Growth and reproduction of cells Knowledge-Based Study

Study notes

1. Why Cells Must Divide

   As a living cell metabolizes and grows, its volume expands substantially faster than its surface area. The plasma membrane provides the only portal through which oxygen, nutrients, and waste materials can exchange with the environment. When a cell becomes excessively large, its decreased surface area-to-volume ratio restricts the rate of exchange, making it impossible to support the metabolic demands of the interior cytoplasm.

   Additionally, a single nucleus contains a limited supply of genetic instructions. Beyond a critical volume, the cell encounters DNA overload (or genomic limitation), where the nucleus can no longer transcribe RNA fast enough to direct all cellular operations. To maintain efficient transport and genetic control, growing cells must replicate their genetic material and divide.

   Cellular Growth and Replication

   The cell cycle consists of ordered stages that prepare a cell for reproduction. A typical dividing eukaryotic cell spends roughly 90% of its life in Interphase, which consists of three distinct phases:

   • G1 Phase (First Gap): The cell experiences rapid physical growth, synthesizes RNA and essential structural proteins, and doubles many of its cytoplasmic organelles.
   • S Phase (Synthesis): The cell replicates its nuclear DNA. At the start of this phase, each chromosome consists of a single linear DNA molecule; by the end, each consists of two genetically identical sister chromatids held together at the centromere.
   • G2 Phase (Second Gap): The cell completes final metabolic preparations, manufactures tubulin and other proteins required for spindle assembly, and duplicates its centrosome.

   Nuclear and Cytoplasmic Division

   Cell reproduction culminates in the M Phase, which involves two coordinated processes:

   1. Mitosis (Nuclear Division)

   Mitosis accurately segregates the duplicated chromosomes into two identical nuclei through four sequential stages:

   • Prophase: Chromatin condenses into visible chromosomes, nucleoli disappear, and the mitotic spindle forms.
   • Metaphase: The spindle apparatus aligns all sister chromatid pairs along the equatorial plane (metaphase plate).
   • Anaphase: Cohesin proteins are cleaved, allowing sister chromatids to separate and move toward opposite spindle poles as individual chromosomes.
   • Telophase: Daughter nuclei assemble, nuclear envelopes reform, and chromatin decondenses.

   2. Cytokinesis (Cytoplasmic Division)

   Cytokinesis physically splits the parent cell into two separate daughter cells. In animal cells, a contractile ring of microfilaments pinches the membrane inward to form a cleavage furrow. In plant cells, rigid cell walls prevent furrowing; instead, Golgi-derived vesicles fuse along the division plane to construct a cell plate, which matures into a new cell wall.

   Signals and Checkpoints

   The progression of growth and division is strictly governed by an internal molecular control system that ensures critical cellular events are completed accurately before the next step begins:

   • G1 Checkpoint (Restriction Point): Evaluates external growth factors, cell size, nutrient reserves, and DNA integrity before committing the cell to replicate its genome.
   • G2 Checkpoint: Confirms that DNA replication during the S phase was complete and accurate before entry into mitosis.
   • M Checkpoint (Spindle Checkpoint): Verifies that all kinetochores are securely attached to spindle microtubules during metaphase before triggering anaphase.

   These transitions are driven by cyclical regulatory proteins called cyclins, which bind to and activate cyclin-dependent kinases (Cdks) to phosphorylate target proteins. Loss of cycle control can lead to unchecked cellular proliferation, characterizing neoplastic disorders such as cancer.