Cell cycle and mitosis US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Cell cycle and mitosis. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Cell cycle and mitosis US Medical PG Question 1: A 33-year-old woman comes to the physician 1 week after noticing a lump in her right breast. Fifteen years ago, she was diagnosed with osteosarcoma of her left distal femur. Her father died of an adrenocortical carcinoma at the age of 41 years. Examination shows a 2-cm, firm, immobile mass in the lower outer quadrant of the right breast. A core needle biopsy of the mass shows adenocarcinoma. Genetic analysis in this patient is most likely to show a defect in which of the following genes?
- A. BRCA1
- B. KRAS
- C. TP53 (Correct Answer)
- D. Rb
- E. PTEN
Cell cycle and mitosis Explanation: ***TP53***
- This patient's presentation with **early-onset breast cancer**, a history of **osteosarcoma** at a young age, and a father's death from **adrenocortical carcinoma** at 41 years strongly suggests **Li-Fraumeni syndrome**.
- Li-Fraumeni syndrome is an autosomal dominant disorder caused by a germline mutation in the **tumor suppressor gene TP53**, increasing the risk for multiple primary cancers at a young age.
*BRCA1*
- While **BRCA1 mutations** are associated with an increased risk of breast and ovarian cancer, they are not typically linked to osteosarcoma or adrenocortical carcinoma.
- The constellation of cancers in this patient is more indicative of Li-Fraumeni syndrome than solely a BRCA1-related cancer syndrome.
*KRAS*
- **KRAS** is an oncogene commonly mutated in several cancers, including pancreatic, colorectal, and lung cancer, but is not primarily associated with either Li-Fraumeni syndrome or the specific tumors seen in this family history.
- Mutations in KRAS are typically somatic mutations acquired during a person's lifetime, not germline mutations causing inherited cancer syndromes like the one suggested here.
*Rb*
- Mutations in the **retinoblastoma (Rb) gene** are associated with retinoblastoma and an increased risk of osteosarcoma, but not typically with adrenocortical carcinoma or breast cancer as part of a classic inherited syndrome.
- The combination of breast cancer, osteosarcoma, and adrenocortical carcinoma points more specifically to TP53.
*PTEN*
- **PTEN mutations** are associated with Cowden syndrome, which increases the risk for breast cancer, thyroid cancer, and endometrial cancer, along with benign growths.
- However, Cowden syndrome does not typically include osteosarcoma or adrenocortical carcinoma as prominent features, making PTEN less likely than TP53 for this specific family history.
Cell cycle and mitosis US Medical PG Question 2: An investigator studying the molecular characteristics of various malignant cell lines collects tissue samples from several families with a known mutation in the TP53 tumor suppressor gene. Immunohistochemical testing performed on one of the cell samples stains positive for desmin. This sample was most likely obtained from which of the following neoplasms?
- A. Squamous cell carcinoma
- B. Rhabdomyosarcoma (Correct Answer)
- C. Prostate cancer
- D. Endometrial carcinoma
- E. Melanoma
Cell cycle and mitosis Explanation: ***Rhabdomyosarcoma***
- **Desmin** is an intermediate filament present in **muscle cells**, and its positive staining is a definitive marker for tumors of muscle origin
- A **rhabdomyosarcoma** is a malignant tumor of **skeletal muscle** differentiation, thus explaining the positive desmin staining.
*Squamous cell carcinoma*
- **Squamous cell carcinomas** are epithelial tumors that typically stain positive for **cytokeratin**, not desmin, as they originate from epithelial cells.
- They are characterized by features such as **intercellular bridges** and **keratinization**.
*Prostate cancer*
- **Prostate cancer** is an adenocarcinoma, meaning it's derived from glandular epithelial cells, and would stain positive for markers like **PSA (prostate-specific antigen)**, not desmin.
- This tumor type is characterized by glandular differentiation.
*Endometrial carcinoma*
- **Endometrial carcinomas** are adenocarcinomas of the uterine lining, derived from glandular epithelial cells, and would express **cytokeratins**, not desmin.
- Histologically, they show glandular structures and atypical endometrial cells.
*Melanoma*
- **Melanomas** are malignant tumors of melanocytes and would stain positive for markers such as **S-100**, **HMB-45**, and **Mart-1**, not desmin.
- These tumors originate from neural crest cells and are not muscle-derived.
Cell cycle and mitosis US Medical PG Question 3: A group of scientists developed a mouse model to study nondisjunction in meiosis. Their mouse model produced gametes in the following ratio: 2 gametes with 24 chromosomes each and 2 gametes with 22 chromosomes each. In which of the following steps of meiosis did the nondisjunction occur?
- A. Telophase I
- B. Metaphase II
- C. Anaphase I (Correct Answer)
- D. Anaphase II
- E. Metaphase I
Cell cycle and mitosis Explanation: ***Anaphase I***
- Nondisjunction during **Anaphase I** occurs when homologous chromosomes fail to separate properly, meaning both homologs of a chromosome pair go to the same pole.
- This results in two secondary gametocytes with abnormal chromosome numbers: one with n+1 chromosomes (24) and one with n-1 chromosomes (22).
- When meiosis II proceeds normally, each abnormal secondary gametocyte divides to produce 2 identical gametes, resulting in **all 4 gametes being abnormal** in a 2:2 ratio (two n+1 and two n-1), matching the observed pattern.
*Telophase I*
- **Telophase I** is the final stage of meiosis I where chromosomes arrive at the poles and the cell divides, but it's not where the initial separation error (nondisjunction) occurs.
- Nondisjunction happens due to a failure of **chromosome segregation**, which is a process of anaphase, not telophase.
*Metaphase II*
- **Metaphase II** involves the alignment of sister chromatids at the metaphase plate in secondary gametocytes. Nondisjunction at this stage would involve sister chromatids failing to separate.
- Nondisjunction in Metaphase II (or Anaphase II) would lead to 2 normal gametes (23 chromosomes), one gamete with n+1 (24 chromosomes), and one gamete with n-1 (22 chromosomes), which differs from the given ratio.
*Anaphase II*
- **Nondisjunction in Anaphase II** would involve the failure of sister chromatids to separate in one of the secondary gametocytes.
- This would produce two normal gametes (23 chromosomes), one gamete with 24 chromosomes (n+1), and one gamete with 22 chromosomes (n-1), which is not the 2:2 ratio observed.
*Metaphase I*
- **Metaphase I** is characterized by the alignment of homologous chromosome pairs at the metaphase plate. While an issue here could precede nondisjunction, the actual event of failed separation occurs during anaphase.
- No separation of chromosomes occurs in Metaphase I; it is the stage of **chromosome alignment** before segregation.
Cell cycle and mitosis US Medical PG Question 4: You are currently employed as a clinical researcher working on clinical trials of a new drug to be used for the treatment of Parkinson's disease. Currently, you have already determined the safe clinical dose of the drug in a healthy patient. You are in the phase of drug development where the drug is studied in patients with the target disease to determine its efficacy. Which of the following phases is this new drug currently in?
- A. Phase 4
- B. Phase 1
- C. Phase 2 (Correct Answer)
- D. Phase 0
- E. Phase 3
Cell cycle and mitosis Explanation: ***Phase 2***
- **Phase 2 trials** involve studying the drug in patients with the target disease to assess its **efficacy** and further evaluate safety, typically involving a few hundred patients.
- The question describes a stage after safe dosing in healthy patients (Phase 1) and before large-scale efficacy confirmation (Phase 3), focusing on efficacy in the target population.
*Phase 4*
- **Phase 4 trials** occur **after a drug has been approved** and marketed, monitoring long-term effects, optimal use, and rare side effects in a diverse patient population.
- This phase is conducted post-market approval, whereas the question describes a drug still in development prior to approval.
*Phase 1*
- **Phase 1 trials** primarily focus on determining the **safety and dosage** of a new drug in a **small group of healthy volunteers** (or sometimes patients with advanced disease if the drug is highly toxic).
- The question states that the safe clinical dose in a healthy patient has already been determined, indicating that Phase 1 has been completed.
*Phase 0*
- **Phase 0 trials** are exploratory, very early-stage studies designed to confirm that the drug reaches the target and acts as intended, typically involving a very small number of doses and participants.
- These trials are conducted much earlier in the development process, preceding the determination of safe clinical doses and large-scale efficacy studies.
*Phase 3*
- **Phase 3 trials** are large-scale studies involving hundreds to thousands of patients to confirm **efficacy**, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug to be used safely.
- While Phase 3 does assess efficacy, it follows Phase 2 and is typically conducted on a much larger scale before submitting for regulatory approval.
Cell cycle and mitosis US Medical PG Question 5: An investigator is studying human genetics and cell division. A molecule is used to inhibit the exchange of genetic material between homologous chromosomes. Which of the following phases of the cell cycle does the molecule target?
- A. Prophase II
- B. Prophase I (Correct Answer)
- C. Metaphase II
- D. Telophase I
- E. Anaphase I
Cell cycle and mitosis Explanation: ***Prophase I***
- **Crossing over** (genetic recombination) occurs specifically during **Prophase I** of meiosis, particularly during the pachytene stage
- During this phase, homologous chromosomes pair up (synapsis) and exchange genetic material through recombination
- Inhibiting this exchange means targeting the phase where this critical genetic recombination takes place
*Prophase II*
- Prophase II is a stage in meiosis II where chromosomes condense again after a brief interkinesis
- **Crossing over does not occur** in Prophase II - genetic recombination has already been completed in Prophase I
- Homologous chromosomes are no longer paired at this stage
*Metaphase II*
- During Metaphase II, individual chromosomes (not homologous pairs) align along the metaphase plate
- There is **no exchange of genetic material** between homologous chromosomes at this stage
- This phase prepares for the separation of sister chromatids
*Telophase I*
- Telophase I involves decondensation of chromosomes and reformation of nuclear envelopes around the separated homologous chromosomes
- This marks the end of meiosis I, **after** genetic exchange has already occurred in Prophase I
- No crossing over occurs during this phase
*Anaphase I*
- In Anaphase I, **homologous chromosomes separate** and move to opposite poles of the cell
- This phase is characterized by segregation of chromosomes, **not genetic exchange**
- Crossing over has already been completed by this stage
Cell cycle and mitosis US Medical PG Question 6: A 12-hour old male infant is seen in the newborn nursery. He was born full term by vaginal delivery to a 40-year-old G4P3-->4 mother. Her pregnancy and delivery were uncomplicated, notable only for declining genetic testing. On exam, her son has a flat face, a fold in the upper eyelid, palpebral fissures that appear to slant upwards, and small ears. The diagnostic test for her son’s most likely condition should be conducted during which of the following phases of the cell cycle?
- A. S-phase
- B. Anaphase
- C. Prophase
- D. Metaphase (Correct Answer)
- E. Telophase
Cell cycle and mitosis Explanation: ***Metaphase***
- The infant's features (flat face, upslanting palpebral fissures, epicanthal folds) are **characteristic of Down syndrome** (Trisomy 21).
- Karyotyping, which visualizes chromosomes to detect trisomies, is best performed during **metaphase** because chromosomes are maximally condensed and aligned at the metaphase plate, making them easily distinguishable.
*S-phase*
- During the **S-phase**, DNA replication occurs, and chromosomes are not yet condensed, making them unsuitable for microscopic visualization and karyotyping.
- This phase is primarily for **DNA synthesis**, not chromosomal analysis.
*Anaphase*
- In **anaphase**, sister chromatids separate and move to opposite poles of the cell.
- While chromosomes are visible, they are in motion, making it challenging to **accurately count and analyze their structure**.
*Prophase*
- **Prophase** involves the condensation of chromosomes and the breakdown of the nuclear envelope.
- Chromosomes are still condensing in prophase and not yet fully aligned, making them less ideal for detailed **karyotype analysis compared to metaphase**.
*Telophase*
- In **telophase**, chromosomes decondense, the nuclear envelope reforms, and the cell divides into two.
- Chromosomes are no longer discrete or condensed enough for **accurate visualization and karyotyping** during this phase.
Cell cycle and mitosis US Medical PG Question 7: A 12-year-old girl is brought to an oncologist, as she was recently diagnosed with a rare form of cancer. Cytogenetic studies reveal that the tumor is responsive to vinblastine, which is a cell-cycle specific anticancer agent. It acts on the M phase of the cell cycle and inhibits the growth of cells. Which of the following statements best describes the regulation of the cell cycle?
- A. Inhibitors of DNA synthesis act in the M phase of the cell cycle.
- B. The G0 phase is the checkpoint before G1.
- C. Cyclin-dependent activation of CDK1 (CDC2) takes place upon the entry of a cell into M phase of the cell cycle. (Correct Answer)
- D. EGF from a blood clot stimulates the growth and proliferation of cells in the healing process.
- E. Replication of the genome occurs in the M phase of the cell cycle.
Cell cycle and mitosis Explanation: ***Cyclin-dependent activation of CDK1 (CDC2) takes place upon the entry of a cell into M phase of the cell cycle.***
- The **M-phase promoting factor (MPF)**, composed of **CDK1 (CDC2)** and **cyclin B**, is activated at the G2/M transition, driving the cell into mitosis.
- Activation of CDK1 by **cyclin B binding** and subsequent dephosphorylation of threonine 161 is crucial for initiation of mitosis.
*Inhibitors of DNA synthesis act in the M phase of the cell cycle.*
- **Inhibitors of DNA synthesis**, such as **hydroxyurea** and **methotrexate**, primarily act during the **S phase** of the cell cycle, when DNA replication occurs.
- The M phase is characterized by **mitosis** (nuclear division) and **cytokinesis** (cytoplasmic division), not DNA synthesis.
*The G0 phase is the checkpoint before G1.*
- The **G0 phase** is a **resting state** where cells exit the cell cycle and cease to divide, not a checkpoint before G1.
- The main checkpoint before G1 is typically referred to as the **restriction point** or **G1 checkpoint**, which determines if a cell will commit to division.
*EGF from a blood clot stimulates the growth and proliferation of cells in the healing process.*
- While **EGF (Epidermal Growth Factor)** does stimulate cell growth and proliferation in healing, it is not primarily associated with blood clots.
- **Platelets** in blood clots release growth factors like **PDGF (Platelet-Derived Growth Factor)** and **TGF-β (Transforming Growth Factor-beta)**, which are critical for wound healing.
*Replication of the genome occurs in the M phase of the cell cycle.*
- **Replication of the genome** (DNA synthesis) occurs during the **S phase** (synthesis phase) of the cell cycle.
- The **M phase** is dedicated to **mitosis** (separation of duplicated chromosomes) and **cytokinesis**, where the cell divides into two daughter cells.
Cell cycle and mitosis US Medical PG Question 8: A 2-year-old boy from a rural community is brought to the pediatrician after his parents noticed a white reflection in both of his eyes in recent pictures. Physical examination reveals bilateral leukocoria, nystagmus, and inflammation. When asked about family history of malignancy, the father of the child reports losing a brother to an eye tumor when they were children. With this in mind, which of the following processes are affected in this patient?
- A. Base excision repair
- B. Regulation of the G1-S transition (Correct Answer)
- C. DNA mismatch repair
- D. Stem cell self-renewal
- E. Nucleotide excision repair
Cell cycle and mitosis Explanation: ***Regulation of the G1-S transition***
- This patient's symptoms (bilateral **leukocoria**, **nystagmus**, family history of eye tumor) are characteristic of **retinoblastoma**, which is often caused by a mutation in the **RB1 gene**.
- The **RB1 gene** product (retinoblastoma protein) is a key **tumor suppressor** that regulates the G1-S cell cycle transition, and its dysfunction leads to uncontrolled cell proliferation.
*Base excision repair*
- This process is primarily involved in repairing damaged bases in DNA, often due to oxidation or alkylation.
- Defects in base excision repair are typically associated with conditions such as **MUTYH-associated polyposis**, not retinoblastoma.
*DNA mismatch repair*
- This system corrects errors that occur during DNA replication, such as incorrect base pairings or small insertions/deletions.
- Impairment of mismatch repair is a hallmark of **Lynch syndrome** (hereditary nonpolyposis colorectal cancer), which does not present with retinoblastoma.
*Stem cell self-renewal*
- While uncontrolled self-renewal can contribute to cancer, retinoblastoma is specifically linked to defects in the **RB1 gene**, which is a cell cycle regulator, not directly a primary regulator of stem cell self-renewal itself.
- Loss of G1-S checkpoint control is a more direct and proximal cause of the tumor formation in retinoblastoma.
*Nucleotide excision repair*
- This pathway is responsible for repairing bulkier DNA lesions, such as those caused by UV radiation.
- Deficiencies in nucleotide excision repair lead to diseases like **xeroderma pigmentosum**, characterized by extreme sensitivity to sunlight and increased skin cancer risk, which is unrelated to the presented case.
Cell cycle and mitosis US Medical PG Question 9: A research lab is investigating the rate of replication of a variety of human cells in order to better understand cancer metastasis. A particular cell line of interest is marked with a high concern for malignant potential due to its chromatin structure characteristics. Which of the following is most closely associated with an increased potential for malignancy?
- A. Methylated DNA
- B. H1 protein
- C. Nucleosomes
- D. Euchromatin (Correct Answer)
- E. Heterochromatin
Cell cycle and mitosis Explanation: ***Euchromatin***
- An increased amount of **euchromatin** suggests that the DNA is less condensed and more accessible for transcription and replication, which is characteristic of rapidly dividing cells, including cancer cells.
- Cancer cells often exhibit **deregulated gene expression** due to alterations in chromatin structure, leading to an increase in euchromatin and higher rates of protein synthesis necessary for rapid proliferation.
*Methylated DNA*
- While DNA methylation is an important **epigenetic modification** involved in cancer, specific patterns of methylation (e.g., hypermethylation of tumor suppressor genes or hypomethylation of oncogenes) are associated with malignancy, not methylation of all DNA.
- Global hypomethylation is linked to genomic instability in cancer, whereas hypermethylation often leads to gene silencing.
*H1 protein*
- **H1 histone protein**, also known as the linker histone, is responsible for compacting nucleosomes and is essential for forming higher-order chromatin structures.
- An abundance of H1 generally indicates a more condensed chromatin state, which would be *less* associated with the active transcription and replication seen in highly malignant cells.
*Nucleosomes*
- **Nucleosomes** are the fundamental building blocks of chromatin, consisting of DNA wrapped around histone proteins. They are present in all eukaryotic cells, irrespective of their malignant potential.
- While alterations in nucleosome positioning and modification can occur in cancer, the presence of nucleosomes themselves is not indicative of malignancy.
*Heterochromatin*
- **Heterochromatin** is a highly condensed form of chromatin that is typically transcriptionally inactive or silenced.
- A high proportion of heterochromatin would suggest a cell with reduced gene expression and replication activity, which is generally *not* characteristic of rapidly dividing malignant cells.
Cell cycle and mitosis US Medical PG Question 10: An investigator is studying the biology of human sperm cells. She isolates spermatogonia obtained on a testicular biopsy from a group of healthy male volunteers. She finds that the DNA of spermatogonia obtained from these men show a large number of TTAGGG sequence repeats. This finding can best be explained by increased activity of an enzyme with which of the following functions?
- A. Ligation of Okazaki fragments
- B. Proofreading of synthesized daughter strands
- C. RNA-dependent synthesis of DNA (Correct Answer)
- D. Production of short RNA sequences
- E. Hemimethylation of DNA strand
Cell cycle and mitosis Explanation: ***RNA-dependent synthesis of DNA***
- The TTAGGG sequence repeats are **telomeric sequences**, which are maintained by **telomerase**, an enzyme that synthesizes DNA from an RNA template.
- **Spermatogonia** are germline stem cells that express high levels of telomerase to maintain telomere length across generations.
*Ligation of Okazaki fragments*
- This function is carried out by **DNA ligase**, which joins discontinuous DNA fragments during replication on the lagging strand.
- This process is essential for general DNA replication but is not specific to the formation or maintenance of telomeric repeats.
*Proofreading of synthesized daughter strands*
- This is a function of **DNA polymerase exonuclease activity**, which corrects errors during DNA replication.
- While important for genetic fidelity, it does not explain the presence or increase of specific TTAGGG repeat sequences at telomeres.
*Production of short RNA sequences*
- This function is performed by **primase**, which synthesizes RNA primers necessary to initiate DNA synthesis during replication.
- These RNA primers are later removed and replaced with DNA, and this process is not directly responsible for generating or extending telomeric repeats.
*Hemimethylation of DNA strand*
- Hemimethylation occurs during **DNA replication** when new DNA strands are unmethylated while parental strands are methylated.
- This phenomenon is involved in DNA repair and gene regulation but is unrelated to the synthesis or regulation of telomeric sequences.
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