Stages of meiosis
Interphase
•Longest part of the cycle
•Cell increases in size
•Synthesis of organelles
•DNA copied, so doubled
•Checking of genetic material
•Chromosomes are not visible because chromosome material (chromatin) is dispersed through the nucleus.
Prophase I
- The chromatin condenses, becoming visible.
- The chromosomes can now be seen as x-shapes. Each side of the ‘x’ is an identical copies of each chromosome in the cell due to replication in interphase. These copies are referred to as sister chromatids and are attached by a centromere.
- The nuclear membrane and nucleolus disappears. The chromosomes line up in homologous pairs in the middle of the cell to form a bivalent.
4. The non-sister chromatids wrap around each other and attach at points known as chiasmata. They may swap sections of chromatids with one another in a process known as crossing over.
Metaphase I
- Bivalents line up across the equator of the cell, attached to the spindle fibres at the centromeres.
Anaphase I
- Chromosomes in each bivalent are pulled by the spindle fibres to opposite poles. The centromeres do not divide.
Telophase and Cytokinesis I
- The cell splits into two, each cell gets one chromosome from each pair. In animal cells, two new nuclear envelopes form.
Prophase II
- The cell splits into two, each cell gets one chromosome from each pair. In animal cells, two new nuclear envelopes form.
- Spindle fibres reform. The next division takes place at 90o to the first division.
Metaphase II
- The chromosomes arrange themselves on the equator of the cell.
- The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate.
Telophase and Cytokinesis II
- The two cells divide to give four haploid cells and the nuclear membrane reforms.
- Each new cell is DIFFERENT to the original and has HALF (n) the original number of chromosomes
During meiosis, what stage do the processes of crossing over and recombination occur within?