M Phase: Mitosis and Cytokinesis (12) The role of proteolysis (continued) Destruction of securin releases separase, which cleaves the subunit that holds the sister chromatids together. Destruction of cyclins leads to a drop in activity of the mitotic Cdk and progression into the G1 phase. Proteolysis moves the cell cycle in an irreversible direction.

Experimental demonstration of proteolysis in a cell’s exit form mitosis

M Phase: Mitosis and Cytokinesis (13) The Events of Anaphase Chromosomes are split in synchrony. As chromosomes move toward a pole, microtubules attached to kinetochores are shortened. Movement of chromosomes toward the poles is called anaphase A. Anaphase B is when the two spindle poles move in opposite directions due to elongation of microtubules.

The mitotic spindle and chromosomes at anaphase

The mitotic spindle and chromosomes at anaphase

M Phase: Mitosis and Cytokinesis (14) Forces Required for Chromosome Movements at Anaphase Both dynein and kinesin are found at kinetochores of chromosomes. Depolymerization of microtubules generates sufficient force to move the chromosomes. In yeast, a protein is pushed by the force released from depolymerization to help move the chromosome toward the spindle pole.

Demonstration that microtubule depolymerization can move attached chromosomes in vitro

Proposed mechanism for the movement of chromosomes during anaphase

Proposed mechanism for the movement of chromosomes during anaphase

M Phase: Mitosis and Cytokinesis (15) The Spindle Checkpoint The spindle checkpoint operates at the metaphase/anaphase transition to check for misaligned chromosomes. Unattached kinetochores contain a protein complex that send a “wait” signal to prevent entry into anaphase.

The spindle checkpoint

M Phase: Mitosis and Cytokinesis (16) Telophase During telophase, the daughter cells return to interphase. Nuclear envelopes of the two nuclei are reassembled. Chromosomes become dispersed. The cytoplasm is partitioned into two cells.

Telophase

M Phase: Mitosis and Cytokinesis (17) Forces Required for Mitotic Movements Mitotic movement is powered by microtubule motors (dynein and kinesin-related proteins). Microtubule motors are located at the spindle poles and kinetochores. Motor proteins have a number of features: Keep the poles apart. Bring chromosomes to the metaphase plate and keep them there. Elongate the spindle during anaphase B.

Proposed activity of motor proteins during mitosis

M Phase: Mitosis and Cytokinesis (18) Cytokinesis in Animal Cells Starts with the indentation of the cell surface. The contractile ring theory suggested that a thin band of actin and myosin filaments generates the force to cleave the cell. The site of filament assembly (the plane of cytokinesis) is determined by a signal coming from the spindle poles.

Cytokinesis

The formation and operation of the contractile ring during cytokinesis

Experimental demonstration of the importance of myosin during cytokinesis

Experimental demonstration of the importance of myosin during cytokinesis

Formation of the cleavage plane

M Phase: Mitosis and Cytokinesis (19) Cytokinesis in Plant Cells Formation of the cell plate, precursor to a new cell wall. Cells build a cell membrane and cell wall in the cell center. Cell plate begins with the appearance of the phragmoplast, which then proceeds laterally. Material for the cell wall is brought to the phragmoplast by Golgi vesicles.

The formation of the a cell plate

14.3 Meiosis (1) During meiosis, chromosome number is halved and haploid cells are formed. Meiosis consists of two divisions. In the first division, homologous chromosomes pair and then segregate ensuring that daughter cells receive a full haploid set of chromosomes. In the second division, the two chromatids are separated.

The stages of meiosis

Meiosis (2) In different eukaryotes meiosis occurs at different points in the life cycle. In gametic meiosis, the process is linked to gamete formation. In zygotic meiosis, the process occurs after fertilization. It occurs only in protists and fungi. In sporic meiosis, the process is independent of gamete formation and fertilization.

A comparison of three groups of organisms based on the stage within the life cycle when meiosis occurs

Meiosis (3) The Stages of Meiosis DNA is replicated prior to meiosis. Prophase I consists of several stages: In leptotene chromosomal condensation starts. During zygotene homologous chromosomes pair. This process is called synapsis, and it is when homologues associate via the synaptonemal complex. The synaptonemal complex allows interacting chromatids to complete crossing-over. Synapsed chromosomes form a bivalent or tetrad.

The stages of prophase I

Association of the telomeres of meiotic chromosomes with the nuclear envelope

The synaptonemal complex

Meiosis (4) Prophase I (continued) In pachytene synapsis ends. During diplotene the synaptonemal complex disappears and homologous chromosomes start moving apart. Chiasmata are the remaining points of attachment between homologous chromosomes. Chiasmata occur where crossing over took place.

Visible evidence of crossing-over

Meiosis (5) Prophase I (continued) The final stage of prophase I is diakinesis, when chromosomes are prepared for attachment to the spindle fibers. Diakinesis ends with the disappearance of the nucleolus and the disassembly of the nuclear envelope. Diakinesis is triggered by an increase in MPF activity.

Meiosis (6) Metaphase I The two homologous chromosomes are aligned at the metaphase plate. Both chromatids of one chromosome face the same pole. Homologous chromosomes are held by one or several chiasmata. Absence of a chiasma can lead to abnormal segregation of chrosomes.

Separation of homologous chromosomes during meiosis I

Meiosis (7) Anaphase I Stage when homologous chromosomes separate. Maternal and paternal chromosomes of each tetrad segregate into the two daughter cells independent of other chromosomes. Behavior of chromosomes during anaphase I corresponds to Mendel’s law of independent assortment.

Meiosis (8) Telophase I Produces less dramatic changes than telophase of mitosis. The nuclear envelope may or may not reform during this stage. The stage between the two divisions is called interkinesis. Cells during this stage have a haploid number of chromosomes. Cells have a diploid amount of DNA.

Meiosis (9) Meiosis II It is simpler than meiosis I. During metaphase II, chromosomes are aligned so that kinetochores of sister chromatids face opposite poles. Sister chromatids separate during anaphase II. Meiosis II produces cells haploid in both amount of DNA and chromosome number.

Meiosis (10) Genetic Recombination During Meiosis Meiosis increases genetic variability by mixing maternal and paternal alleles between homologous chromosomes. Recombination occurs by the physical breakage of and ligation of individual DNA molecules. It occurs without the addition or loss of a single base pair. DNA repair enzymes fill gaps that develop during the exchange process.

Meiosis (11) Genetic recombination (continued) Prior to recombination, DNA strands are aligned by homology search, in which homologous DNA molecules associate with one another. Breaks are introduced into one strand of each duplex at corresponding sites. The gap is subsequently widened. The two duplexes are joined to each other by Holliday junctions (pairs of DNA crossovers).

Proposed mechanism for genetic recombination initiated by double-strand breaks

The Human Perspective: Meiotic Nondisjunction and Its Consequences (1) Meiotic nondisjunction occurs when homologous chromosomes do not separate during meiosis I or sister chromatids do not separate during meiosis II. Nondisjunction leads to the formation of gametes and zygotes with abnormal number of chromosomes, or aneuploidy.

Meiotic nondisjunction

The Human Perspective: Meiotic Nondisjunction and Its Consequences (2) An extra chromosome is referred to as a trisomy. Down syndrome is the resulty of trisomy of chromosome 21. A missing chromosome is referred to as a monosomy.

The Human Perspective: Meiotic Nondisjunction and Its Consequences (3) The presence of an abnormal chromosome number is less disruptive to human development. A zygote with only one X chromosome leads to Turner syndrome, in which genitalia development is arrested. A male with an extra X chromosome develops Klinefelter syndrome, which leads to the presence of secondary female sex characteristics.

The Human Perspective: Meiotic Nondisjunction and Its Consequences (4) There is no precise answer as to why meiosis I is more susceptible to nondisjunction than meiosis II. A possibility is that sister chromatid cohesion is not fully maintained over an extended period thus allowing homologous chromosomes to separate prematurely.

Experimental Pathways: The Discovery and Characterization of MPF (1) MPF was first observed in studies of the effect of cytoplasm on the state of the nucleus in oocytes.

Experimental Pathways: The Discovery and Characterization of MPF (2) In developing embryos, MPF activity fluctuates following the stages of the cell cycle.

Experimental Pathways: The Discovery and Characterization of MPF (3)

Experimental Pathways: The Discovery and Characterization of MPF (4) Levels of cyclin were found to correlate with levels of MPF activity. Purified MPF was shown to have kinase activity and stimulate nuclei to prepare for entry into mitosis.