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)Diploid (2n)  haploid (n) (paired chromosomes) (single chromosome for each characteristic) Meiosissexual reproduction.Meiosis is necessary for sexual reproduction. Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II).

Meiosis Sex cellshalfchromosomesSex cells have half the # of chromosomes. SimilarmitosisSimilar 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 IINo interphase II DNA replication (no more DNA replication)

Prophase II prophasemitosissame as prophase in mitosis

Metaphase II metaphasemitosissame as metaphase in mitosis

Anaphase II anaphasemitosissame as anaphase in mitosis sister chromatids separatesister chromatids separate

Telophase II telophasemitosisSame as telophase in mitosis. Nuclei form. Afterwards, CytokinesisAfterwards, Cytokinesis occurs. Remember: Four haploid daughter cells are produced.Remember: Four haploid daughter cells are produced.

Telophase II Cytokinesis II

Meiosis 2 Review

Meiosis

Question: 20 chromosomes(diploid) chromosomesA cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 Chromosomes (haploid)

Fertilization sperm egg zygoteThe fusion of a sperm (gamete or “S” Cell) and egg (gamete or “S” Cell) to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote human

Were you Paying Attention 1.In asexual reproduction the genes of the offspring are _____________ those of the parent. 2.In what stage of Meiosis are the homologous pairs pulled apart? __________________ 3.What is another name for reproductive cells or sex cells? _________________

. How many gametes are produced at the end of Meiosis from one parent cell? 4. How many gametes are produced at the end of Meiosis from one parent cell?___________ 5. A sperm cell from a frog has 14 chromosomes how many chromosomes will a fertilized frog egg have? _________ 6. Why is Meiosis important to sexual reproduction?_________________________ _____________________________________ __________________________