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Stages of Meiosis

Stages of Meiosis. The Phases of Meiosis. Interphase. Interphase the cell replicates its chromosomes . After replication, each chromosome consists of two identical sister chromatids , held together by a centromere . The Phases of Meiosis. Prophase I. Prophase I

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Stages of Meiosis

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  1. Stages of Meiosis

  2. The Phases of Meiosis Interphase Interphase • the cell replicates itschromosomes. • After replication, eachchromosome consists of twoidentical sisterchromatids, heldtogether by acentromere.

  3. The Phases of Meiosis Prophase I Prophase I • The chromosomes coil up and a spindle forms. • As the chromosomescoil,homologous chromosomesline up with each other gene by gene along their length, to form a four-part structure called atetrad.

  4. The Phases of Meiosis Prophase I Prophase I • The chromatids in a tetrad pair tightly • In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process knownas crossing over.

  5. The Phases of Meiosis Metaphase I Metaphase I • During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber. • The spindle fibers pull the tetrads (homologous pairs) into the middle

  6. The Phases of Meiosis Prophase I + Metaphase 1 • Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. • Genetic recombination results from crossing over during prophase I of meiosis • This increases variation further

  7. The Phases of Meiosis Anaphase I Anaphase I • begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell. • This critical step ensures that each new cell will receive only one chromosome from each homologous pair. *Random assortment*

  8. The Phases of Meiosis Telophase I Telophase I • Nucleus reforms, chromosomes may or may not unwind • The spindle is broken down and the cytoplasm divides to yield two new cells • Cells are now HAPLOID- half the amount of genetic information

  9. Meiosis 1 overview Divide 1 • 1st division of meiosis Line Up in tetrads Copy DNA during interphase prophase 1 metaphase 1 • 4 chromosomes • diploid • 2n telophase 1 • 2 chromosomes • haploid • 1n

  10. The Phases of Meiosis Prophase II Meiosis II • The second division in meiosis is simply a mitotic division of the products of meiosis I. • During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes. • Nucleus dissolves

  11. The Phases of Meiosis Metaphase II Metaphase II • The chromosomes, still made up of sister chromatids, are pulled to the center of each cell and line up randomly at the equator.

  12. The Phases of Meiosis Anaphase II • Anaphase II • the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles of each cell.

  13. The Phases of Meiosis Telophase II Telophase II • nuclei reform, the spindles break down, and the cytoplasm divides. • At the end of meiosis II, four haploid cells have been formed from one diploid cell. • These haploid cells will become gametes, transmitting the genes they contain to offspring.

  14. Meiosis 2 overview • 2nd division of meiosis • looks like mitosis telophase 2 telophase 1 Line Up 2 metaphase 2 • 2 chromosomes • haploid • 1n gametes

  15. The Phases of Meiosis • At the end of meiosis II, four haploid cells have been formed from one diploid cell. • These haploid cells will become gametes, transmitting the genes they contain to offspring.

  16. Gamete Formation • Formation of Gametes (egg and sperm- sexual reproductive cells) • Meiosis in males occurs in the testes and produces 4 sperm cells. The process is called spermatogenesis. • Meiosis in females occurs in the ovaries and produces 1 egg and 3 polar bodies, which disintegrate. The process is called oogenesis. • Meiosis allows for crossover and genetic recombination (gene mixing), which creates the variation between parents and offspring.

  17. Meiosis = reduction division • Meiosis • special cell division in sexually reproducing organisms • reduce number of chromosomes • 2n  1n • diploid  haploid • half • makes gametes • sperm, eggs

  18. Meiosis & mitosis • Meiosis to make gametes • sperm & egg • Mitosis to make copies of cells • growth • repair • development

  19. Comparison of Mitosis & Meiosis • MITOSIS MEIOSIS

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