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Meiosis

Meiosis. Creation of Gametes. Asexual reproduction yields two identical organisms (with exception of mutations) Sexual reproduction merges the genes of the two parents Result: two versions of each chromosome Called homologous chromosomes Organism is diploid (2n=46)

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Meiosis

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  1. Meiosis

  2. Creation of Gametes • Asexual reproduction yields two identical organisms (with exception of mutations) • Sexual reproduction merges the genes of the two parents • Result: two versions of each chromosome • Called homologous chromosomes • Organism is diploid (2n=46) • All somatic cells are diploid

  3. All can be lined up based on size, creates a karyotype

  4. First 22 are autosomes • Chromosome 23 is the sex chromosome • Haploid (n=23) sex cells merge to create a zygote in sexual reproduction • Meiosis creates these sex cells (gametes) • The overall process is the sexual life cycle

  5. Interphase I of Meiosis Homologous chromosomes Chromosomes replicate Homologous chromosomes separate Meiosis I Meiosis II Sister chromatids separate

  6. Interphase I • Similar to mitosis, chromatin replicates • Centrioles replicate, proteins form around to make the centrosomes • Also called the microtubule organizing centre • Collectively, with microtubules, is called mitotic spindle

  7. Prophase I • Longest part of meiosis • Chromatin condenses, chromosomes become visible • Homologous chromosomes come pair up, forming a tetrad – called synapsis • The chromatids can overlap, called a chiasmata • All other features are similar to mitosis

  8. Metaphase I • Almost identical to metaphase in mitosis, with the tetrad exception

  9. Anaphase I • Sister chromatids remain attached, tetrads separate (cells are now haploid)

  10. Telophase I • Homologous chromosomes are at opposite poles of cell • Each end is now haploid • Cytokinesis takes place simultaneously • Some cells proceed to interphase II, others directly to prophase II • No cells replicate chromatin again

  11. Prophase II • Spindle forms again, chromosomes move towards equator

  12. Metaphase II • Closely resembling mitosis, metaphase plate is formed

  13. Anaphase II • Sister chromatids finally separate

  14. Telophase II • Mitotic spindle breaks down, nucleus reforms around two daughter nuclei

  15. Errors • Some of the processes can go awry • If anaphase I or II does not proceed correctly, the chromosomes may not be shared evenly • This is called nondisjunction • More of a problem if it occurs during meiosis I • Can take place in mitosis, but not as much of a concern

  16. An abnormal chromosome number is called aneuploidy • If faulty gamete succeeds in fertilization, the offspring with have health consequences • If a gamate with an extra chromosome fertilizes, the zygote is trisomic (2n+1) • If the gamete has a chromosome missing, the zygote is monosomic (2n-1) • Ex. Extra chromosome 21 – Down syndrome

  17. If the extra chromosome is an X in a male (XXY), called Klinefelter syndrome • The individual will have male sex organs, but the testes will be abnormally small, and he will be sterile • As well, femine characteristics such as breast enlargement is common

  18. Final example is Turner syndrome • Example of monosomyX • Individuals are classified X0, and are female • However, sex organs do not mature at adolescence and secondary sex characteristics fail to develop

  19. Polyploidy • Also possible for anaphase I or II to completely fail and all chromosomes move to one cell • Referred to as polyploidy • Could have three chromosome sets (triploidy, 3n) or four (tetraploidy, 4n)

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