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  1. Meiosis Chapter 13: Meiosis and Sexual Life Cycles

  2. Variation • Living organisms are distinguished by their ability to reproduce their own kind • Geneticsis the scientific study of heredity and variation • Heredityis the transmission of traits from one generation to the next • Variationis demonstrated by the differences in appearance that offspring show from parents and siblings

  3. Comparison of Reproductions • In asexual reproduction, one parent produces genetically identical offspring by mitosis • A cloneis a group of genetically identical individuals from the same parent • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents • A life cycleis the generation-to-generation sequence of stages in the reproductive history of an organism

  4. Genes • Genesare the units of heredity, and are made up of segments of DNA • Genes are passed to the next generation through reproductive cells called gametes(sperm and eggs) • Each gene has a specific location called a locuson a certain chromosome • Most DNA is packaged into chromosomes • One set of chromosomes is inherited from each parent

  5. Chromosomes • Every organism has its own chromosome number • Somatic (body) cell chromosomes come in pairs • Called diploid (2n) number of chromosomes • The two chromosomes in each pair are called homologous chromosomes, or homologs • They are the same length and carry genes controlling the same inherited characters • 1 from mom • 1 from dad • In humans, somatic cells have 46 chromosomes

  6. Chromosomes • Gametes have only 1 of each chromosome • Called haploid (n) number of chromosomes • In humans, sex cells are haploid: n=23 • A karyotypeis an ordered display of the pairs of chromosomes from a cell

  7. Homologous Chromosomes Exception • Sex chromosomesare called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes(found in homologous pairs)

  8. Fertilization • Each set of 23 chromosomes consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y • Fertilizationis the union of gametes (the sperm and the egg) • The fertilized egg is called a zygoteand has one set of chromosomes from each parent • The zygote produces somatic cells by mitosis and develops into an adult

  9. Meiosis • Meiosis is a type of cell division used to make gametes (sex cells) • 2 nuclear divisions • Meiosis I • Meiosis II • Begins with 1 diploid (2n) cell • Ends with 4 haploid (n) cells

  10. Meiosis • Gametes are the only types of human cells produced by meiosis • Meiosis results in one set of chromosomes in each gamete (23) • Gametes are the only haploid cells in animals • Gametes fuse (23 + 23) to form a diploid zygote (46) that divides by mitosis to develop into a multicellular organism

  11. Cell Cycle Review • G1, S, G2, M • Mitosis has 4 main phases • Prophase • Metaphase • Anaphase • Telophase • Cells split by cytokinesis • Produce 2 identical cells • Growth and repair

  12. Meiosis I • Division in meiosis I occurs in four phases: • Prophase I • Metaphase I • Anaphase I • Telophase I and cytokinesis • Meiosis I results in two haploid daughter cells with replicated chromosomes • Focus is on splitting homologous chromosomes

  13. Interphase • Meiosis I is preceded by interphase, in which chromosomes are replicated • Each replicated chromosome consists of two identical sister chromatids • Sister chromatids held together by centromere • The sister chromatids are genetically identical

  14. Prophase I • Chromosomes begin to condense • Nuclear membrane & nucleoli dissolve • Homologous chromosomes pair up (align gene by gene); this is called synapsis • Each pair of chromosomes forms a tetrad, a group of four chromatids • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurs • In crossing over, nonsister chromatids exchange DNA segments

  15. Prophase I: Tetrads Homologous chromosomes (each with sister chromatids) A Tetrad

  16. Metaphase I • In metaphase I, tetrads independently line up across from each other“sandwiching” the equator (metaphase plate) • In mitosis the homologs made one single line on the equator • Microtubules from the poles are attached to the kinetochores of each chromosome of each tetrad

  17. Anaphase I • Pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle fibers • Sister chromatids remain attached at the centromere and move as one unit toward the pole

  18. Telophase I • Reverse of prophase I • Spindle fibers breaks down • Chromosomes uncoil • Nuclear envelope reforms • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes • Each chromosome still consists of two sister chromatids

  19. Cytokinesis & Interkinesis • Cytokinesis forms two haploid daughter cells • In animal cells, a cleavage furrow forms • In plant cells, a cell plate forms • Each new cell has ½ the genetic information as the original • 1 chromosome from each pair • Need a second division for sister chromatids to split • Each chromosome from the pair is still doubled • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

  20. Meiosis I

  21. Meiosis II • Division in meiosis II also occurs in four phases: • Prophase II • Metaphase II • Anaphase II • Telophase II and cytokinesis • Meiosis II is very similar to mitosis • Focus is on splitting sister chromatids

  22. Prophase II & Metaphase II Prophase II • Spindle apparatus forms; nuclear membrane and nuclelous dissolve • Chromosomes condense (each still composed of two chromatids) and move toward the metaphase plate Metaphase II • Sister chromatids are arranged at the metaphase plate • The two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles

  23. Anaphase II & Telophase II Anaphase II • Sister chromatids separate and move to opposite poles • Each chromatid is now its own chromosome Telophase II • Nuclei reform • Spindles break down • Chromosomes uncoil • Cytokinesis again separates the cytoplasm

  24. Final Products • At the end of meiosis, there are four haploid daughter cells • Each daughter cell is genetically distinct from the others and from the parent cell • Each has 1 chromosome from each homologous pair • Each will mature into eggs or sperm (gametogenesis)

  25. Meiosis II

  26. Meiosis Distinctions • Three events are unique to meiosis, and all three occur in meiosis I: • Synapsis and crossing over in prophase I • In metaphase I, paired homologous chromosomes (tetrads) independently arrange on either side of the equator • In anaphase I, homologous chromosomes, instead of sister chromatids, separate

  27. Meiosis & Variation • Three mechanisms contribute to genetic variation: • Independent assortment of chromosomes • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • Crossing over • Nonsister chromatids of a tetrad exchange genetic information • Random fertilization • Over 8 million different gametes possible (2^23) • 70 trillion chromosome combinations possible for zygotes! (2^23)x(2^23)

  28. Mitosis vs. Meiosis

  29. Mitosis 1 division 2 daughter cells Exact copies of parent cells Diploid to diploid Purpose Growth Repair Asexual reproduction Meiosis 2 divisions 1st separates pairs 4 daughter cells Each unique Diploid to haploid Purpose Make gametes/ sex cells Leads to genetic variation Mitosis vs. Meiosis

  30. Review Questions Define genetics and differentiate between heredity and variation. Differentiate between asexual and sexual forms of reproduction in regards to the life cycles of various organisms. Define the following vocabulary associated with meiosis: gene, gamete, locus, somatic cell, karyotype, homologous chromosomes, & zygote. Differentiate between autosomes and sex chromosomes. Describe the process of fertilization. Define meiosis and explain why there must be 2 divisions. Define the 4 major phases of meiosis I, along with the important events that occur during those phases and how they are unique from those phases of mitosis. Explain the relationship between synapsis, tetrads, chiasmata, and crossing over. Define the 4 major phases of meiosis II, along with the important events that occur during those phases and how they are unique from those phases of mitosis. Describe the 3 events that are unique to meiosis. Name and describe 3 mechanisms that contribute to genetic variation. Describe 5 major differences between mitosis and meiosis as processes.