Mitosis is a cell division in which two daughter cells arise. Each time a cell divides, the DNA double helix splits along into two single strands. Every strand serves as a template for a new counter strand until two complete helices have been created from the original double helix. Thus, there are two exact copies of the “old” DNA strand that remain connected at one point, the centromere. This process takes place (in parallel) in all chromosomes of a cell and prepares the cell division. In the next step, the nuclear membrane that separates the cell nucleus from the cytoplasm dissolves and the chromosomes store themselves next to each other in one plane. The two identical chromosome strands, which are called chromatids, are separated and pulled apart at the centromere. They are each drawn into a different half of the cell. Subsequently, a new cell membrane is formed between the cell halves, resulting in two daughter cells that are equipped with identical genes.

In this way, new body cells are created when an organism grows and develops its organs or when cells have to be replaced in a mature body. However, shoots, offshoot, brood bulbils and the brood buds of bryophyllum are also formed during such cell divisions (for the specialization of cells, see Section 2. 2). Completely new plants can grow from these specialized plant parts but also from others like e.g. cuttings. In doing so, the daughter plants are genetically identical to the mother plant. Even if one speaks of “mother plant”, there is no “father plant“ in this special case. Not only plants, but also some animals form offspring in this manner. For example, the females of some insect species, such as certain aphids and gall wasps, form eggs through the mitotic cell division described above, from which genetically identical young females emerge.


As described in Section 1.1, body cells normally contain a double set of chromosomes, but gametes (oocytes, sperm) contain a single set. This means that in cell divisions used to form gametes, the chromosome pairs must be separated. The duplication of DNA in this form of cell division largely corresponds to that of mitosis. However, the chromosomes do not all accumulate in the same plane. Instead, the pairs of chromosomes bundle on each other and arrange themselves as pairs in the plane so that a bilayer is formed. The chromosomes frequently exchange sections with each other, often for only one of the two chromatids.

This is done by the chromatids of a chromosome pair crossing and assemble each other new during the pairwise arrangement in the equatorial plane (see Fig. 109). This process is called cross-over. During cell division, the chromosome pairs are separated. Which chromosome of each pair reaches which cell is left to chance. The number of chromosomes per cell is halved by this division (meiosis I). In a further division cycle (meiosis II), which is similar to mitosis, the two chromatids of each chromosome are distributed to daughter cells. Because of the cross-over and the random distribution four genetically different gametes are produced.

Pic 109: Meiotic cell division occurs in two phases. Red chromosomes come from one

parent, blue from the other. The chromosomes are separated at the first division (Fig. 1-5) and randomly distributed to the daughter cells. The number of chromosomes per cell is halved. In a further division cycle similar to mitosis, the two chromatids of each chromosome are distributed to daughter cells.

(Source: Grafik von Ali Zifan - Eigenes Werk; Used information from Campbell Biology (10th Edition) by: Jane B. Reece & Steven A. Wasserman., CC-BY-SA 4.0,