MEIOSIS
Two types of reproduction Asexual: only one parent cell needed Parent cell divides using mitosis to produce new, exactly identical offspring
Sexual reproduction: Two parent cells join together Parent cells known as sex cells or reproductive cells Different from ordinary body cells in that they have only HALF the usual amount of DNA (chromosomes) WHY?
How are these sex cells made? Through a process called MEIOSIS Diploid (2n) haploid (n) (paired chromosomes) (single chromosome for each characteristic) Meiosis is necessary for sexual reproduction. Two divisions (meiosis I and meiosis II).
Meiosis Sex cells have half the # of chromosomes. Similar to mitosis with some chromosomal differences. Below is model of mitosis.
So how do we get half the number of chromosomes? Chromosomes are copied ONCE (replication), but the nucleus divides TWICE. Two stages: Meiosis I and Meiosis II Each stage still has prophase, metaphase, anaphase and telophase, where the same things occur as in mitosis
Remember… Prophase: Prepare: Chromosomes form, nucleus disappears Metaphase: chromosomes line up in MIDDLE Anaphase: Chromosomes move APART Telophase and Cytokinesis : Cell TEARS in two
This is a model of Meiosis
Interphase DNA is Replicated (copied).
Prophase I
Metaphase I
Anaphase I Note that the sister chromatids are still attached at the centromere (different from mitosis)
Telophase I Cytokinesis I
Meiosis 1 Review
Meiosis II No interphase II (no more DNA replication)
Prophase II same as prophase in mitosis
Metaphase II same as metaphase in mitosis
Anaphase II same as anaphase in mitosis sister chromatids separate
Telophase II Same as telophase in mitosis. Nuclei form. Afterwards, Cytokinesis occurs. Remember: Four haploid daughter cells are produced.
Telophase II Cytokinesis II
Meiosis 2 Review
Meiosis
Fertilization n=23 egg 2n=46 zygote The fusion of a sperm (gamete or “S” Cell) and egg (gamete or “S” Cell) to form a zygote. A zygote is a fertilized egg sperm n=23 human n=23 egg 2n=46 zygote human human