MEIOSIS The formation of sex cells Basic Meiosis
MEIOSIS The formation of sex cells
Basic Meiosis: Differences from Mitosis?
Meiosis • Forms sex cells or gametes. Sperm and egg cells are examples of gametes. • Takes place in the testis or ovaries
Meiosis • Involves 2 stages of cell division so that chromosome number in daughter cells is half of that of the parent cells. • The number of chromosomes in a daughter cell is called haploid or n. In humans this is 23 • The number of chromosomes in all other cells is called diploid or 2 n. In humans this is 46
• Each of the 23 chromosomes you get from your dad is matched to 23 chromosomes from your mom. The paired chromosomes are called Homologous Chromosomes. Each pair carries the same genes.
Homologous Chromosomes
Stages of Meiosis • Meiosis involves 2 nuclear divisions. Meiosis I and Meiosis II • Meiosis I reduces the chromosome number from diploid to haploid • Meiosis II separates the double stranded chromosomes into chromatids • In the end 4 haploid cells are produced.
Meiosis I: Prophase I • Nuclear membrane dissolves • Centrioles move to poles, spindle fibres formed. • Chromosomes come together in homologous pairs called a tetrad • Synapsis and crossing over occur
Synapsis
Meiosis I: Metaphase I • Homologous chromosomes attach to spindle fibres and line up along the equatorial plate
Meiosis I: Anaphase I • Homologous chromosomes move towards the poles. • Chromosome number is now half. (Reduction Division)
Meiosis I: Telophase I • Chromosomes reach poles and nuclear membrane reforms. • Daughter cells are not identical!!!
Meiosis II: Prophase II • There is NO replication of chromosomes before Prophase II • Nuclear membrane dissolves, spindle forms
Meiosis II: Metaphase II • Sister chromatids align on equatorial plate still attached at centromere • There is NO pairing of chromosomes.
Meiosis II: Anaphase II • Chromatids are broken apart at centromere region and move to opposite poles.
Meiosis II: Telophase II • Nuclear membrane reforms • Division of cytoplasm occurs • 4 haploid daughter cells are produced.
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