Meiosis What does this picture tell us about meiosis? How is the purpose of meiosis different from the purpose of mitosis?

Why is meiosis necessary? Why can’t mitosis be the cell division process that runs sexual reproduction? What would happen to the chromosome number from one generation to the next?

In sexual reproduction, meiosis and fertilization are complementary processes; Meiosis produces the haploid chromosome number (n) and… Fertilization restores the diploid (2n) chromosome number in the zygote. Meiosis is often called ‘Reduction Division’ since the process reduces the number of chromosomes in half (2n  n) (Mitosis would be 2n  2n or n  n)

Meiosis involves two divisions Each division includes prophase, metaphase, anaphase, and telophase. A Roman Numeral is added to each phase. Prophase I  Metaphase I  Anaphase I  Telophase I …  Interphase (no DNA replication) …  Prophase II  Metaphase II  Anaphase II  Telophase II

Stage Descriptions/Characteristics Prophase I Synapsis occurs – Homologous chromosomes pair up Each group is made up of 4 chromatids. Each group of 4 chromatids is called a… TETRAD.

Crossing Over When parts of non-sister chromatids change places… (increases genetic variety)

Metaphase I The tetrads are aligned at the equator. [note the crossing over…]

Anaphase I Tetrads split and chromosomes (in replicated form) move to opposite poles.

Telophase I Two cells form – each cell is haploid but each chromosome is in replicated form.

Interphase (INterkinesis) DNA is in chromatin form. No DNA replication occurs! Why?  because each cell contains chromosomes already in replicated form – no need to copy…

Prophase II Each cell is haploid with chromosomes in replicated form.  note the Crossing over…

Metaphase II Chromosomes align at the equator in both cells.

Anaphase II Chromosomes separate – Chromatids move towards opposite poles. (Centromeres split.)

Telophase II 4 cells result – each cell is haploid with unreplicated chromosomes. Each cell will become a gamete (sperm/egg)

What phase of meiosis?

Telophase I or Telophase II?

Meiosis I or II just like Mitosis?