Cell division as the base for life cycles and alternation of generation
How can a meter-high corn plant arise from a seedling of just a few centimeters? How does it come that each child has iets own,individual properties?In other words, how does it come that each child, despite resemblances, is different from his/her parents and his/her borthers and sisters and other children?
Not even an attempt will be made to explain the how and why of these true miracles here. What will be presented in these webpages is a simplified description of how mitosis (normal cell division) and meiosis (special division leading tot the formation of gametes) according to the current scientific concept contribute to the formation of new cells, even entire new organisms, or also offspring through sexual reproduction. , ertoe bijdragen dat er nieuwe cellen en compleet nieuwe organisme ontstaan en ook nakomelingen via een stap van geslachtelijke voortplanting. The two types of cell division together form the base for the progress of life cycles and alternation of generation in protists, plants, animal and man.
As carriers of information for genetic characteristics and metabolism in cells, DNA molecules play a central role in cell division.
Mitosis: the "normal" cell division
The coming webpages will comment on the mechanisms of normal cell division, called mitosis, in which the original cell (called mothercell) and the two newly formed cells (called daughter cells, although the term is misleading because no alternation of generation occurs) are identical. Vegetative propagation by means of mitotic division can lead to the formation of new individuals which are then genetically identical to each other and the mother organism (e.g. plants arosen from cuttings are genetically identical to the "mother plant"). Daughtercells run through the cell cycle and generate new daughtercells or they follow the line of growth, differentiation and cell death.
Meiosis: reduction division
Furtheron, attention will be paid to meiosis, a two-step process that leads to the formation of gametes which carry the half of the number of chromosomes with respect to the mothercell (hence the name reduction division) and which also differ in genetical constitution with respect to that same original mothercell. The phenomenon of crossing-over
is the key feature for this second particularity. The path of sexual reproduction thus involves meiosis, fertilization
through fusons of a male and a female gamete and further mitotic divisions. In this way genetical variation among organims can occur in a population and offspring organims may be unique individuals.