Meiosis is a form of cell division that gives rise to genetic variation. The main role of meiosis is production of haploid gametes from diploid cells, through two divisions. The steps involved are:
Meiosis 1:
Prophase 1
- the chromosomes super coil, the nuclear envelope breaks down, and the spindle fibres form from the centrioles (allowing genetic material to be moved around the cell.)
-Then, the chromosomes undergo a process called crossing over= the exchange of sections of DNA between chromatids on homologous chromosomes (explained at bottom)
Metaphase 1
- Chromosomes line up at the equator of the cell (center)
- Chromosomes are attached to the spindle fibres by centromeres
- Each chromosome lies next to its homologous pair at the equator.
- Within this homologous pair, the chromosomes are randomly organised on either side of the equator- this is independent assortment.
Anaphase 1
- Each chromosome is pulled apart to opposite ends (poles) of the cell
- One of the pair of homologous chromosomes will end up on each side randomly
Telophase 1 + Cytokinesis
- Animal cells reform a nucleus for a short period of time after anaphase
- Also undergo cytoplasm splitting to form two new cells.
Meiosis 2
After Meiosis 1, we are left with two sets of segregated chromosomes
Prophase 2
- The DNA supercoils, the nucleus disintegrates and new spindle fibres form.
Metaphase 2
- the chromosomes line up at the centre through the attachment of spindle fibres to their centromeres
- Chromatids are randomly assorted to either side of the equator
Anaphase 2
- The genetically different chromatids are randomly segregated to opposite poles
Telophase 2
- Nuclei starts to reform+ cell splits
END OF MEIOSIS
Crossing over
= the exchange of sections of DNA between chromatids on homologous chromosomes in Prophase 1 of Meiosis.
- The homologous chromosomes line up and twist around each other, with the same genes in the same place.
- This wrapping can cause sections of chromatid to break off, which than can then be swapped over to the other homologous chromosome.
- This means that whilst crossed over chromosomes have the same genes as before, the alleles are shuffled around to different chromosomes- this leads to genetic variation.
No comments:
Post a Comment