Meiosis Reduction Division Mike Clark, M.D.. Meiosis Meiosis is nicknamed reduction division It is a process where a cell divides (division) but reduces

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 (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

Figure 27.5 (1 of 2) Mother cell (before chromosome replication) Chromosome replication Chromosome replication 2n = 4 MITOSIS Replicated chromosome Prophase Chromosomes align at the metaphase plate Sister chromatids separate during anaphase 2n2n2n2n Metaphase Daughter cells of mitosis Tetrad formed by synapsis of replicated homologous chromosomes Tetrads align at the metaphase plate Homologous chromosomes separate but sister chromatids remain together during anaphase I No further chromosomal replication; sister chromatids separate during anaphase II Daughter cells of meiosis II (usually gametes) nnnn Prophase I Metaphase I Daughter cells of meiosis I Meiosis II MEIOSIS

Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 46 pieces of genetic material in parent cell 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. Interphase in meiosis occurs prior to the start of meiosis I. It consists of the same three Phases as in mitosis G1,S and G2.

Fig. 13-7-2 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated 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

Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes At the end of meiosis II have 4 daughter cells each with the amount of genetic material (haploid). At the completion of meiosis I (after cytokinesis I) - the two cells enter into a phase termed Interkinesis. Interkinesis is similar to Interphase but it lacks the S-phase thus DNA is not replicated it is already enough DNA for 4 haploid cells.

Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes 46 pieces of genetic material in parent cell S- phase in interphase duplicates DNA (but stays attached chromatid/ chromosome thus enough genetic material for 4 haploid (gamete) cells 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 the amount of genetic material (haploid).

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

Metaphase I Fig. 13-8a Prophase IAnaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate 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, X- shaped 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 Nonsister chromatids held together during synapsis Chiasma Centromere TEM

Fig. 13-12-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I TEM

Fig. 13-12-4 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II TEM

Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes TEM

Possibility 1 Possibility 2 Metaphase II Without crossing over the newly formed cells would inherit either a full chromosome containing only moms or dads genes on that chromosome. By crossing over the situation above would not happen in that each chromosome would have a piece of dads genetic material and a piece of moms genetic material.

Without crossing over the 4 daughter cells below would have no genetic recombination. Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 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 IMetaphase 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 Anaphase I Telophase I and Cytokinesis 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 Telophase II and Cytokinesis Sister chromatids separate 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 IIMetaphase 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 Telephase II and Cytokinesis Sister chromatids separate 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 Downs Syndrome in the womans offspring

Fig. 15-16

Fig. 15-16b

Fig. 15-17 Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Error crossing over occurred improperly the exchange was with non-homologous chromosomes.

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

Figure 27.17 Meiotic eventsFollicle development in ovary Before birth Infancy and childhood (ovary inactive) Primary oocyte Primary oocyte (still arrested in prophase I) Vesicular (Graafian) follicle Primary follicle Primordial follicle Oocyte Ovulated secondary oocyte In absence of fertilization, ruptured follicle becomes a corpus luteum and ultimately degenerates. Degenating corpus luteum Secondary follicle Primary oocyte (arrested in prophase I; present at birth) Oogonium (stem cell) Each month from puberty to menopause Meiosis I (completed by one primary oocyte each month in response to LH surge) First polar body Mitosis Growth Meiosis II of polar body (may or may not occur) Polar bodies (all polar bodies degenerate) OvumSecond polar body Meiosis II completed (only if sperm penetration occurs) Sperm Ovulation Secondary oocyte (arrested in metaphase II) Follicle cells Spindle

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 non- functional 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 ovums 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

Figure 27.7b Basal lamina Spermatogonium (stem cell) Mitosis Growth Late spermatids Early spermatids Secondary spermatocytes Primary spermatocyte Spermatozoa Type B daughter cell Enters meiosis I and moves to adluminal compartment Meiosis I completed Meiosis II Type A daughter cell remains at basal lamina as a stem cell (b) Events of spermatogenesis, showing the relative position of various spermatogenic cells

Figure 27.8a, b Centrioles Spermatid nucleus Golgi apparatus Acrosomal vesicle Mitochondria Approximately 24 days Excess cytoplasm Nucleus Acrosome Microtubules Flagellum Tail MidpieceHead (a) (b) 1 2 3 4 5 6 7

Final Result of Spermatogenesis All the four cells (sperm) are viable thus differing from the female situation

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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