Molecular Physiology Practice Questions

Q: A genetic study identifies a family with multiple members having early-onset colorectal cancer. DNA mismatch repair gene testing reveals a mutation in MLH1 gene. Which molecular phenomenon would be most prominently observed in tumor cells from these patients?

Microsatellite instability (MSI). ***Microsatellite instability (MSI)*** - Mutations in **MLH1** lead to defective **DNA mismatch repair (MMR)**, which is the hallmark of **Lynch syndrome** (HNPCC). - Failure to repair errors in repetitive DNA sequences results in **microsatellite instability**, characterized by the expansion or contraction of short tandem repeats in tumor cells. *Chromosomal translocations* - These are typical of **hematologic malignancies** (e.g., Philadelphia chromosome in CML) or certain sarcomas, rather than MMR defects. - Lynch syndrome tumors usually exhibit **diploidy** and stable karyotypes compared to tumors following the **chromosomal instability (CIN)** pathway. *Telomere shortening* - This is a feature of **cellular senescence** and aging, where repeating sequences at the end of chromosomes are lost over time. - Cancer cells generally find ways to **maintain telomere length** (via telomerase) rather than experiencing shortening, which would limit their proliferative capacity. *DNA methylation* - While **hypermethylation** of the MLH1 promoter can cause sporadic MSI, this case specifies a **germline mutation** in a genetic study context. - Methylation is an **epigenetic modification** that affects gene expression but is not the primary molecular consequence arising directly from the loss of DNA repair enzyme function.

Q: A 45-year-old woman with breast cancer is being treated with trastuzumab (Herceptin). Laboratory analysis of her tumor cells shows 100-fold amplification of a specific gene. Analysis of the signal transduction pathway reveals constitutive activation of PI3K/AKT pathway. Which receptor is most likely overexpressed?

Human epidermal growth factor receptor 2 (HER2). ***Human epidermal growth factor receptor 2 (HER2)*** - **Trastuzumab** is a monoclonal antibody specifically designed to target the **HER2/neu (ERBB2)** receptor, which is overexpressed in approximately 20-30% of breast cancers. - **HER2** is a receptor tyrosine kinase that, when amplified, leads to constitutive activation of the **PI3K/AKT** and **MAPK** pathways, driving uncontrolled cell proliferation. *Vascular endothelial growth factor receptor (VEGFR)* - **VEGFR** is primarily involved in **angiogenesis** (the formation of new blood vessels) and is the target of drugs like **bevacizumab**, not trastuzumab. - While important in many cancers, its signaling pathway is not the primary target in the standard clinical use of trastuzumab for breast cancer. *Progesterone receptor (PR)* - **PR** is a **nuclear receptor** that functions as a transcription factor, not a cell surface receptor tyrosine kinase that activates the **PI3K/AKT** pathway directly. - Overexpression of PR is managed with **hormonal therapies** (like aromatase inhibitors), but it is not the target of the antibody **trastuzumab**. *Estrogen receptor (ER)* - Similar to the progesterone receptor, the **ER** is an intracellular receptor used to guide treatment with drugs like **Tamoxifen**. - Genes for ER are not typically subject to the **100-fold amplification** described, as ER status is usually determined by ligand-dependent signaling rather than massive gene duplication.

Q: A researcher is studying a cell line that shows resistance to apoptosis despite DNA damage. Analysis reveals overexpression of a protein that normally prevents p53 from inducing cell cycle arrest. Which protein is most likely overexpressed?

MDM2. ***MDM2*** - **MDM2** functions as an **E3 ubiquitin ligase** that directly binds to **p53**, promoting its degradation and inhibiting its transcriptional activity. - Overexpression leads to the loss of **p53-mediated cell cycle arrest** (via p21) and apoptosis, allowing cells with **DNA damage** to survive and proliferate. *Caspase-3* - **Caspase-3** is an **executioner caspase** that plays a central role in the final stages of the apoptotic cascade, not the regulation of p53. - Overexpression would typically **promote apoptosis** rather than providing resistance to it after DNA damage. *Cytochrome C* - **Cytochrome C** is released from the **mitochondria** into the cytosol to activate the **apoptosome** during the intrinsic pathway of apoptosis. - Its release occurs **downstream** of p53 activation and is not a protein that inhibits p53-induced cell cycle arrest. *Bcl-2* - **Bcl-2** is an **anti-apoptotic** protein that prevents the release of Cytochrome C, but it does not directly regulate p53-mediated **cell cycle arrest**. - While its overexpression prevents apoptosis, the specific mechanism of inhibiting **p53's ability to induce arrest** points towards MDM2.

Q: Which enzyme is responsible for unwinding the DNA double helix during replication?

Helicase. ***Helicase*** - **Helicase** is the specific enzyme responsible for **unwinding** the DNA double helix by breaking **hydrogen bonds** between complementary nitrogenous bases. - This action creates the **replication fork**, providing the single-stranded templates necessary for the replication process to proceed. *DNA polymerase* - Its primary role is the **synthesis** of new DNA strands by adding **deoxyribonucleotides** to a pre-existing primer. - It possesses **proofreading** capabilities but cannot physically separate the two parent strands of the DNA molecule. *Topoisomerase* - This enzyme functions to relieve **torsional strain** and prevent **supercoiling** of the DNA ahead of the replication fork. - It works by creating transient **single or double-stranded breaks** in the DNA backbone rather than unwinding the helix for template access. *DNA ligase* - Its main function is to join **Okazaki fragments** on the lagging strand by catalyzing the formation of **phosphodiester bonds**. - It acts as a molecular glue during the final stages of replication and **DNA repair**, not during the initial unwinding phase.

Q: Which type of RNA carries amino acids to the ribosome during protein synthesis?

Transfer RNA (tRNA). ***Transfer RNA (tRNA)*** - **tRNA** acts as an adapter molecule that matches specific **amino acids** to their corresponding **codons** on the mRNA template. - It contains an **anticodon loop** that base-pairs with mRNA and a 3' end that covalently binds the amino acid via **aminoacyl-tRNA synthetase**. *Ribosomal RNA (rRNA)* - **rRNA** is a structural and catalytic component of the **ribosome**, providing the scaffold for protein assembly. - It facilitates the formation of **peptide bonds** through its **peptidyl transferase** activity but does not transport amino acids. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in the **splicing** of pre-messenger RNA within the nucleus. - It combines with proteins to form **snRNPs** (spliceosomes) and is not directly involved in the translation process at the ribosome. *Messenger RNA (mRNA)* - **mRNA** serves as the genetic blueprint that carries the **coding sequence** from DNA to the ribosome for translation. - It provides the **template** of codons that dictates the specific sequence of amino acids rather than transporting them.

Question 1

A research team is developing a gene therapy approach using CRISPR-Cas9 to correct a point mutation causing sickle cell disease. They must decide between two strategies: (A) correcting the mutation in hematopoietic stem cells ex vivo, or (B) in vivo correction in bone marrow. Considering molecular physiology principles, what is the most significant advantage of strategy A over strategy B?

Accepted Answer

Strategy A allows for screening and selection of successfully edited cells before transplantation, minimizing off-target effects

Answer

Strategy A requires lower doses of viral vectors

Answer

Strategy A produces faster clinical improvement

Answer

Strategy A is less expensive to implement

Question 2

A novel drug is designed to treat a genetic disorder caused by a nonsense mutation in the dystrophin gene. The drug works by allowing the ribosome to skip over the premature stop codon and continue translation. Evaluation of this therapeutic strategy reveals partial restoration of dystrophin protein with 60% of normal length but sufficient function. What is the most critical molecular consideration in determining if this approach will be clinically beneficial?

Accepted Answer

Whether the truncated protein retains the actin-binding domain and maintains membrane stability

Answer

Whether the drug prevents degradation of dystrophin mRNA

Answer

Whether the drug enhances ribosomal binding to the start codon

Answer

Whether the drug increases transcription of the dystrophin gene

Question 3

A genetic study identifies a family with multiple members having early-onset colorectal cancer. DNA mismatch repair gene testing reveals a mutation in MLH1 gene. Which molecular phenomenon would be most prominently observed in tumor cells from these patients?

Accepted Answer

Microsatellite instability (MSI)

Answer

Chromosomal translocations

Answer

Telomere shortening

Answer

DNA methylation

Question 4

A 45-year-old woman with breast cancer is being treated with trastuzumab (Herceptin). Laboratory analysis of her tumor cells shows 100-fold amplification of a specific gene. Analysis of the signal transduction pathway reveals constitutive activation of PI3K/AKT pathway. Which receptor is most likely overexpressed?

Accepted Answer

Human epidermal growth factor receptor 2 (HER2)

Answer

Vascular endothelial growth factor receptor (VEGFR)

Answer

Progesterone receptor (PR)

Answer

Estrogen receptor (ER)

Question 5

A researcher is studying a cell line that shows resistance to apoptosis despite DNA damage. Analysis reveals overexpression of a protein that normally prevents p53 from inducing cell cycle arrest. Which protein is most likely overexpressed?

Accepted Answer

MDM2

Answer

Caspase-3

Answer

Cytochrome C

Answer

Bcl-2

Question 6

A 6-month-old infant presents with severe anemia, jaundice, and hepatosplenomegaly. Hemoglobin electrophoresis shows predominantly HbF (fetal hemoglobin) with absent HbA. Which molecular defect best explains this clinical picture?

Accepted Answer

Deletion or mutation in beta-globin gene causing beta-thalassemia major

Answer

Deletion in alpha-globin gene causing alpha-thalassemia

Answer

Point mutation in beta-globin gene causing sickle cell disease

Answer

Mutation in gamma-globin gene

Question 7

How does alternative splicing contribute to protein diversity in humans despite having approximately 20,000 genes?

Accepted Answer

By allowing different combinations of exons to be included in mature mRNA

Answer

By modifying amino acids after translation

Answer

By changing the sequence of codons in the mRNA

Answer

By producing different tRNA molecules

Question 8

Why does DNA replication occur in a 5' to 3' direction only?

Accepted Answer

Because DNA polymerase can only add nucleotides to the 3'-OH group

Answer

Because primase can only synthesize primers in that direction

Answer

Because the template strand is read in 3' to 5' direction only

Answer

Because helicase unwinds DNA in only one direction

Question 9

Which enzyme is responsible for unwinding the DNA double helix during replication?

Accepted Answer

Helicase

Answer

DNA polymerase

Answer

Topoisomerase

Answer

DNA ligase

Question 10

Which type of RNA carries amino acids to the ribosome during protein synthesis?

Accepted Answer

Transfer RNA (tRNA)

Answer

Ribosomal RNA (rRNA)

Answer

Small nuclear RNA (snRNA)

Answer

Messenger RNA (mRNA)

Molecular Physiology — MCQs

Molecular Physiology — MCQs

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