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Meiosis

Premedical- Biology. Meiosis. Reproduction The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms . Asexual reproduction is not limited to single-celled organ isms - m ost plants

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Meiosis

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  1. Premedical- Biology Meiosis

  2. Reproduction The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms. Asexual reproduction is notlimited to single-celled organisms - most plants - binary fission- Bacteria - budding - yeasts and Hydras - conjugation- bacteria may exchange genetic information • parthogenesis, fragmentation and spore formation

  3. Aphis Aphid Green-fly Wingless female giving birth baby hammerhead Parthenogenesis • lower plants (where it is called apomixis) • invertebrates - water fleas, aphids, some bees and parasitic wasps • vertebrates- some reptiles,fish, and very rarely birds and sharks

  4. Sexual reproduction bycombination of genetic material contributed from two different members of the species Each contributes half of the offspring's genetic material male produce sperm or microsporein anisogamous species andfemale produce ovum or megaspore

  5. Sexually reproducing organisms have two sets of genes for every trait called alleles,offspring inherit one allele from each parent • the offspring is combination of parental genes • diploid and haploid phases alternation in cell lines inbreeding = is the reproduction from the mating of two genetically related parents inbred line – origin of homozygote offsprings by sexual reproduction

  6. Human reproduction cycle

  7. Meoisis = division of germ cells -reduction of diploid chromosomal number to haploid - origin of haploid gamets

  8. First meiotic division n M I = reduction2n = heterotypic n Prophase I: • leptotene– spiralisation of chromosomes • zygotene– pairing of homologous chromosomes – synapsis -bivalents synaptonemal complex

  9. 3. pachytene – each chromosome has two chromatids – tetrads crossing-over = reciprocal exchange of homologous parts of non-sister chromatids = recombination of maternal and paternal genetic material Recombination possibilities between non-sister chromatids and result products- two chiasmata on one bivalent a - recombination only between 2 chromatids; b - 4 chromatids in recombination; c - 3 chromatids in recombination.

  10. 4. diplotene – separation of homologs, connected only in sites of chiasmata. prerequisite of crossing-over = chiasma formation 5. diakinesis – terminalisation of chiasmataand maximal contraction of chromosomes Metaphase I – lining of bivalents in equatorial plane of the cell. Chromosomes from the pairs split.

  11. Anaphase I– migration of homologous chromosomes to the opposite poles randomly according their parental origin Telophase I- chromosomes in the opposite poles Cytokinesis – division of cytoplasm- equal in spermiogenesis, unequal in oogenesis Interkinesis – a phase without replication

  12. Second meiotic division n M II – homeotypic n n inProphase II division and separation of centriols – spindle of microtubules in Metaphase II centromers of chromosomes split in Anaphase IIseparation of chromatids

  13. Gametogenesis – formation of gametes • migration of primordial germ cells to the gonads • during early fetal development number of mitotic division

  14. Spermatogenesis is continuous process, which starts in the time of sexual maturity (top off testosteron threshold) growth MI mitotic division - spermatogonia primary spermatocyte (dipl) M II 2 secondary spermatocytes (hapl) 4 spermatids diferentiation-spermiogenesis 4 sperm 1 cycle = about 10 weeks

  15. Oogenesis M I to the end of prophase 3 month of fetal life diplotene=dictyotene growth at the time of birth Oogonia primary oocyte (dipl) mitotic division in the time of sexual maturity M II M I continues secondary oocyte ovulation in metaphase II +1st polar body anaphase II + telophase II only after fertilization oocyte + 2nd polar body

  16. Oogenesis a fertilization I. meiotic division polar body oogonia primary secondary oocyte n 2n fertilization a II. Meiotic division

  17. Differences in gametogenesis Male Female Initiation Puberty Early embryonic life Duration cca 72 days 10-50 years Numbers of mitoses 30 - 500 20-30 in gamete formation Gamete production 4 spermatids 1 ovum+3 polar bodies Gamete production 100-200 million 1 ovum per menstrual per ejaculate cycle

  18. Degeneration of germ cells in ovary 5th month of fetal life 7 x 106 of cells time of birth 2 x 106 of cells puberty 200 000 of cells ovulated 400 of cells Long period between M I and ovulation = factor of high probability of nondisjunction

  19. Consequences of meiosis • reduction of diploid chromosomal number to haploid • segregation of alleles in M I, M II (2st Mendel´s law) 3. random assortment of homologues – random combination maternal and paternal chromosomes in gametes (3nd Mendel´s law) 4. increase of genetic variability by crossing-over (chromatids with segments of maternal and paternal origin)

  20. Genetic determination of sex 23,X 23,X 23,Y

  21. If you feel small, low-spirited and useless, remember: Thanks to perfect Ovum and the fastest Sperm and their genetic information, YOU ARE ABSOLUTELY UNIQUE and NO REPEATABLE ELEMENT THE WHOLE UNIVERSE

  22. Thank you for your attention Campbell, Neil A., Reece, Jane B., Cain Michael L., Jackson, Robert B., Minorsky, Peter V., Biology, Benjamin-Cummings Publishing Company, 1996 –2010.

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