Cell Biology Fundamentals: Cycle, Control, and Structure

Posted on Dec 28, 2025 in Biology

Cell Cycle: Growth, Replication, and Division

The cell cycle is an ordered sequence of events enabling a cell to grow, replicate its DNA, and divide into two genetically identical daughter cells, ensuring accurate genetic transmission. It comprises interphase (G1, S, G2 phases) and the mitotic (M) phase (mitosis and cytokinesis).

  • G1 Phase: The cell grows metabolically, synthesizing proteins and organelles while assessing environmental conditions at the restriction point before committing to division.
  • S Phase: Involves DNA replication, duplicating each chromosome into two sister chromatids.
  • G2 Phase: Prepares for mitosis by producing RNA, proteins for spindle formation, and checking DNA integrity at the G2 checkpoint.
  • M Phase: Includes prophase (chromosome condensation, nuclear envelope breakdown), metaphase (chromosome alignment at the equator, metaphase checkpoint), anaphase (sister chromatid separation via shortening microtubules), telophase (nuclear reformation), and cytokinesis (cytoplasmic division). [1]

Components of Cell Cycle Control System

The control system relies on cyclin-dependent kinases (CDKs) and cyclins, which form complexes activating phase transitions. Cyclins fluctuate in concentration:

  • G1 cyclins pair with CDK4/6 for G1/S progression.
  • S-phase cyclins pair with CDK2 for DNA replication.
  • Mitotic cyclins (cyclin B) pair with CDK1 for M phase entry.

Checkpoints enforce order:

  1. G1 checkpoint: Verifies cell size, nutrients, and DNA damage.
  2. G2 checkpoint: Ensures complete replication.
  3. Metaphase checkpoint (Spindle Assembly): Confirms chromosome attachment to spindles.

Proteins like p53 detect damage, halting the cycle for repair, while the anaphase-promoting complex (APC) ubiquitinates securin and cyclins for degradation, triggering anaphase. [4][9][1]

Intracellular and Extracellular Control of Cell Division

Intracellular controls monitor internal states via tumor suppressors (e.g., Rb protein inhibits E2F transcription factors until phosphorylated by G1 CDK-cyclin) and proto-oncogenes (e.g., cyclin D overexpression accelerates progression). DNA damage activates ATM/ATR kinases, stabilizing p53 to induce repair genes or apoptosis.

Extracellular controls involve growth factors (e.g., PDGF, EGF) binding receptor tyrosine kinases, activating Ras-MAPK pathways that upregulate cyclin D expression, promoting G1 progression. Contact inhibition via cadherins halts division at high density, and nutrients like glucose influence division via the TOR pathway. [7][1][4]

Programmed Cell Death (Apoptosis)

Apoptosis eliminates unnecessary or damaged cells through an energy-dependent cascade, preventing inflammation unlike necrosis.

  • Intrinsic pathway (mitochondrial): Releases cytochrome c upon stress (DNA damage, hypoxia), forming the apoptosome with Apaf-1 and activating initiator caspase-9, which cleaves executioner caspases (3,6,7) leading to DNA fragmentation, chromatin condensation, membrane blebbing, and apoptotic body formation.
  • Extrinsic pathway: Uses death receptors (e.g., Fas) recruiting FADD and caspase-8.

Inhibitors like the Bcl-2 family regulate this balance; phagocytosis by macrophages clears debris. Dysregulation links to cancer (e.g., p53 mutations) or neurodegeneration. [9][1][4]

Discovery of Cell and Cell Theory

The discovery of the cell began in 1665 when Robert Hooke first observed and described cell-like compartments in cork using a primitive microscope, coining the term “cells.” Antonie van Leeuwenhoek later observed living microscopic organisms (protozoa, bacteria) in 1674, expanding cellular awareness.

In 1838, Matthias Schleiden proposed that all plants are composed of cells, followed by Theodor Schwann in 1839 who extended this idea to animals. Rudolph Virchow, in 1855, established that all cells arise from pre-existing cells, culminating in the Cell Theory. This fundamental biological principle states:

  1. All living organisms are composed of cells.
  2. The cell is the basic unit of structure and function in organisms.
  3. All cells come from pre-existing cells by division. [1][2][4][7]

Structure and Function of Cell Components

Protoplasm

Protoplasm is the living substance within a cell, comprising the cytoplasm and nucleoplasm. It contains organelles, cytosol, and inclusions, facilitating metabolic activities, growth, and response to stimuli.

Cell Wall

A rigid extracellular structure mainly in plants, fungi, and bacteria, composed of cellulose in plants. It provides mechanical support, shape maintenance, and protection against osmotic pressure.

Plasma Membrane

A selectively permeable phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It controls the movement of substances in and out, helps cell communication, and maintains the internal environment. Modifications include glycoproteins for cell recognition and lipid rafts for signaling platforms.

Intracellular Junctions

  • Tight junctions: Seal adjacent cells, preventing leakage.
  • Adherens junctions: Connect actin cytoskeletons of neighboring cells.
  • Desmosomes: Anchor intermediate filaments for mechanical stability.
  • Gap junctions: Enable direct cytoplasmic communication for molecule passage.

Cytoskeleton

A dynamic network of:

  • Microtubules (tubulin): Provide structural support, intracellular transport, and form spindle fibers in mitosis.
  • Microfilaments (actin): Enable cell movement and cytokinesis.
  • Intermediate filaments: Offer tensile strength and maintain cell integrity.

Mitochondria

Double-membraned organelles responsible for ATP production via oxidative phosphorylation. The inner membrane folds into cristae increasing surface area, while the matrix contains mitochondrial DNA and enzymes for the Krebs cycle.

Chloroplast

Found only in plants and algae, chloroplasts have double membranes enclosing thylakoids stacked into grana where light-dependent reactions of photosynthesis occur. The stroma contains enzymes for the Calvin cycle and their own DNA, supporting the endosymbiotic origin theory.

Endoplasmic Reticulum (ER)

  • Rough ER: Studded with ribosomes, synthesizing and modifying proteins destined for membranes or secretion.
  • Smooth ER: Synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons.

Golgi Complex

Composed of flattened cisternae, the Golgi apparatus modifies, sorts, and packages proteins and lipids from the ER, preparing them for transport to destinations like lysosomes or the plasma membrane.

This comprehensive view integrates the historic discovery of cells, the foundational cell theory, and detailed structures and functions of key cellular components essential for life processes. [2][4][12][13][1]