRISPR-cas9?

Non-homologous end joining. ***Non-homologous end joining*** - **CRISPR-Cas9** creates a **double-strand break (DSB)** in DNA, which is primarily repaired by **non-homologous end joining (NHEJ)**. - **NHEJ** is the **predominant repair pathway** in most cells, accounting for 60-90% of DSB repairs. - **NHEJ** is an error-prone repair mechanism that ligates the broken ends directly, often leading to small **insertions or deletions (indels)** causing gene knockout. - Note: **Homology-directed repair (HDR)** is another CRISPR-associated mechanism used for precise editing when a donor template is provided, but **NHEJ is the primary endogenous repair pathway**. *Mismatch repair* - This mechanism corrects errors that arise during **DNA replication**, such as incorrect base pairing. - It does not repair **double-strand breaks** induced by CRISPR-Cas9. *Nucleotide excision repair* - This pathway removes **bulky DNA adducts** and lesions, such as those caused by UV radiation. - It is not involved in repairing **CRISPR-Cas9 induced double-strand breaks**. *Base excision repair* - This mechanism corrects **damaged or modified bases**, typically single base changes, without affecting the sugar-phosphate backbone significantly. - It handles different types of DNA damage than the **double-strand breaks** generated by CRISPR-Cas9.

Q: Which of the following molecular analysis methods is used to detect RNA?

RT-PCR. ***RT-PCR*** - **Reverse transcriptase polymerase chain reaction (RT-PCR)** is a molecular method that first converts **RNA into complementary DNA (cDNA)** using reverse transcriptase before amplification via PCR. - This technique is commonly used to detect and quantify specific **RNA sequences**, such as viral RNA or gene expression levels. *Western blot* - **Western blot** is a laboratory technique used to detect specific **proteins** in a sample. - It involves separating proteins by size using gel electrophoresis, transferring them to a membrane, and then detecting the target protein using specific antibodies. *G banding* - **G banding** is a cytogenetic technique used to produce a visible **karyotype** by staining condensed chromosomes. - This method is primarily used to detect large-scale structural changes in chromosomes, not to detect RNA. *Sanger's method* - **Sanger's method**, also known as **dideoxy sequencing**, is a DNA sequencing technique. - It is used to determine the exact order of **nucleotides within a DNA molecule**, not to detect RNA directly.

Question 1

A patient presents with skin cancer and hyperpigmentation that worsens with sunlight exposure. Which of the following DNA repair mechanisms is most likely defective in this condition?

Accepted Answer

Nucleotide excision repair

Answer

Base excision repair

Answer

Mismatch repair

Answer

Non-homologous end joining (NHEJ)

Question 2

A frameshift mutation occurs due to the insertion of a new nucleotide at the 4th position of an mRNA sequence with 900 nucleotides. What is the most likely outcome of this mutation?

Accepted Answer

Partial protein production

Answer

Complete change in protein production

Answer

No protein production

Answer

No change in the final protein

Question 3

Which of the following correctly describes gene therapy ?

Accepted Answer

Introduction of gene sequence into a cell

Answer

Mapping and isolation of gene sequence

Answer

Synthesis of DNA probes with specific sequence

Answer

Use of polymerase chain reaction technique

Question 4

Which PCR technique is best suited for identifying a syndrome with multiple causative agents?

Accepted Answer

Multiplex PCR

Answer

RT-PCR

Answer

Nested PCR

Answer

Conventional PCR

Question 5

A patient presents with a skin rash that is exaggerated on sun exposure. What is the repair mechanism involved in this condition?

Accepted Answer

Nucleotide excision repair

Answer

Base excision repair

Answer

Mismatch repair

Answer

Double stranded DNA break repair

Question 6

Which of the following is used to detect abnormal gene sequences EXCEPT?

Accepted Answer

Flow cytometry

Answer

RFLP analysis

Answer

Pyrosequencing

Answer

FISH

Question 7

DNA sequence is determined by?

Accepted Answer

Sanger sequencing

Answer

PCR

Answer

FISH

Answer

Gel electrophoresis

Question 8

miRNA binds to which part of the mRNA to inhibit translation?

Accepted Answer

3'UTR

Answer

Gene promoter

Answer

Gene body

Answer

5'UTR

Question 9

What is the repair mechanism associated with CRISPR-cas9?

Accepted Answer

Non-homologous end joining

Answer

Mismatch repair

Answer

Nucleotide excision repair

Answer

Base excision repair

Question 10

Which of the following molecular analysis methods is used to detect RNA?

Accepted Answer

RT-PCR

Answer

Western blot

Answer

G banding

Answer

Sanger's method

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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