Binary Fission > Transverse Binary Fission

When a unicellular organism divides across the transverse (horizontal) axis, it is called transverse binary fission.

or

Transverse binary fission is an asexual reproduction process where the plane of division runs along the transverse (horizontal) axis of the individual, splitting it into two parts.

Example: Paramecium

Paramecium (Protozoa):

• Nuclear Division: The Paramecium contains two types of nuclei. During fission, the larger macronucleus divides amitotically, while the smaller micronucleus divides mitotically (passing through all stages of mitosis).
• Segregation: The division results in two macronucleoli and two micronucleoli. One pair (one macronucleus and one micronucleus) moves to the anterior (front) part of the cell, and the other pair moves to the posterior (back) part.
• Cytokinesis: A constriction appears in the middle of the Paramecium's body along the transverse axis.
• Formation of Daughter Cells: The constriction deepens until the cell splits, forming two independent daughter paramecia.

Paramecium: stages in transverse binary fission

Planaria (Platyhelminthes):

• Constriction: During fission, a constriction appears slightly behind the pharynx area of the flatworm.
• Division and Regeneration: The body splits along this transverse plane into two pieces. Each of the two resulting parts then regenerates its missing body segments as it grows into a complete individual.

Planaria: stages in transverse binary fission

• Transverse binary fission is an asexual reproduction process in which an organism divides horizontally along its transverse axis to produce two distinct individuals.
• In Paramecium, the larger macronucleus divides amitotically while the smaller micronucleus divides mitotically, distributing one pair of each to the anterior and posterior ends of the cell.
• Following nuclear division, a horizontal constriction forms in the middle of the Paramecium and deepens until the cell splits completely into two independent daughter paramecia.
• Planaria (flatworms) also utilise this method by developing a constriction behind the pharynx and splitting in two, with each resulting half regenerating its missing segments to form a complete organism.