Crossing over: when does it make something new?
"Crossing over: when does it make something new?" is a free, interactive Class 11 Biology simulation for NEET, built on the lesson "Prophase I" in Cell Cycle and Cell Division. Top: one bivalent in pachytene, four chromatids joined at their centromeres (left). The red dot is the recombination nodule where two chromatids exchange everything beyond that point. Bottom: the four chromatids as they end up in the four cells after meiosis. Crossing over happens in pachytene between non-sister chromatids of homologues, by the enzyme recombinase. It gives two recombinant chromatids out of four, and that is one way meiosis adds variation.
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.
Questions
2 answers
- What does the "Crossing over: when does it make something new?" simulation show?
- Top: one bivalent in pachytene, four chromatids joined at their centromeres (left). The red dot is the recombination nodule where two chromatids exchange everything beyond that point. Bottom: the four chromatids as they end up in the four cells after meiosis.
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Prophase I" in Cell Cycle and Cell Division (Class 11 Biology).
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