Meiosis Formation of Gametes
Facts About Meiosis Similar in many ways to mitosis, but division occurs twice Original cell is diploid (2n), resulting cells are haploid (n) Results in 4 non-identical cells, each with ½ the number of chromosomes Start with 1 cell with 46 double-stranded chromosomes (2n), then divides to make 2 cells with 23 double stranded chromosomes (n), then divides a second time to make 4 cells with 23 single stranded chromosomes (n)
Facts About Meiosis The cells that are produced are the gametes (or sex cells) – eggs (ova) in females and sperm in males Ova are produced in the ovaries (this is called oogenesis) Sperm are produced in the testes (spermatogenesis) Called Reduction-Division We are dividing cells twice We are reducing the number of chromosomes in each cell
Facts About Meiosis
Why is Meiosis Important? It is the fundamental basis of sexual reproduction Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote
Prior to Meiosis Just like in mitosis, Interphase must occur first This proceeds just like it does with mitosis: G1, S, G2 During the S phase, the homologous DNA chromosomes are replicated The replicated copies are called sister chromatids, and are held together at the centromere
Meiosis I Proceeds like mitosis – prophase, metaphase, anaphase telophase In prophase I the nucleus breaks down A critical component of prophase I is the event crossing over when homologous chromosomes pair together (tetrads) in the middle of the cell. They connect in an area called the chiasma
Crossing Over This leads to an exchange of genes that results in new gene combinations (increased variation/diversity) This is the main advantage of sexual reproduction
Meiosis I In metaphase I, chromosomes line up in the middle (in 2 lines) In anaphase I the homologous chromosomes move apart In telophase I, 2 new nuclei form (1 cell becomes 2 cells, each with 23 paired chromosomes)
Meiosis I
Meiosis II Prophase II – both nuclei break down Metaphase II – chromosomes line up in the middle (single file) Anaphase II – sister chromatids are ripped apart Telophase II – New nuclei form *Cytokinesis occurs after meiosis II (2 cells become 4 cells, each with 23 single chromosomes)
Meiosis II
Overall Meiosis
Meiosis Animation
Mitosis Vs. Meiosis