1. Bio 2970 Lab Weeks 2-3: Mitosis and Meiosis Sarah VanVickle-Chavez

2. Phases of the Cell Cycle The cell cycle consists of – Mitotic (M) phase (mitosis and cytokinesis) – Interphase (cell growth and copying of chromosomes in preparation for cell division) Interphase (about 90% of the cell cycle) can be divided into subphases – G 1 phase (“first gap”) – S phase (“synthesis”) – G 2 phase (“second gap”) The cell grows during all three phases, but chromosomes are duplicated only during the S phase

3. INTERPHASE G1G1 G2G2 S (DNA synthesis) MITOTIC (M) PHASE Cytokinesis Mitosis

4. G 2 of Interphase Prophase Prometaphase Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids Fragments of nuclear envelope Nonkinetochore microtubules Kinetochore Kinetochore microtubule

5. Metaphase Metaphase plate Anaphase Telophase and Cytokinesis Spindle Centrosome at one spindle pole Daughter chromosomes Cleavage furrow Nucleolus forming Nuclear envelope forming

6. The Mitotic Spindle The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase An aster (a radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Kinetochores are protein complexes associated with centromeres At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell In telophase, genetically identical daughter nuclei form at opposite ends of the cell Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles

8. Sister chromatids Aster Centrosome Metaphase plate (imaginary) Kineto- chores Overlapping nonkinetochore microtubules Kinetochore microtubules Microtubules Chromosomes Centrosome 0.5  m 1  m

9. (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM) Cleavage furrow Contractile ring of microfilaments Daughter cells Vesicles forming cell plate Wall of parent cell Cell plate New cell wall Daughter cells 100  m 1  m In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis

10. Figure 12.11 Chromatin condensing Nucleus NucleolusChromosomes Cell plate 10  m Prophase Prometaphase Metaphase Anaphase Telophase 1 2 345

12. Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome 5  m APPLICATION TECHNIQUE A karyotype is an ordered display of the pairs of chromosomes from a cell. The two chromosomes in each pair are called homologous chromosomes, or homologs. Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters.

13. Key Haploid (n) Diploid (2n) Gametes MEIOSISFERTILIZATION Zygote Mitosis Diploid multicellular organism (a) Animals n n n 2n2n 2n2n 2n2n 2n2n 2n2n n n n n n n n n n n MEIOSIS FERTILIZATION Mitosis Gametes Spores Gametes Zygote Haploid multi- cellular organism (gametophyte) Diploid multicellular organism (sporophyte) Haploid unicellular or multicellular organism (b) Plants and some algae (c) Most fungi and some protists Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles differ in the timing of meiosis and fertilization

14. Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell After chromosomes duplicate, two divisions follow – Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes – Meiosis II (equational division) sister chromatids separate The result is four haploid daughter cells with unreplicated chromosomes

15. Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes Interphase Meiosis I Meiosis II 21 The Stages of Meiosis

16. Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes Division in meiosis I occurs in four phases – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis The Stages of Meiosis

18. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids.

19. Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene In crossing over, nonsister chromatids exchange DNA segments 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 occurred

20. Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome

21. Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole

22. Telophase I and Cytokinesis 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 Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

23. Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

24. Meiosis II In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated 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

25. Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

26. Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles

27. Anaphase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

28. Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell

29. F 1 Generation All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R r r r r r r r r r r r r YY Y Y Y Y Y Y Y Y Y Y yy y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 12 21 1/41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR

30. Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes Nondisjunction of homo- logous chromosomes in meiosis I (a) Nondisjunction of sister chromatids in meiosis II (b) n  1 n  1 n  1 n  1 n  1 n n

31. Comparison of Mitosis & Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell Three events are unique to meiosis, and all three occur in meiosis l –Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information –At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes –At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)

32. Prophase Duplicated chromosome MITOSIS Chromosome duplication Parent cell 2n  6 Metaphase Anaphase Telophase 2n2n2n2n Daughter cells of mitosis MEIOSIS MEIOSIS I MEIOSIS II Prophase I Metaphase I Anaphase I Telophase I Haploid n  3 Chiasma Chromosome duplication Homologous chromosome pair Daughter cells of meiosis I Daughter cells of meiosis II n n n n

33. SUMMARY PropertyMitosis Meiosis DNA replication Number of divisions Synapsis of homologous chromosomes Number of daughter cells and genetic composition Role in the animal body Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Does not occur Two, each diploid (2n) and genetically identical to the parent cell Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes