1 Cellular Reproduction Part II: Meiosis

2 Inheritance of Genes Each gene in an organism’s DNA has a specific locus on a certain chromosome We inherit one set of chromosomes from our mother and one set from our father Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

3 Mitosis vs Meiosis Mitosis produces two identical daughter cells, each with 2 copies of DNA (46 chromosomes), the same exact DNA as the parent cell. For sexual reproduction, each gamete must have only 1 set of DNA; otherwise the chromosome count would double each generation. Therefore, meiosis produces 4 daughter cells (gametes), each with only 1 copy of DNA

4 Sex Cells - Gametes Germ cells are also diploid, but they are found only in the gonads. Gonads are the ovaries in females and testes in males. In these organs, females make gametes called eggs, and males make gametes called sperm. Gametes are haploid cells, which means that they have only one set of chromosomes.

5 Meiosis Phases Meiosis involves the same four phases seen in mitosis  prophase  metaphase  anaphase  telophase They are repeated during both meiosis I and meiosis II.

6 Prophase I Prophase I occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE IMETAPHASE IANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

7 Metaphase I At 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 Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE IMETAPHASE IANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

8 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 Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad PROPHASE IMETAPHASE IANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

9 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 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

10 Prophase II Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Cleavage furrow PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming

11 Metaphase II At metaphase II, the sister chromatids are 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 Cleavage furrow PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming

12 Anaphase II At anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Cleavage furrow PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming

13 Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing 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 Cleavage furrow PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming

14 A Comparison of Mitosis and 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 The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

15 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 of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate A Comparison of Mitosis and Meiosis

16 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6 A Comparison Of Mitosis And Meiosis

17 Comparison Meiosis DNA duplication followed by 2 cell divisions Sysnapsis Crossing-over One diploid cell produces 4 haploid cells Each new cell has a unique combination of genes Mitosis Homologous chromosomes do not pair up No genetic exchange between homologous chromosomes One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells New cells are genetically identical to original cell (except for mutation)

18 Spermatocytes to Spermatids Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids Spermatids are small round cells seen close to the lumen of the tubule Late in spermatogenesis, spermatids are nonmotile Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming motile sperm

19 Spermatogenesis Figure 27.8b, c

20 Oogenesis Production of female sex cells by meiosis In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients Primordial follicles appear as oogonia are transformed into primary oocytes Primary oocytes begin meiosis but stall in prophase I From puberty, each month one activated primary oocyte completes meiosis one to produce two haploid cells – The first polar body – The secondary oocyte The secondary oocyte arrests in metaphase II and is ovulated If penetrated by sperm the second oocyte completes meiosis II, yielding: – One large ovum (the functional gamete) – A tiny second polar body

21 Oogenesis