Cell Division

Why is Cell Division Required?

Cell division is a fundamental biological process necessary for the growth, development, and maintenance of all living organisms. It enables organisms to reproduce, grow, and repair damaged tissues.

• Unicellular organisms (e.g., prokaryotes or protists):

  • Cell division gives rise to new individuals.

  • Serves as a method of reproduction.

• Multicellular organisms:

  • Cell division is essential for growth and development.

  • Allows for tissue repair and renewal.

  • Enables reproduction (e.g., formation of gametes).

• Example: A fertilized egg divides to form a multicellular embryo, and tissue repair occurs via cell division.

Types of Cell Division

Mitosis, Meiosis, and Cytokinesis

Cell division in eukaryotes involves highly organized processes to ensure accurate distribution of genetic material.

• Mitosis:

  • Distribution of identical genetic material (DNA) to two daughter cells.

  • Produces two daughter cells genetically identical to the parent cell.

• Meiosis:

  • Occurs in specialized cells (e.g., gametes).

  • Produces four non-identical daughter cells with half the genetic material of the parent cell.

• Cytokinesis:

  • Division of the cytoplasm, resulting in separate daughter cells.

• Example: Mitosis is used for growth and repair, while meiosis is used for sexual reproduction.

Bacterial Cell Division

Binary Fission

Bacteria, which lack a nucleus, divide by a simpler process called binary fission.

• Binary fission:

  • DNA is faithfully replicated.

  • The cell splits into two identical cells.

• Process:

  • DNA replication: DNA is copied and moves to opposite ends of the cell.

  • Septation: Formation of a septum (cell wall) divides the cell into two.

• Example: Each daughter cell receives an identical copy of the bacterial chromosome.

Chromosomes in Eukaryotes

Chromatin Structure and Organization

Eukaryotic chromosomes are composed of DNA associated with proteins, forming chromatin fibers that are further organized into chromosomes.

• Chromatin:

  • DNA associates with proteins to form chromatin fibers.

  • Chromatin fibers are the basis for chromosome structure.

• Genes:

  • Functional units of heredity located on chromosomes.

  • Humans have thousands of genes organized on chromosomes.

• Chromosome Packaging:

  • DNA is associated with histone proteins.

  • Chromosomes are organized into looped domains held together by non-histone scaffolding proteins.

• Example: Human chromosomes are visible under a microscope during cell division due to their condensed structure.

Chromosome States in the Cell Cycle

The structure and activity of chromosomes change depending on the stage of the cell cycle.

• Non-dividing cell:

  • Chromosomes are loosely packaged and extended as chromatin fibers.

  • Heterochromatin: Non-active regions of chromatin.

  • Euchromatin: Active regions of chromatin.

• Dividing cell:

  • Chromatin fibers condense to form visible chromosomes.

  • Condensin: Family of proteins that facilitate chromosome condensation.

• Example: Chromosomes become highly condensed and visible during mitosis.

Additional info:

• Chromosome Number: Human somatic cells are diploid (2n = 46 chromosomes), while gametes are haploid (n = 23 chromosomes).

• Homologous Chromosomes: Chromosomes exist in pairs, one inherited from each parent.