Practice 5 structured short- and long-answer medical questions on Weekly Year 2: Molecular Genetics and Cytogenetics Exam - July 3, 2026 (Sections B & C). Re...
Answer: DNA replication is a semi-conservative process where each parent strand serves as a template for the synthesis of a new complementary strand. The process begins with the unwinding of the double helix by helicase, creating a replication fork. DNA polymerase then adds new nucleotides to the growing strands, following base pairing rules (A with T, G with C). DNA ligase joins Okazaki fragments on the lagging strand. Topoisomerase relieves torsional stress.
Answer: A gene mutation is a permanent alteration in the DNA sequence. Point mutations involve changes to a single nucleotide base, such as substitutions (e.g., missense, nonsense, silent), insertions, or deletions. Chromosomal aberrations are larger-scale alterations affecting the structure or number of chromosomes, including deletions, duplications, translocations, and aneuploidy.
Answer: Transcription is the synthesis of RNA from a DNA template, occurring in the nucleus. RNA polymerase binds to the promoter region of a gene and synthesizes a complementary RNA strand (mRNA). Translation is the synthesis of protein from mRNA, occurring in the cytoplasm on ribosomes. Ribosomes read the mRNA codons, and tRNA molecules bring specific amino acids to the ribosome, which are then linked together to form a polypeptide chain.
Answer: DNA repair mechanisms correct damage to DNA that can arise from replication errors, environmental mutagens, or spontaneous chemical changes. Key mechanisms include base excision repair (BER) for small base damage, nucleotide excision repair (NER) for bulky lesions, mismatch repair (MMR) for replication errors, and double-strand break repair (DSBR) via homologous recombination or non-homologous end joining. Essential for preventing mutations, genetic instability, cancer, and cell death.
Answer: Autosomal dominant inheritance involves a single copy of an altered gene on a non-sex chromosome causing disease. Affected individuals have a 50% chance of passing the altered gene to each child. Mutations in genes like APP can lead to Alzheimer's through various mechanisms, such as altered amyloid precursor protein processing, increased production of toxic amyloid-beta peptides, or impaired clearance, leading to neuronal dysfunction and death. Cytogenetic analysis examines chromosome structure and number. Karyotyping involves visualizing stained chromosomes under a microscope to detect large-scale structural abnormalities (deletions, duplications, translocations) and numerical abnormalities (aneuploidy). Limitations include resolution for smaller deletions/duplications and interpretation of complex rearrangements. Comparative Genomic Hybridization (CGH) compares the amount of DNA from a patient sample to a reference sample across the genome, detecting copy number variations (gains or losses of DNA segments). Array CGH (aCGH) offers higher resolution than traditional CGH and can detect smaller deletions/duplications. However, it cannot detect balanced rearrangements (translocations or inversions) or mosaicism effectively. In this context, karyotyping would be useful to rule out gross chromosomal imbalances, while aCGH could identify microdeletions or microduplications in candidate genes or chromosomal regions associated with Alzheimer's, especially if the mutation identified is a de novo event or if other family members present with different phenotypes suggesting a larger chromosomal anomaly.