Cell Division: Stages of Meiosis | A-level Biology | OCR, AQA, Edexcel

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The key points covered of this video include:

1. Meiosis Overview
2. Prophase 1
3. Metaphase 1
4. Anaphase 1
5. Telophase 1 and Cytokinesis
6. Prophase 2
7. Metaphase 2
8. Anaphase 2
9. Telophase 2

Meiosis Overview

Meiosis is the process of creating haploid cells - it does this through two divisions. The first division - meiosis 1 - comes just after chromosomes have duplicated in interphase. In meiosis 1, homologous chromosomes are separated from each other. A short time after the first division meiosis 2 happens - in meiosis 2 the sister chromatids from each chromosome are separated. At the end of meiosis 2 we end up with 4 sets of genetically different cells, each with a haploid number of chromatids in them.

Prophase 1

Prophase 1 is the first stage in meiosis. The chromosomes supercoil, the nuclear envelope breaks down, and the spindle fibres form from the centrioles. Then the chromosomes undergo a process called crossing over. Crossing over is the exchange of DNA between chromatids on homologous chromosomes in prophase 1 of meiosis. In crossing over, sections of DNA are exchanged between homologous chromosomes. The homologous chromosomes line up and twist around each other. The wrapping can cause sections of chromatid to break off, they can then be swapped over to the homologous chromosome. Because the chromosomes are homologous, crossed over chromosomes have the same genes as before. Genetic variation is introduced in crossing over since all of the alleles are shuffled around on different chromosomes.

Metaphase 1

The next stage of meiosis 1 - metaphase 1 - involves chromosomes lining up at the equator. The first step involves the attachment of the chromosomes to spindle fibres by the centromeres. Each chromosome lies next to its homologous partner at the equator. Within the homologous pair, the chromosomes are randomly organised on either side of the equator - this is independent assortment.

Anaphase 1

The next stage of meiosis 1 - anaphase 1 - involves the separation of homologous chromosomes to opposite poles. In anaphase 1 each chromosome from the homologous pair is pulled apart to opposite ends of the cell. One out of the pair of homologous chromosomes will end up on each side - which of them it is, is random.

Telophase 1 and Cytokinesis

The next stage of meiosis 1 - telophase 1 - involves the separation of homologous chromosomes to opposite poles. Animal cells reform a nucleus for a short period after anaphase 1 in telophase 1. Animal cells also undergo cytoplasm splitting in cytokinesis. Plant cells on the other hand go directly into the next stage of meiosis without splitting.

Prophase 2

After meiosis 1 we are left with two sets of segregated chromosomes. In prophase 2 the DNA supercoils, the nucleus disintegrates and a new spindle fibres form. This is the same as mitosis - except that the chromatids on each chromosome are no longer identical due to crossing over.

Metaphase 2

In metaphase 2 the chromosomes line up at the centre through the attachment of spindle fibres to their centromeres. Chromatids are randomly assorted onto either side of the equator - they will be pulled to the poles in anaphase 2.

Anaphase 2

In anaphase 2 the genetically different chromatids are randomly segregated to opposite poles.

Telophase 2

After anaphase 2 the nuclei start to reform in telophase 2 and the cell splits in cytokinesis. In animals the two cells split in two - making four cells. In plant cells the nuclei aren't separated by plasma membrane yet, so the cell splits four ways to make four cells.

Summary

Meiosis involves the splitting of a diploid cell into four haploid cells in two phases
In prophase 1 homologous chromosomes cross over - shuffling the alleles
In the rest of meiosis 1 the chromosomes are independently assorted to opposite poles
In meiosis 2 the chromatids are separated to form four genetically different haploid cells

Видео Cell Division: Stages of Meiosis | A-level Biology | OCR, AQA, Edexcel канала SnapRevise