Meiosis
Meiosis is a nuclear division process. It differs from mitosis, in that it includes two sequenced divisions, and results in four genetically distinct daughter nuclei. These four daughter nuclei each have one half the chromosome number of the original parent nucleus. Meiosis together with fertilization (the union of gametes) constitute sexuality in eukaryotes.
Students should learn to recognize cells in interphase and in any of the stages of meiosis. Students should understand the mechanisms of meiosis that resulted in the various asci of Sordaria observed in lab, and be able to relate the model of the pop beads to the process. Further, students should relate what they learn in lecture to their observations in lab as outlined below.
Microsporogenesis in lily:
In plants meiosis results in the formation of spores. These spores produce a haploid generation. These haploid individuals in turn produce gametes by mitosis. The images linked below are of microspore formation (these go on to produce pollen grains).
View raw images all stages of meiosis in lily pollen development
Interphase: The non dividing cell: Includes three stages, G1, S, and G2 described in lecture. The cell's chromosomes are dispersed as chromatin inside the nucleus.
Link to view of lily flower
Link section of lily flower
Link to view of interphase cell
The Stages of Meiosis:
Meiosis I
In meiosis I, the chromosome number is reduced by one half, but the two chromatids that make up each chromosome remain together.
Prophase I: The chromatin becomes condensed and the chromosomes become visible. Each chromosome consists of two chromatids. Unlike in mitosis, homologous chromosomes are paired. These pairs are tightly associated with each other.These complexes apear to have two parts and are called bivalents. During this stage sections of chromosome are exchanged between homologs. This stage ends with the breakdown of the nuclear envelope.
Link to view of early prophase I cells of lily anther
Link to view of late prophase I cells of lily anther
Metaphase I: A spindle forms. Individual microtubules connect to the kinetochore of each chromosome at the centromere. These rearrange the paired homologous chromosomes (bivalents) so that they line-up along the cell equator.
Link to view of metaphase I cells of of lily anther
Anaphase I: Homologs separate and are pulled to opposite ends of the cell. Each pair of homologs segregate independently of one another.
Link to view of anaphase cell of lily anther
TelophaseI: Begins when the chromosomes stop moving. During this stage chromosomes may disperse to form chromatin and the nuclear envelope reforms. Telophase in pollen formation is coupled with cytokinesis. Cell division is accomplished through the formation of a cell plate. This action is facilitated by a a phragmoplast which directs the movement of vesicles formed by dictyosomes.These vesicles deliver pectic materials to the forming cell plate. The membranes of the vesicles coalesce to form a continuous new membrane that will divide the mother cell in two.
Link to view of telophase cell of lily anther
Meiosis II
In meiosis II, the two nuclei derived from meiosis I can be seen dividing in synchrony with each other. In these divisions, the two chromatids that make up each chromosome separate and move in opposite directions to form two new nuclei.
Prophase II: If the chromosomes became dispersed after meiosis I, the chromatin becomes condensed and the chromosomes become visible. Each chromosome consists of two chromatids. This stage ends with the breakdown of the nuclear envelope.
Link to view of prophase II cells of lily anther
Metaphase II: A spindle forms. Individual microtubules connect to the kinetochore of each chromatid at the centromere. These rearrange the chromosomes so that they line-up along the cell equator.
Link to view of metaphase II cell of of lily anther
Anaphase II: Chromatids separate and are pulled to opposite ends of the cell. Because of crossing over during prophase I these new chromosomes are genetially different.
Link to view of early anaphase II cell of lily anther
Link to view of anaphase II cell of lily anther
TelophaseII: Begins when the chromosomes stop moving. During this stage the chromosomes disperse to form chromatin and the nuclear envelope reforms. Telophase II in microsporogenesis is coupled with cytokinesis. Cell division is accomplished through the formation of a cell plate. This action is facilitated by a a phragmoplast which directs the movement of vesicles formed by dictyosomes.These vesicles deliver pectic materials to the forming cell plate. The membranes of the vesicles coalesce to form a continuous new membrane that will divide the mother cell in two.
Link to view of telophase II cell of lily anther
The four cells that result from meiosis in the lily anther are microspores and can be seen as four cells attached together (a tetrad of cell). Each of these is considered a type of spore and will become a pollen grain. The pollen grain in turn will produce sperm nuclei by mitosis.
Link to view of tetrad of microspores
Link to view of pollen grain