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Meiosis. An example of reduction division. During sexual reproduction, a gamete (23 chromosomes, X or Y) from the male organism and a gamete from the female organism (23 chromosomes) fuse to create a new cell. United Streaming: Mitosis and Meiosis Part 9. Prophase I.

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meiosis

Meiosis

An example of reduction division

slide2
During sexual reproduction, a gamete (23 chromosomes, X or Y) from the male organism and a gamete from the female organism (23 chromosomes) fuse to create a new cell.

United Streaming: Mitosis and Meiosis Part 9

slide3

Prophase I

  • Most of the significant processes of Meiosis occur during Prophase I
  • The chromosomes condense and become visible
  • The centrioles form and move toward the poles
  • The nuclear membrane begins to dissolve
  • The homologs pair up, forming a tetrad
    • Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid
  • Homologous chromosomes will swap genetic material in a process known as crossing over
    • Crossing over serves to increase genetic diversity by creating four unique chromatids

United Streaming: Mitosis and Meiosis Part 10-11

slide4

Crossing Over

  • Genetic material from the homologous chromosomes is randomly swapped
  • This creates four unique chromatids
  • Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased
metaphase i
Metaphase I
  • Microtubules grow from the centrioles and attach to the centromeres
  • The tetrads line up along the cell equator

United Streaming

anaphase i
Anaphase I
  • The centromeres break and homologous chromosomes separate (note that the sister chromatids are still attached)
  • Cytokinesis begins

United Streaming

telophase i
Telophase I
  • The chromosomes may decondense (depends on species)
  • Cytokinesis reaches completion, creating two haploid daughter cells

United Streaming

slide9
In Cytokinesis I, the cells finally split, with one copy of each chromosome in each one.
  • Prophase II, Metaphase II, Anaphase II, and Telophase II repeats the same steps as Prophase I-Telophase I, with half as much genetic material.
  • Cytokinesis II is the final step of meiosis, where each cell splits into two daughter cells, for a total of four gametes, each with half the number of chromosomes.