What is the difference to mitosis?
Meiosis is very similar to mitosis in terms of the second meiotic division, but there are some differences between the two nuclear divisions. The result of the meiosis are germ cells with a simple set of chromosomes, which are suitable for sexual reproduction. In mitosis, identical daughter cells with a double set of chromosomes are formed.
These cells do not have the function of reproduction, but replace old, dead or no longer fully functional body cells. Another difference between meiosis and mitosis is the different number of divisions. In meiosis, two divisions are necessary.
In the first reduction division the two pairs of chromosomes are separated, in the following equational division the two sister chromatids are separated from each other. In contrast, one division is sufficient in mitosis. In this one division, the sister chromatids are separated so that two genetically identical daughter cells are formed.
Meiosis and mitosis differ not only in their function and the number of divisions, but also in their duration. Mitosis is a relatively fast process that takes about one hour. Meiosis, on the other hand, takes much longer and can also stagnate in one phase for several years or even decades.
This is the case with oocytes that are already created at birth, but are all in a dormant mode until they reach sexual maturity. The development of male gametes, the sperm, also takes about 64 days. Of these, about 24 days are devoted to meiosis. More information on this topic can be found here: Mitosis – Simply explained!
- Result
- Number of divisions
- Duration
What is the crossing-over?
Crossing-over describes the exchange of genetic material between two chromatids. In the process, the chromatids approach each other, cross over each other and then exchange certain DNA fragments. This process takes place during germ cell division (meiosis).
The crossing-over can be assigned to Prophase I, which can again be divided into five phases. Before the first division of meiosis begins, the DNA is doubled so that there are four chromatids in the cell. The first phase of prophase I is the leptotene, in which the chromosomes are condensed and thus become visible under the light microscope.Next comes the zygotene, in which the chromosomes converge and a homologous chromosome pairing occurs.
This spatial proximity of both chromosomes is the prerequisite for the exchange of genetic material. In parallel, the synaptonemal complex is formed. This is a complex of several proteins that forms between the chromosomes and ensures the exact position of the chromosomes.
In the following pachytän now the actual crossing-over takes place. Already in the two preceding phases, there were already breaks in the DNA. Now two chromatids cross over and the breaks are repaired.
During this process, the maternal and paternal chromosomes exchange small segments of the DNA. These crossings are visible under the light microscope as chiasmata. In the diplotene the synaptonemal complex dissolves and the chromosomes are only connected at the chiasmata. In the last phase of prophase I, diakinesis, the nuclear membrane dissolves, the formation of the mitosis spindle begins and meiosis can proceed in the usual sequence. The crossing-over serves the purpose of intrachromosomal recombination and, together with the random assignment of the genetic material to the germ cells, plays a soft role in the diversity of characteristics.
