The Cell Cycle and Cell Division

G2 Phase (Gap 2)

Following DNA replication, the cell enters the G2 phase. Here, the cell replicates additional organelles and performs a final "quality control" check, assessing the newly replicated DNA for any errors. The cell's ability to progress from G2 into the M phase is regulated by the levels of the Maturation Promoting Factor (MPF); a sufficient concentration of this factor is required to trigger the next stage.

Exam Tip: Remember the mnemonic "Go, Sam, Go, Make, Cake" to recall the order: G1, S (Sam), G2, M (Mitosis), and C (Cytokinesis).

The M Phase: Division in Action

The M phase is the stage where the cell physically divides. This phase is composed of two distinct processes: karyokinesis and cytokinesis.

• Karyokinesis: The division of the genetic material (the nucleus). In eukaryotes, this occurs via mitosis (producing two identical diploid cells) or meiosis (producing four genetically distinct haploid cells).
• Cytokinesis: The division of the cytoplasm and the physical splitting of the cell into two separate entities.

Prokaryotic vs. Eukaryotic Division

It is important to distinguish between how different organisms divide. While eukaryotes use mitosis or meiosis, prokaryotes (like bacteria) undergo binary fission. In binary fission, the genome replicates simultaneously with the cell division, and there is no separate S phase or microtubule organizing center.

The Machinery of Division: Centrosomes and Microtubules

In animal cells, the complex movement of chromosomes during karyokinesis is facilitated by Microtubule Organizing Centers (MTOCs). The primary MTOC in animal cells is the centrosome.

Centrosomes and Centrioles

The centrosome replicates during the S phase so that each future daughter cell receives one. It is composed of a pair of centrioles, which are hollow cylinders arranged in a specific "$9 \times 3$" pattern (nine triplets of microtubules). Surrounding these centrioles is the pericentriolar material, which is responsible for microtubule nucleation.

The Role of Microtubules

Microtubules are structural fibers made of the protein tubulin. They form the spindle apparatus, which acts as a guide for chromosomes during division. There are three functional types of microtubules:

1. Polar Microtubules: These connect the two centrosomes and push them toward opposite sides of the cell.
2. Astral Microtubules: These attach to the cell membrane to help orient the centrosome within the cell.
3. Kinetochore Microtubules: These attach directly to the kinetochore (a protein structure on the chromosome) to pull the genetic material apart during karyokinesis.

Summary Table: Mitosis vs. Meiosis

To help distinguish between the two types of eukaryotic karyokinesis, refer to the following comparison:

Note on Ploidy: Diploid ($2n$) means the cell has two sets of chromosomes (one from each parent), while Haploid ($n$) means it has only one set.

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References Ch. 5_ Cell Division Handout.pdf