Chapter 18: The Molecular Biology of Cancer

DNA packaging begins with the nucleosome, the fundamental repeating unit in which DNA wraps around a histone octamer composed of two copies each of H2A, H2B, H3, and H4 proteins. Higher-order chromatin structure is stabilized by linker histone H1, creating a solenoid architecture that condenses into either transcriptionally active euchromatin or tightly packed, transcriptionally silent heterochromatin. Epigenetic modifications including histone acetylation, methylation, and phosphorylation regulate chromatin accessibility and remodeling, with dysregulation implicated in diseases such as Rett syndrome, fragile X syndrome, and various malignancies. Chromosomal DNA replication involves multiple origins of replication organized into replicons, with bidirectional replication forks synthesizing leading and lagging strands through the coordinated action of polymerases alpha, delta, and epsilon. Processivity during synthesis is maintained by PCNA, the sliding clamp protein, while FEN1 endonuclease removes RNA primers synthesized by polymerase alpha. Telomeres protect chromosomal termini from degradation, and telomerase enzyme activity counteracts progressive shortening associated with cell division, linking telomere dynamics to cellular aging, premature aging syndromes, and cancer cell immortalization. The chapter details multiple DNA repair pathways: mismatch repair corrects errors introduced during replication, base excision repair addresses oxidative damage and single-nucleotide lesions, and nucleotide excision repair removes bulky DNA adducts such as thymine dimers induced by ultraviolet radiation. Double-strand breaks are repaired through homologous recombination or nonhomologous end joining pathways, with defects in BRCA1 and BRCA2 proteins conferring hereditary susceptibility to breast and ovarian cancers, and mutations in ATM causing ataxia-telangiectasia with impaired double-strand break recognition. Clinical case studies illustrate melanoma development from compromised UV damage repair, lung cancer from tobacco carcinogen exposure, and xeroderma pigmentosum from nucleotide excision repair deficiency. Trinucleotide repeat expansions disrupting chromatin function underlie fragile X syndrome and Huntington disease, while chromosomal rearrangements including deletions, duplications, translocations, and inversions contribute to malignant transformation, exemplified by the Philadelphia chromosome in chronic myelogenous leukemia. The chapter integrates these topics to demonstrate how chromatin organization, replication fidelity, and repair mechanisms collectively preserve genomic stability essential for normal development and disease prevention.