Simulation · Biology · Class 11
Crossing over: when does it make something new?
From the lesson Prophase I in Cell Cycle and Cell Division. Change the values and watch what happens.
The idea behind it
NCERT §10.4.1
- Prophase I is longer and more complex than the prophase of mitosis. By how the chromosomes behave it is split into five stages, in order: leptotene, zygotene, pachytene, diplotene and diakinesis.
- Leptotene: the chromosomes gradually become visible under the light microscope, and they keep compacting all through this stage.
- Zygotene: homologous chromosomes begin to pair up, a process called synapsis. Electron micrographs show that synapsis comes with a complex structure, the synaptonemal complex.
- Two homologues joined by synapsis form a bivalent, also called a tetrad; bivalents become clearer in the next stage. Leptotene and zygotene are short compared with pachytene.
- Pachytene: the four chromatids of each bivalent become distinct and clearly appear as tetrads. Recombination nodules appear; these are the sites where crossing over happens between non-sister chromatids of the homologues.
- Crossing over means homologous chromosomes swap pieces of genetic material. It is enzyme-mediated, the enzyme being recombinase, and it recombines the genetic material on the two chromosomes.
- Recombination is finished by the end of pachytene, and the homologues stay linked at the crossover sites.
- Diplotene begins when the synaptonemal complex dissolves. The recombined homologues of each bivalent tend to move apart, except where they crossed over; these X-shaped links are the chiasmata. In some vertebrates, an oocyte can stay in diplotene for months or even years.
- Diakinesis, the last stage, is marked by terminalisation of the chiasmata. Condensation of the chromosomes is now complete, and the meiotic spindle is assembled to get the homologues ready to separate.
- By the end of diakinesis the nucleolus has disappeared and the nuclear envelope has broken down; diakinesis is the transition into metaphase I.
More simulations in Cell Cycle and Cell Division
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