Biology Cell Division Overview

Introduction

• Organisms need new cells for growth, replacement, and repair.
• New cells are formed through asexual reproduction, where one cell divides into two identical cells (e.g., mitosis and binary fission)
• Sexual reproduction involves the joining of two cells (e.g., egg and sperm) to create a new cell (zygote).

Cell Division

• Prokaryotes: Division occurs through binary fission.
  • Prokaryotes have a single circular DNA molecule.
  • DNA replicates, and each copy attaches to the cell membrane.
  • The cell pulls apart, separating the DNA copies.
  • Two identical daughter cells are formed.
• Eukaryotes: Division occurs when the cell reaches a certain size and energy stores are sufficient.
  • The process is controlled by the cell cycle.

The Cell Cycle

• The cell cycle represents the stages a cell goes through from division to the next division.
• It consists of three phases:
  • Interphase: The period between cell divisions, where the cell grows and copies its DNA.
  • Mitosis: Nuclear division, where the replicated chromosomes are separated into two daughter nuclei.
  • Cytokinesis: The division of the cytoplasm, resulting in two daughter cells.

Interphase

• G1 (Growth 1) Phase:
  • Cell grows and increases organelle production.
  • Prepares for DNA replication.
• S (Synthesis) Phase:
  • DNA amount is doubled.
  • Chromosomes are replicated to form sister chromatids.
• G2 (Growth 2) Phase:
  • Cell continues to grow and produce proteins and organelles needed for cell division.
  • Prepares for mitosis.

Mitosis

• Nuclear division that occurs in eukaryotes.
• Consists of four phases:
  • Prophase:
    • Chromatin condenses into chromosomes.
    • Nucleolus and nuclear membrane disappear.
    • Mitotic spindle forms from centrioles.
  • Metaphase:
    • Chromosomes align at the cell equator.
    • Centromeres are attached to kinetochore fibers.
  • Anaphase:
    • Sister chromatids are pulled apart by spindle fibers.
    • Each pole receives a complete set of original genes.
  • Telophase:
    • Chromosomes reappear as chromatin.
    • Nuclear envelope reforms around each set of chromosomes.
    • Nucleolus becomes visible.
    • Spindle breaks apart.

Cytokinesis

• The division of the cytoplasm.
• Involves the formation of a cleavage furrow in animal cells.
• Acts to separate the daughter cells after mitosis.

Regulation of the Cell Cycle

• Aims to prevent uncontrolled cell division.
• Controlled by cyclins and cyclin-dependent kinases (CDKs).
  • They form heterodimers that activate or deactivate target proteins.
• Checkpoints: Monitor and regulate the cell cycle to ensure proper progression.
  • G1 Checkpoint: Ensures cell is ready for DNA replication.
  • G2 Checkpoint: Ensures cell is ready for mitosis.
  • Metaphase Checkpoint: Ensures all chromosomes are attached to spindle fibers before anaphase.

Uncontrolled Cell Division

• Uncontrolled cell division leads to tumor formation.
• Defective p53 gene can cause loss of cell cycle control.
• Cancer cells lack p53 function, leading to uncontrolled growth and multiplication.

Cell Death

• Necrosis:
  • Unregulated cell death due to injury, disease, or blood supply failure.
• Apoptosis:
  • Programmed cell death, a regulated process for removing unwanted cells.

Necrosis vs Apoptosis

• Necrosis:
  • Uncontrolled cell death.
  • Causes cell swelling and lysis.
  • Can damage surrounding tissues.
• Apoptosis:
  • Controlled cell death.
  • Involves cell shrinkage and fragmentation.
  • Minimizes damage to surrounding tissues.

Apoptosis in Physiological Situations

• Important for embryogenesis, morphogenesis, and tissue remodeling.

Apoptosis in Adults

• Plays a role in tissue remodeling and hormone-regulated processes.

Apoptosis in Disease

• Too much apoptosis: Tissue atrophy, neurodegeneration.
• Too little apoptosis: Hyperplasia, atherosclerosis.

Why Cell Division is Important

• Cell division allows for growth and repair, enabling organisms to maintain their size and function.
• Cell division is more efficient than cell enlargement for maintaining cellular function.
• Larger cells have a lower surface area to volume ratio, making it difficult for nutrients and wastes to be exchanged efficiently.
• Cell division maintains a more optimal surface area to volume ratio, ensuring efficient function.

Explore the fascinating processes of cell division in organisms. This quiz covers asexual reproduction, binary fission in prokaryotes, and the cell cycle in eukaryotes. Test your understanding of how cells grow, replace, and repair themselves.