Cell Division¶

Part of Cell Biology.

Cell division explains how organisms grow, repair damage and replace worn-out cells. The central idea is that genetic information must be copied accurately before one cell becomes two.

What You Need to Learn¶

Growth and repair depend on copying genetic information before cells divide. Follow how chromosomes are shared during the cell cycle, then compare the possibilities offered by embryo, adult and plant stem cells. Weigh their uses in treatment and plant propagation against biological risks and ethical concerns.

Chromosomes¶

Chromosomes are long DNA molecules found in the nucleus. Genes are short sections of DNA carried on chromosomes.
Human body cells contain chromosomes in pairs, with one chromosome of each pair inherited from each parent.
Before a cell divides, the DNA must be copied so each new cell can receive a full set of genetic instructions.

The Cell Cycle¶

The cell cycle is a continuous process consisting of three main stages: interphase, mitosis and cytokinesis.
The length of the cell cycle varies between organisms and cell types. Dividing cells in hair follicles, skin and the gut lining, together with blood-forming cells in bone marrow, replace cells frequently. Mature red blood cells themselves do not divide.

Interphase¶

Interphase is the longest stage of the cell cycle during which the cell grows, replicates its DNA and produces more organelles.
DNA replication ensures that when the cell divides, each daughter cell receives a complete copy of the genetic instructions.
Organelle production is essential: mitochondria are made (needed for energy during growth), and ribosomes are made (needed for protein synthesis to support cell growth).
The cell membrane extends outward and the cytoplasm grows. The cell becomes larger and doubles its DNA content, preparing for division.

Mitosis and Cytokinesis¶

Before mitosis, DNA replication produces two copies of each chromosome.
During mitosis, one copy of every chromosome moves to each end of the cell, and the nucleus divides. Each new nucleus receives the same complete set of genetic information.
Finally, the cytoplasm and cell membrane divide in cytokinesis, producing two genetically identical daughter cells. This allows growth, repair and replacement while preserving the genetic instructions.
If the DNA is not copied accurately, new cells may not function properly, which is why control of the cell cycle matters.

Stem Cells¶

Stem cells are unspecialised cells that can divide by mitosis to form more stem cells, and then differentiate into specialised cell types.
For organisms to grow and repair damage, cells must divide (through mitosis) and differentiate. In plants, elongation also occurs at the root and shoot tips.
Embryonic stem cells are found in early embryos and are completely undifferentiated. They can become virtually any cell type and are pluripotent (can form many types).
Adult stem cells are found in limited locations such as bone marrow and teeth. They can only differentiate into a limited range of cell types and are multipotent (e.g., forming different blood cells).
Plant stem cells are found in meristems (growing regions) and can keep dividing and differentiating throughout the plant's life.
Stem cells offer possible medical benefits such as replacing damaged tissues and treating diseases, but their use raises ethical questions (especially regarding embryonic stem cells) and biological challenges (controlling their differentiation, ensuring safety).

Cultured embryonic stem cells could replace insulin-producing cells in diabetes or damaged nerve cells in paralysis. Therapeutic cloning aims to provide embryonic cells with the patient’s genes, reducing immune rejection. Possible transfer of viruses remains a risk, and using embryos raises ethical or religious objections.

Meristem cells can generate any plant cell type and can produce large numbers of genetically identical plants quickly. This allows propagation of rare species and of crops with useful characteristics such as disease resistance.

Common Confusions¶

Mitosis vs. division: Mitosis is nuclear division; cytokinesis is cytoplasmic division. Mitosis must happen before the cell can physically split into two.
Embryonic vs. adult stem cells: Embryonic stem cells are pluripotent (can become almost any cell); adult stem cells are multipotent (limited to certain cell types). This difference limits their medical use.
Copying vs. sharing DNA: DNA is copied before mitosis. Mitosis shares those copies between the two new nuclei rather than making the copies.

Key Terms¶

Chromosome: a structure made of DNA that carries genes.
Gene: a short section of DNA that codes for a particular characteristic or protein.
Mitosis: nuclear division that produces two genetically identical nuclei.
Cell cycle: the sequence of growth, DNA replication and division in a cell.
Interphase: the stage of the cell cycle during which the cell grows, replicates DNA and produces organelles.
Cytokinesis: the final stage where the cytoplasm and cell membrane divide to form two separate cells.
Stem cell: an unspecialised cell that can divide and become specialised.
Pluripotent: able to differentiate into almost any cell type (describing embryonic stem cells).
Multipotent: able to differentiate into a limited range of cell types (describing adult stem cells).
Meristem: plant tissue containing cells that can divide and differentiate.