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

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

Interphase in meiosis occurs prior to the start of meiosis I. Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell 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. 46 pieces of genetic material in parent cell Chromosomes replicate Homologous pair of replicated chromosomes Diploid cell with replicated chromosomes Sister chromatids 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. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

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 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 Meiosis I Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1 Haploid cells with replicated chromosomes Homologous chromosomes separate

Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell 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. Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid). Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

46 pieces of genetic material in parent cell Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell 46 pieces of genetic material in parent cell 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 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 Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid). 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

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: – 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Fig. 13-8a Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Cleavage furrow Homologous chromosomes Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore

Chromosomes begin to condense 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

In crossing over, nonsister chromatids exchange DNA segments 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

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-1 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure 13.12 The results of crossing over during meiosis

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-2 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-3 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis

Prophase I Nonsister of meiosis chromatids held together Fig. 13-12-4 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis Anaphase II

Recombinant chromosomes Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis 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 1 Possibility 2 Metaphase II Figure 13.11 The independent assortment of homologous chromosomes in meiosis 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 Figure 13.11 The independent assortment of homologous chromosomes in meiosis 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

Prophase I Metaphase I Centrosome (with centriole pair) Centromere Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

In anaphase I, pairs of homologous chromosomes separate 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

Telophase I and Cytokinesis Fig. 13-8c Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Figure 13.8 The meiotic division of an animal cell 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

Telophase II and Cytokinesis Fig. 13-8d Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Sister chromatids separate Haploid daughter cells forming Figure 13.8 The meiotic division of an animal cell

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

Prophase II Metaphase II Fig. 13-8e Figure 13.8 The meiotic division of an animal cell

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

Telephase II and Cytokinesis Fig. 13-8f Telephase II and Cytokinesis Anaphase II Sister chromatids separate Haploid daughter cells forming Figure 13.8 The meiotic division of an animal cell

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 Figure 15.16 Down syndrome

Fig. 15-16b Figure 15.16 Down syndrome

Translocated chromosome 22 (Philadelphia chromosome) Fig. 15-17 Error – crossing over occurred improperly – the exchange was with non-homologous chromosomes. Reciprocal translocation Normal chromosome 9 Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Normal chromosome 22 Figure 15.17 Translocation associated with chronic myelogenous leukemia (CML)

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

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

Golgi apparatus Acrosomal vesicle Approximately 24 days Golgi apparatus Acrosomal vesicle Mitochondria Acrosome Nucleus 1 2 Centrioles Spermatid nucleus Microtubules Midpiece Head (a) 3 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