Botany: Meiosis and Life Cycles

Sporogenesis

The process of spore formation in plants, fungi, and algae that gives rise to mitosis, a separate plant that forms gametes (gametophyte). Fertilization produces the zygote that develops into a diploid which will again bear spores.

Meiosis I

Sister chromatids align during synapsis to form homologous chromosomes. While paired, the chromosomes exchange genetic material; the sister chromatids are no longer identical.

Meiosis II

The second meiotic division functions similarly to mitosis without intervening DNA replication. Chromosomes align individually along the metaphase plate, centromeres divide during anaphase II, and sister chromatids separate toward opposite poles. Concluding with telophase II and cytokinesis, it produces four genetically distinct haploid ($n$) daughter cells, each containing a chromatid.

When does DNA replicate?

Replicates once before mitosis and meiosis

Synaptonemal complex

A zipper-like protein structure that connects the two homologous chromosomes.

Chiasmata

The chiasma is the place where two chromosomes’ arms overlap; during cross-over

Metaphase I

The spindle fibers from opposite sides of the poles attach to the kinetochores at the centromeres of each homologous chromosome. The orientation is random: each homolog could face either pole, so the maternal and paternal homologues may be on either side of the metaphase plate - centromeres on opposite sides of the plane. The random alignment is the basis for independent assortment.

Anaphase I

Homologous chromosomes move toward the poles. In contrast to mitosis, the centromeres have not separated. The event of crossing over means the chromatids are not identical.

Telophase I

1. chromosomes decondense

2. Nuclear envelope and nucleolus reform

3. Cytokines produce two daughter cells.

Prophase II

The nuclear envelope and nucleus disappear, while the chromatin condenses into chromosomes.

Metaphase II

The Spindle complex forms and attaches to centromeres. The chromosomes, each consisting of two sister chromatids, line up with their centromeres on the metaphase (II) plate.

Anaphase II

The centromeres separate, leaving the daughter chromosomes to move towards the poles.

Telophase II

Produces four genetically distinct (n) daughter cells.

Prophase I

A bivalent forms when homologous chromosomes pair via synapsis and are held together by one chiasma (site of crossing over). Each bivalent contains four chromatids and is physically connected at the centromeres.

The Gametic Life Cycle

The reproductive cell produces haploid gametes that undergo fertilization to form a diploid zygote. The zygote undergoes mitosis to produce a multicellular diploid organism. Gametes are the only haploid stage found in the life cycle.

Zygotic Life Cycle

The simplest sexual life cycle; common among protists and fungi. The zygote is the only diploid phase; it undergoes meiosis to produce haploid cells. The cells undergo mitosis to increase in number or produce a multicellular haploid organism.

Sporic Life Cycle

There is an alternation between adult haploid and diploid stages. The diploid zygote undergoes mitosis to produce a diploid adult called a sporophyte. The cells that undergo meiosis produce spores. Spores develop into haploid adults known as gametophytes. This produces gametes by mitosis.