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Meiosis Reduction Division Mike Clark, M.D. Meiosis • Meiosis is nicknamed reduction division • It is a process where a cell divides (division) but reduces the genetic material to ½ (reduction) • This type of cell division occurs in the gametes (sex cells) • The original parent gamete cells (spermatogonium and oogonium) are diploid (2n) like a somatic cell but the final daughter gamete cells (sperm and egg term ovum) are haploid (1n) Differences between Mitosis and Meiosis • Mitosis occurs in somatic cells – meiosis occurs in gametes • Mitosis has one nuclear division – meiosis has two nuclear divisions • Mitosis produces two new daughter cells – meiosis produces four new daughter cells • The resultant daughter cells in mitosis have 46 pieces of genetic material – the resultant daughter cells in meiosis has 23 pieces of genetic material Mother cell (before chromosome replication) Chromosome replication Chromosome replication 2n = 4 MITOSIS MEIOSIS Replicated chromosome Prophase Metaphase Chromosomes align at the metaphase plate Sister chromatids separate during anaphase Metaphase I Tetrads align at the metaphase plate Homologous chromosomes separate but sister chromatids remain together during anaphase I Daughter cells of mitosis 2n Tetrad formed by synapsis of replicated homologous chromosomes Prophase I Daughter cells of meiosis I 2n No further chromosomal replication; sister chromatids Meiosis II separate during anaphase II n n n Daughter cells of meiosis II (usually gametes) n Figure 27.5 (1 of 2) Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell Interphase in meiosis occurs prior to the start of meiosis I. 46 pieces of genetic material It consists of the same three in parent cell Phases as in mitosis – G1,S and Chromosomes replicate G2. Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes In the S- phase of interphase DNA is duplicated. As noted before the new DNA stays attached to the old (chromatid/chromosome) – thus though we say there are 46 chromosomes – there is actually enough genetic material for 92 chromosomes since one chromosome contains two chromatids. When the chromatids separate they are considered full chromosomes thus there is enough genetic material for 4 haploid (gamete) cells. 92 divided by 4 equals 23 – thus 23 chromosomes in a cell is termed haploid (1n). This is the amount of genetic material that the sperm and egg contain. Fig. 13-7-2 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Diploid cell with replicated chromosomes Sister chromatids Meiosis I 1 Haploid cells with replicated chromosomes Homologous chromosomes separate At the end of meiosis I have two daughter cells with 23 doublets of genetic material (23 chromosomes) but each chromosome has two chromatids – thus enough for 46 singlet chromosomes Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid). At the completion of meiosis I (after Chromosomes cytokinesis I) - the two replicate Homologous pair of replicated chromosomes cells enter into a phase termed Interkinesis. Interkinesis is similar to Sister Diploid cell with chromatids replicated Interphase – but it lacks chromosomes Meiosis I the S-phase – thus DNA is not replicated – it is Homologous already enough DNA for chromosomes separate 4 haploid cells. Haploid cells with 1 replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell 46 pieces of genetic material in parent cell Chromosomes replicate Homologous pair of replicated chromosomes S- phase in interphase duplicates DNA (but stays attached chromatid/ chromosome– thus enough genetic material for 4 haploid (gamete) cells Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes At the end of meiosis I have two daughter cells with 23 doublets of genetic material (23 chromosomes) but each chromosome has two chromatids – thus enough for 46 singlet chromosomes At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid). • Division in meiosis I occurs in four phases: – – – – Prophase I Metaphase I Anaphase I Telophase I and cytokinesis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Three events are unique to meiosis, and all three occur in meiosis l: – 1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes 3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8a Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Sister chromatids remain attached Metaphase plate Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Telophase I and Cytokinesis Anaphase I Microtubule attached to kinetochore Cleavage furrow 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information • 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, Xshaped regions where crossing over occurred Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Crossing Over • Crossing over produces recombinant chromosomes, which combine genes inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • In crossing over, homologous portions of two nonsister chromatids trade places • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-12-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Fig. 13-12-2 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Nonsister chromatids held together during synapsis Fig. 13-12-3 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase I Nonsister chromatids held together during synapsis Fig. 13-12-4 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Nonsister chromatids held together during synapsis Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes Without crossing over the newly formed cells would inherit either a full chromosome containing only mom’s or dad’s genes on that chromosome. Possibility 2 Possibility 1 Metaphase II By crossing over the situation above would not happen in that each chromosome would have a piece of dad’s genetic material and a piece of mom’s genetic material. Without crossing over the 4 daughter cells below would have no genetic recombination. Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore 3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8c Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow • 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8d Prophase II Metaphase II Anaphase II Sister chromatids separate Telophase II and Cytokinesis Haploid daughter cells forming 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8e Prophase II Metaphase II 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Telophase II and Cytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8f Anaphase II Sister chromatids separate Telephase II and Cytokinesis Haploid daughter cells forming Oogenesis • Production of female gametes • Begins in the fetal period – Oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients – Primary oocytes develop in primordial follicles – Primary oocytes begin meiosis but stall in prophase I and stay there for years – until the woman ovulates – This suspended prophase 1 can late in life lead to Down’s Syndrome in the woman’s offspring Fig. 15-16 Fig. 15-16b Fig. 15-17 Error – crossing over occurred improperly – the exchange was with non-homologous chromosomes. Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Oogenesis • Each month after puberty, a few primary oocytes are activated • One is selected each month to resume meiosis I (the one to be ovulated) • Result is two haploid cells – Secondary oocyte – First polar body Oogenesis • The secondary oocyte arrests in metaphase II and is ovulated • If penetrated by sperm the second oocyte completes meiosis II, yielding – Ovum (the functional gamete) – Second polar body Follicle development in ovary Meiotic events Before birth Oogonium (stem cell) Follicle cells Oocyte Mitosis Primary oocyte Primordial follicle Primary oocyte (arrested in prophase I; present at birth) Primordial follicle Growth Infancy and childhood (ovary inactive) Each month from puberty to menopause Primary follicle Primary oocyte (still arrested in prophase I) Secondary follicle Spindle Meiosis I (completed by one primary oocyte each month in response to LH surge) First polar body Meiosis II of polar body (may or may not occur) Polar bodies (all polar bodies degenerate) Vesicular (Graafian) follicle Secondary oocyte (arrested in metaphase II) Ovulation Sperm Second Ovum polar body Meiosis II completed (only if sperm penetration occurs) Degenating Ovulated secondary oocyte In absence of fertilization, ruptured follicle becomes a corpus luteum and ultimately degenerates. corpus luteum Figure 27.17 Final Result of Oogenesis (formation of the egg) • Four cells are produced – all 4 with a haploid set of genetic material - but three of the cells are nonfunctional – termed polar bodies • Only one viable cell is produced - the egg cell (termed the ovum) – this is the cell to be ovulated for the month • The one viable cell (ovum) receives most of the cell cytoplasm • Inasmuch as the placenta will not develop till much later if the egg is fertilized – the developing embryo must live off the food in the ovum’s cytoplasm till the after birth (placenta) is formed Mitosis of Spermatogonia • Begins at puberty • Spermatogonia – Stem cells in contact with the epithelial basal lamina – Each mitotic division a type A daughter cell and a type B daughter cell Spermatogonium (stem cell) Mitosis Growth Enters meiosis I and moves to adluminal compartment Meiosis I completed Meiosis II Basal lamina Type A daughter cell remains at basal lamina as a stem cell Type B daughter cell Primary spermatocyte Secondary spermatocytes Early spermatids Late spermatids Spermatozoa (b) Events of spermatogenesis, showing the relative position of various spermatogenic cells Figure 27.7b Approximately 24 days Golgi apparatus Acrosomal vesicle Mitochondria Acrosome Nucleus 1 (a) 2 Spermatid nucleus Centrioles 3 Midpiece Head Microtubules Flagellum Excess cytoplasm 4 Tail 5 6 7 (b) Figure 27.8a, b Final Result of Spermatogenesis • All the four cells (sperm) are viable – thus differing from the female situation