MEIOSIS

Meiosis A type of nuclear division Meiosis is also called reduction division because it results in the production of daughter cells with only *1 set of chromosomes (the parent cells has 2 sets).

Meiosis - Reduction Division Eukaryotic cells are diploid (have 2n chromosomes). Mitosis (asexual reproduction) produces diploid daughter cells. Sexual reproduction involves the fusion of 2 gametes = fertilization, to produce a zygote. If the gametes each have 2n chromosomes then the zygote would have ?

Meiosis – Reduction Division Preservation of karyotype (number and physical properties of chromosomes) is essential to survival of organism. All of our cells are diploid, 2n chromosomes. Those not destined to become gametes are called somatic (body) cells, those destined to produce gametes are called germ cells.

Meiosis Meiosis is similar to mitosis but differs in 4 ways: Meiosis has 1 round of duplication of DNA; 2 rounds of nuclear division = Meiosis I and II. Homologous chromosomes are paired through metaphase of meiosis I. They are physically joined = synapsis.

Meiosis Differences from mitosis cont’d Homologous chromosomes exchange genetic material (some of father’s DNA is exchanged with mother’s DNA). This is called crossing over. Crossing over can result in recombination = production of unique chromosomes, unlike those from the mother or father.

Chromosome Replication, Synapsis, and Crossing Over A CLOSER LOOK AT THREE KEY EVENTS IN MEIOSIS Centromere during interphase. Sister chromatids are held together by proteins along the chromosome “arms” and at the centromere. Shown: early prophase of meiosis I, when chromosomes have condensed. 1. Replication, Sister chromatids Chromosomes One homolog during prophase I. Homologous chromosome are held together by proteins in the synaptonemal complex. 2. Synapsis, Synaptonemal complex Second homolog during prophase I. Complex of proteins forms where crossing over will occur. Chromosome segments are swapped between non-sister chromatids. 3. Crossing over, Non-sister chromatids Protein complex Crossing over usually occurs at least once in each non-sister chromatid, but is only shown on 1 pair here

Meiosis Sequence of Events. Chromosomes are duplicated – S phase (Interphase) Prophase I Homologous chromosomes pair and are physically joined (synapsis) until Anaphase I. *Crossing over can occur only in this phase. Nuclear membrane disappears, spindle forms.

Meiosis Sequence of events cont’d Metaphase I Homologous chromosome pairs line up at center of spindle randomly. Microtubules of the spindle attach to kinetochores. Anaphase I – homologous chromosome pairs split and migrate to opposite poles of spindle.*** Telophase I – spindle disappears and nuclear membranes reappear. Cytokinesis – separation into 2 cells, may occur.

The Phases of Meiosis PRIOR TO MEIOSIS MEIOSIS I 1. Interphase: Chromosomes replicate, forming sister chromatids. Homologous chromosomes separate. Tetrad (4 chromatids from homologous chromosomes) Nuclear envelope Chromatin Non-sister chromatids Spindle apparatus Chiasma 1. Interphase: 2. Early Prophase I: 3. Late Prophase I: Chromosomes replicate in parent cell, in uncondensed state. Chromosomes condense, nuclear envelope breaks up, spindle apparatus forms. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids (often multiple cross- overs between the same chromatids).

The Phases of Meiosis MEIOSIS I 4. Metaphase I: 5. Anaphase I: Homologous chromosomes separate. 4. Metaphase I: 5. Anaphase I: Tetrads migrate to metaphase plate. Homologs separate and begin moving to opposite sides of cell. 6. Telophase I and Cytokinesis: Chromosomes move to opposite sides of cell, then cell divides.

Sequence of events cont’d Brief interphase with no DNA duplication. Prophase II: nuclear membranes disappear, spindles form in each cell. Metaphase II: chromosomes line up on spindles; sister chromatids attached to opposite poles of spindle. Anaphase II: sister chromatids split and move to opposite poles of spindle. Telophase II: nuclear membranes reappear. Cytokinesis results in 4 haploid cells, with “n” number of chromosomes.

The Phases of Meiosis: Meiosis II Sister chromatids separate. 7. Prophase II: 8. Metaphase II: 9. Anaphase II: 10. Telophase II and Cytokinesis: Spindle apparatus forms. Chromosomes line up at middle of cell (metaphase plate). Sister chromatids separate, begin moving to opposite sides of cell. Chromosomes move to opposite sides of cell, then cell divides.

Meiosis KM

Meiosis KM

Meiosis KM