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In this schapter of AS/A-Level Biology, we will learn about the process and significant of meiosis
Prophase I
- Chromatin condenses, supercoil, shorten and thicken. Come together in homologous pairs forming bivalents
- Non-sister chromatids wrap around each other and attach at chiasmata where crossing over may occur.
- Nucleolus disappears, nuclear envelope disintegrates.
- Spindle forms.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/403894_596279.jpeg)
Metaphase I
- Bivalents line up across the equator of the spindle attached to fibres at centromeres.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/436184_160439.jpeg)
Anaphase I
- Homologous chromosomes pulled by spindle fibres to opposite poles—centromeres do not divide.
- Chiasmata separate.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/630718_956005.jpeg)
Telophase I
- two new nuclear envelopes form around each set of chromosomes and the cell divides by cytokinesis. Brief interphase and chromosomes uncoil.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/846478_652158.jpeg)
Prophase II
- Nuclear envelope breaks down again.
- Nucleolus disappears, chromosomes condense, spindles form.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/771452_224134.jpeg)
Metaphase II
- Chromosomes arranged on equator of spindle, attached by centromeres.
- Random assortment
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![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/415475_391644.jpeg)
Anaphase II
- Centromeres divide, chromatids pulled to opposite poles by spindle fibres.
- Chromatids randomly segregate.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/955169_826065.jpeg)
Telophase II
- Nuclear envelopes reform around the haploid daughter nuclei.
- two cells now divide to give four haploid cells.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/555670_722606.jpeg)
This is the end of this topic
Drafted by Harris