Practice 5 structured short- and long-answer medical questions on Weekly Year 2: Molecular Genetics and Cytogenetics Exam - July 17, 2026 (Sections B & C). R...
Answer: Telomeres are protective caps at the ends of chromosomes that shorten with each cell division. This shortening acts as a mitotic clock, eventually leading to cellular senescence or apoptosis. Telomerase is the enzyme responsible for adding repetitive DNA sequences to the ends of telomeres, counteracting this shortening and allowing for continued cell division. Its activity is typically high in germ cells and cancer cells but low in most somatic cells.
Answer: A frameshift mutation occurs when the insertion or deletion of nucleotides in a gene's coding sequence is not a multiple of three. This alters the reading frame of the codons during translation, leading to a completely different amino acid sequence downstream of the mutation. An example would be a single base deletion in the gene for hemoglobin, which can cause a frameshift and result in the production of a non-functional or truncated protein, leading to conditions like thalassemia.
Answer: Aneuploidy refers to an abnormal number of chromosomes, where there is an extra chromosome (trisomy) or a missing chromosome (monosomy) for a specific pair. It commonly arises from non-disjunction during meiosis. Polyploidy, on the other hand, involves having extra sets of chromosomes, such as triploidy (three sets) or tetraploidy (four sets). It can result from failures in cytokinesis after DNA replication or fusion of gametes with duplicated chromosome sets.
Answer: cDNA synthesis from mRNA is achieved using the enzyme reverse transcriptase. First, mRNA serves as a template, and reverse transcriptase synthesizes a complementary DNA strand (cDNA) using free nucleotides. Then, the mRNA template is degraded, and a second DNA strand is synthesized to form double-stranded cDNA. This process is significant because it allows researchers to study the expressed genes in a cell (genes that are being transcribed into mRNA) and is crucial for techniques like gene cloning, PCR, and creating cDNA libraries.
Answer: The most likely chromosomal abnormality in this child is Down syndrome, caused by Trisomy 21. **Genetic Basis and Pathogenesis:** Down syndrome is most commonly caused by a nondisjunction event during meiosis, leading to the presence of an extra copy of chromosome 21 in all cells of the body (standard trisomy 21, accounting for about 95% of cases). Less commonly, it can be caused by translocation (where part of chromosome 21 is attached to another chromosome) or mosaicism (where only some cells have the extra chromosome). The extra genetic material from chromosome 21 leads to an overexpression of genes located on this chromosome, which in turn disrupts normal development and leads to the characteristic phenotype. **Associated Clinical Features:** The clinical features are varied but commonly include: intellectual disability (ranging from mild to moderate), characteristic facial features (e.g., brachycephaly, epicanthal folds, upslanting palpebral fissures, flat nasal bridge, small ears, protruding tongue), short stature, hypotonia (poor muscle tone), single palmar crease, and congenital heart defects (such as atrioventricular septal defects and ventricular septal defects). Other potential issues include gastrointestinal abnormalities (e.g., duodenal atresia), increased risk of leukemia, hearing and vision problems, and thyroid dysfunction. **Implications for Genetic Counseling and Management:** Genetic counseling is crucial for families of children with Down syndrome. It involves explaining the diagnosis, the cause of the condition (including recurrence risks for future pregnancies), and the range of potential health issues. Management is multidisciplinary and focuses on early intervention, addressing specific medical concerns (e.g., cardiac surgery, thyroid treatment, vision/hearing support), and providing educational and therapeutic support to maximize the child's potential. Support for the family is also a key component.