EM

DNA Packaging in Bacteria and Eukaryotes

DNA Packaging in Bacteria

  • Chromosomal DNA
    • Most bacteria possess circular chromosomal DNA.
    • Typical chromosome length: Few million base pairs.
    • Usually, a single chromosome type exists but may be present in multiple copies.
    • Chromosomes contain thousands of genes with short intergenic regions between them.
  • Replication Origin
    • One origin of replication is essential for DNA replication initiation.
    • Repetitive sequences can be found sporadically throughout the chromosome.

DNA Packaging in Eukaryotes

  • Chromosomal Structure
    • Eukaryotic chromosomes are linear.
    • Typical chromosome length ranges from tens of millions to hundreds of millions of base pairs.
    • Chromosomes usually occur in sets (e.g., diploid organisms have two sets).
    • Typically, several hundred to thousands of different genes are distributed throughout.
  • Replication and Features
    • Origins of replication are present every ~100,000 base pairs.
    • Each chromosome has a centromere for kinetochore protein recognition.
    • Telomeres at each end contain specialized sequences.
    • Common repetitive sequences near centromeres/telomeres and interspersed across chromosomes.

DNA Sequence Composition in the Human Genome

  • Classifications
    • Unique sequences: Code for proteins, enhancers, etc.
    • Repetitive DNA: Non-coding regions, regions of genes not found within coding sequences (exons).
    • Introns constitute an additional category.
    • Graph Data: Unique sequences constitute approximately 24%, while repetitive DNA includes a substantial 59%.

Transposable Elements (TEs)

  • Types of TEs
    • Transposons (TES): Move through simple transposition or retrotransposition processes.
    • Simple Transposition: Directly moves DNA segments.
    • Retrotransposition: Involves an RNA intermediate and reverse transcriptase.
  • Consequences of Transposition
    • Can lead to mutagenesis:
    • Causes chromosome breakage and rearrangements.
    • Induces mutations and may result in gene inactivation or alteration of gene expression.
    • Exon Shuffling: Inserts exons into existing gene sequences through TEs.

Abundance of Transposable Elements Across Species

  • Species and TE Genome Percentage:
    • Xenopus laevis: 77%
    • Zea mays: 60%
    • Homo sapiens: 45%
    • Mus musculus: 40%
    • Drosophila melanogaster: 20%
    • Caenorhabditis elegans: 20%
    • Saccharomyces cerevisiae: 12%
    • Escherichia coli: 4%
    • Note: Variation in TE abundance can exist among strains within a species.

Levels of DNA Packaging in Eukaryotes

  1. Nucleosome Structure
    • Comprised of histone octamers wrapped around DNA (146-147 bp).
    • Nucleosomes present the DNA as "beads on a string".
  2. 30 nm Fiber
    • Further coiling into a thicker structure, also known as the solenoid structure.
  3. Radial Looping
    • Loop domains anchored to protein filaments, creating higher-order structures.
  4. Metaphase Chromosomes
    • Highly condensed and accessible for individual chromosomal analysis during cell division.

Summary of Chromosome Features

  • Basic Structure: Telomeres, Centromeres, Euchromatin, and Heterochromatin.
  • Compaction Levels: From 2 nm (DNA helix) to 1400 nm chromosomal structures during metaphase, each stage involves significant structural formations to enable effective gene deployment and protection during cellular processes.