Organisms, from little moss to man, go during their life through a number of biological phases, generation after generation. The following "biological phases" are encountered during the life cycle
: the formation of gametes
(the sexual reproductive cells), the fusion of male and female gametes (~ fertilization
) to a zygote
, and a period of growth and development (cell differentiation and morphogenesis) that occurs at various time sequences, depending on the phylogenetic group to which the organism belongs. Within the life cycle, meiotic divisions (more
) take place, in which cell proceed from a diploid
to a haploid
stage; in the diploid stage two homologous copies of each chromosome are present in the cells and in the haploid stage only one sample of each chromosome in present in each cell. Gametes are always haploid, whereas zygotes are diploid. Furthermore, during the life cycle mitotic divisions
about mitosis). Depending on the phylum to which the organism belongs, these mitotic divisions occur in the haploid, or diploid phase, or in both the haploid and diploid stages. More here about here below.
In these web modules one finds descriptions of the life cycles of brown, red and green algae and that of various phyla of land plants. The characteristics are shown according to typical representants of each group, the same as those that are discussed in the course "Evolution and Development of the Plant" (Radboud University Nijmegen). The classification and terminology in these webpages follows the handbook that is used during lectures and practical courses: "Biology, Campbell, Reece, Urry, Cain, Wasserman, Minorsky en Jackson, 2008. Pearson International Edition (eds).San Francisco. 1267 pp".
|Classification of brown, red, green algae and land plants within the Eukaryotes
|Zoom view of the classification of the Eukaryotes|
Zoom view of the classification of Land plants
Phyla that are discussed in this module are indicated by a red dot
Three types of life cycles can be distinguished on account of the timing of the mitoses, in the haploid and/or diploid phase:
In diplonts mitotic divisions only occur in diploid cells. Gametes (arisen through meiosis) are the only occurance of the haploid phase. The diploid zygote formed after fertilization can however divide mitotically. This process results in the production of a multicellular diploid organism or in the production of many diploid single cells.
Animals, for example, belong to the diplonts.
In haplonts the mitoses only occur in haploid cells. This can result in the formation of single haploid cells or a multicellular haploid organism. The haplontic life forms produce the gametes through mitosis. After fertilization a zygote is formed: this cell is the only diploid cell in the entire life cycle. It is thus that same zygotic cell that later undergoes meiosis.
A haploid life cycle is found in most fungi and in some green algae like Chlamydomonas.
In haplo-diplonts the mitoses occur in both diploid and haploid cells. Such organims go during their life cycle through a phase in which they are multicellular and haploid (the gametophyte), and a phase in which they are multicellular and diploid (the sporophyte). The phenomenon is called "alternation of generation".
This haplo-diplontic type of cycle is found in all land plants and in many algae.
This series webpages on the life cycle in plants is based on course notes and other teaching material of Mieke Wolters-Arts, Celestina Mariani, Jan Derksen, the late G. van den Ende and Luud J.W. Gilissen. Dr Gerard van den Ende had developed an extended manual enriched with numerous drawings of life cycles, which went along with the practical courses that he taught. The microscopical work and the layout are by Liesbeth Pierson (contact
), Wilbert Janssen and Ines Schulten. The web structure was developed by Remco Aalbers.