Genes, Genomics, and Chromosomes

Eukaryotic Gene Structure

  • Definition of a Gene: Whole nucleic acid sequence necessary for the synthesis of functional gene product (polypeptide or RNA), including protein-coding, enhancer, and promoter regions.
  • Single Human Cell DNA: Measures about 2 m total length, contained within nuclei with diameters of less than 10 μm.
  • Compaction Ratio: Greater than 105 to 1.
  • Chromosome Composition:
    • Each chromosome consists of a single DNA molecule, as long as 280 Mb in humans.
    • Organized into nucleosomes, higher-order chromatin folding.
    • Compact DNA is accessible for transcription, replication, and repair without tangling.

Structural Organization of Eukaryotic Chromosomes

  • Levels of Chromatin Organization:
    • Nucleosomes: DNA wrapped around histone octamer, the basic unit of chromatin structure.
    • 30 nm Fibers: Comprising nucleosomes, compacting further into larger, organized structures.
    • Mitotic Chromosomes: Maximum compaction, packing ratio of 10,000:1.

Morphology and Functional Elements of Eukaryotic Chromosomes

  • Chromosome Components:
    • Telomeres: End regions of chromosomes, critical in preventing degradation; added by telomerase.
    • Centromeres: Allows microtubule attachment during mitosis, located at indented site on chromosomes.

Gene Duplication and Evolutionary Innovations

  • Gene Duplication Mechanisms:
    • Exon Duplication: Formation of additional exons through unequal crossing over during meiosis, can create new functions (neofunctionalization).
    • Gene Family: Groups of genes with similar sequences, often arising from duplication events.
  • Functional Consequences:
    • Redundancy: Duplicated genes retain function and may increase expression levels.
    • Subfunctionalization: Both copies adapt different functions.
    • Pseudogenes: Genes that become nonfunctional over time due to mutations.

Levels of Chromatic Organization and Histone Code

  • Histone Modifications:
    • Modifications such as acetylation, methylation alter interactions with DNA, regulating gene expression by loosening or compacting chromatin.
    • Histone Code: Specific combinations of modifications influence chromatin function and create binding sites for additional proteins.
  • Nuclear Structure:
    • Chromatin is not random, but highly organized within the nucleus.
    • Functional regions engaged in biological processes interact within transcription factories.

Telomeres and Chromosomal Stability

  • Function of Telomeres:
    • Protect chromosome ends from degradation.
    • Length is reduced in somatic cells with each division; critical limit results in growth arrest.

Epigenetics

  • Definition: Study of heritable changes in gene function that do not involve changes in the DNA sequence.
  • Importance: Could explain differences in gene expression in genetically identical organisms.
  • X-Chromosome Inactivation: Example of epigenetic control in females where one X chromosome is inactivated.

Summary of Learning Objectives

  • Understanding:
    • Differentiates between simple and complex transcription units, gene families, and chromatin organization levels.
    • Recognize evolutionary significance of gene duplication, mechanisms of transcription regulation, and chromosomal stability factors.
  • Key Terms: Homologs, orthologs, paralogs, euchromatin, heterochromatin.