Prokaryotic and Eukaryotic Genome Structure

Prokaryotic Genome Structure

• Prokaryotes have a single, circular chromosome located in the nucleoid region.
• They lack a membrane-bound nucleus.
• DNA is compacted by supercoiling and proteins, not histones.
• Plasmids, small independent DNA molecules, may also be present.

Eukaryotic Genome Structure

• Eukaryotic genomes consist of multiple linear chromosomes enclosed within a nucleus.
• Chromosomes are associated with histones to form chromatin.
• The DNA structure is highly organized and changes throughout the cell cycle.
• Non-coding DNA, including introns and regulatory sequences, and repetitive elements are present.

Cell Cycle Phases

• Interphase (90%):
  • G1 Phase: Cell growth, normal function, protein and organelle synthesis, preparation for DNA replication.
  • S Phase: DNA replication, formation of sister chromatids.
  • G2 Phase: Cell prepares for mitosis, synthesis of microtubules and proteins, DNA error checking, organelle duplication.
• M (Mitotic) Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).
• G0 Phase: Quiescent stage, cell exits cycle, non-dividing cells.

Mitosis Phases

• Prophase: Chromatin condenses, mitotic spindle forms, nuclear envelope breaks down.
• Prometaphase: Nuclear envelope fragments, spindle fibers attach to kinetochores.
• Metaphase: Chromosomes align at the metaphase plate, spindle fibers are fully formed.
• Anaphase: Sister chromatids separate, kinetochore microtubules shorten, non-kinetochore microtubules elongate the cell.
• Telophase: Chromosomes decondense, nuclear envelopes reform, spindle fibers disassemble.

Mitotic Spindle Structure

• Centrosomes: Microtubule-organizing centers forming spindle poles.
• Kinetochore Microtubules: Attach to kinetochores, pull chromosomes in anaphase.
• Non-Kinetochore Microtubules: Overlap, push against each other, elongate the cell.
• Asters: Short microtubules radiating from centrosomes, anchoring them to the plasma membrane.

Cytokinesis Differences (Animals vs. Plants)

• Animals: Cleavage furrow (actin and myosin filaments) forms, plasma membrane pinches inward.
• Plants: Cell plate forms in the middle of the cell. Vesicles from Golgi fuse to create the cell plate, which eventually becomes a new cell wall.

Binary Fission in Bacteria

• Steps:

  • Chromosomal replication at the origin of replication.
  • Two DNA copies move to opposite ends.
  • Inward pinching of the plasma membrane, formation of a new cell wall.

• Evolutionary Perspective: Binary fission is the ancestral process for mitosis. Protein similarities exist between bacterial chromosome segregation and mitotic spindle proteins. Complexity increased in eukaryotes with linear chromosomes and a nuclear envelope.

This quiz covers the fundamental differences between prokaryotic and eukaryotic genome structures, including chromosome organization and DNA packaging. It also explores the phases of the cell cycle and the key processes that occur during each phase, providing a comprehensive overview of cellular biology.