Chapter 6: Genes & Genomes: Structure & Chromatin

Genes & Genomes: Structure & Chromatin details the split structure of eukaryotic genes, where coding sequences known as exons are interrupted by noncoding intervening sequences called introns, which are removed via RNA splicing to produce functional mRNA. The text explains how this structure allows for alternative splicing, a mechanism that significantly expands the proteome by enabling a single gene to encode multiple distinct proteins. Beyond protein-coding genes, the summary highlights the critical role of noncoding sequences, referencing the ENCODE project which revealed that the vast majority of the genome is functional and transcribed into noncoding RNAs. These include microRNAs (miRNAs) that regulate translation and stability, and long noncoding RNAs (lncRNAs) like Xist, which is essential for X chromosome inactivation and dosage compensation. The chapter further explores the prevalence of repetitive DNA, including simple-sequence repeats and transposable elements such as SINEs and LINEs, which move through the genome via reverse transcription and contribute to evolutionary diversity. The origins of gene families, such as the alpha and beta-globins, are traced back to gene duplication events, which also lead to the formation of nonfunctional pseudogenes and processed pseudogenes. Finally, the text describes the physical packaging of DNA into chromatin, where DNA is wrapped around histone octamers to form nucleosomes. It differentiates between active euchromatin and condensed heterochromatin and concludes with the specialized structures of centromeres, which rely on the histone variant CENP-A for epigenetic inheritance, and telomeres, which utilize shelterin and telomerase to protect chromosome ends.