Meiosis
What is meiosis?
Meiosis occurs during reproduction to create cells with half the chromosome number to create a new phase of the life cycle
Meiosis is associated with sexual reproduction
It is similar to mitosis, but the process has two successive divisions
Meiosis I: reduction division (reduces the number of chromosomes)
Meiosis II: equatorial division (similar to mitosis)
Why is meiosis important?
It is part of the sexual life-cycle of an organism
A single, diploid (2N) cell creates four, haploid (1N) cells (e.g. spores or pollen in plants)
Where does meiosis occur?
In plants, meiosis occurs in specialized reproductive structures called sporangia, which create spores
In seed plants, the male sporangia (microsporangia) are called pollen sacs. The pollen sacs create pollen, also called microspores, which each contain a male gametophyte (known as the pollen tube)
The female sporangia are more complicated. Inside each ovule, there is a female sporangium (megasporangium) called the nucellus. The nucellus creates megaspores, which each contain a female gametophyte
In animals, meiosis occurs in the gonads to create sperm or eggs.
N.B. Please note that meiosis creates spores in plants (sporic meiosis), but meiosis creates gametes in animals (gametic meiosis). In plants, gametes are created by mitosis.
Stages of Meiosis
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Prophase I
Chromosomes continue to condense
Homologous chromosomes align
Nuclear envelope and nucleolus disassociate
Crossing over occurs
Prophase II:
Chromosomes become shorter and thicker
Metaphase I
Spindle visible
Chromosomes aligned in pairs at equator
Metaphase II
Spindle visible
Chromosomes aligned in pairs at equator
Anaphase I
Whole chromosomes migrate to opposite ends of cell
Anaphase II
Chromatids migrate to opposite ends of cell
Telophase I
Cell plate begins to form
Nucleolus forms
Telophase II
Cell plate begins to form
Nucleolus forms
New nuclear envelope forms
Additional Resources
Pollen and Plant Stress: podcast discussion of how env't factors like UV radiation can disrupt meiosis and make gymnosperms sterile, and indicators of this same mechanism in the fossil record during extinction events (In Defense of Plants 13Feb2022)