Sickle Cell Anemia: Caused by a single base change in the beta-globin gene: GAG (Glu) → GUG (Val) at codon 6. Results in abnormal hemoglobin (HbS) that polymerizes under low oxygen, causing sickle-shaped RBCs, vaso-occlusive crises, and chronic anemia. Classic example of a missense mutation causing human disease.

Antibiotics Target Ribosomes: Streptomycin and tetracycline inhibit bacterial 30S subunit; chloramphenicol and erythromycin inhibit bacterial 50S subunit. Selectivity for 70S (bacterial) over 80S (eukaryotic) ribosomes is the basis of their therapeutic use.

Reverse Transcriptase and HIV: HIV is a retrovirus that uses reverse transcriptase (RNA→DNA) to integrate its genome into host DNA (the provirus). Drugs like AZT (zidovudine) inhibit reverse transcriptase, blocking viral replication. This is a direct application of the central dogma exception.

Cancer and Telomerase: Most somatic cells lack telomerase; telomeres shorten with each division → cellular senescence. Cancer cells reactivate telomerase, enabling unlimited division (immortalization). Telomerase inhibitors are being explored as anti-cancer therapy.

PCR in Diagnostics: PCR amplifies specific DNA sequences exponentially. Used in: COVID-19 testing (RT-PCR), genetic disease diagnosis (prenatal testing), forensics, paternity testing, and cancer mutation detection. Based on same principles as DNA replication.

Genetic Code in Medicine: Understanding the genetic code allows prediction of protein changes from gene mutations. Silent mutations (synonymous) don't change protein; missense mutations change one amino acid; nonsense mutations create premature stop codons producing truncated proteins; frameshift mutations (insertions/deletions of non-multiple-of-3 nt) alter the entire reading frame downstream of the mutation.

DNA Fingerprinting in Forensics: VNTRs vary between individuals. Used in criminal investigations, paternity disputes, identification of disaster victims, and immigration cases. Probability of false match: ~1 in 10^20 (all but identical twins).

Gene Therapy Principles: Understanding transcription and translation enables rational design of gene therapy vectors. Correcting mutations in the DNA of affected cells can potentially cure genetic diseases (e.g., SCID, cystic fibrosis).