Meiosis Formation of Gametes Facts About Meiosis Similar
Meiosis Formation of Gametes
Facts About Meiosis Similar in many ways to mitosis, but division occurs twice � Original cell is diploid (2 n), 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 (2 n), 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 (2 n) zygote
Prior to Meiosis � Just like in mitosis, Interphase must occur first � This proceeds just like it does with mitosis: G 1, S, G 2 � 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
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