Lillium Meiosis

in Anthers

Meiosis is reduction division; the cells produced contain half the number of chromosomes as before meiosis.

1 Diploid Cell, chromosomes in pairs

¯

4 Haploid Cells, chromosome pairs split

In higher plants and animals, meiosis occurs to produce the gametophyte generation or the gametes themselves gametes. Gametes fuse to produce the diploid generation, which grows into the adults. In lower plants, the haploid generation produced by meiosis is the dominant form. For example, mosses are primarily composed of haploid cells.

Meiosis occurs in two steps. During meiosis I, the chromosome pairs are divided between the two cells produced. During meiosis II, the two sister chromatids are segregated into each daughter cell. Meiosis II is analogous to mitosis, because the chromosome number stays the same.

Lillium sp. anthers have nice meiotic figures as they produce the male gametophyte, that is, the chromosomes are more easily observed than in many other species.

Meiosis I

Prophase I

Prophase I takes the most time.

Chromosomes begin to condense.

Cell is 2N (diploid) and 2 chromatids/chromosomes (like before mitosis).

Synapsis occurs: homologous chromosomes pair, their DNA aligns.

Crossing over may occur: chromosomes exchange homologous loci (between homologous pairs). The point of crossing over is called the chiasma.

Metaphase I

Homologous chromosomes align at cell equator to form tetrads.

Anaphase I

Chromosomes are pulled to either side of the dividing cell; one of each pair is pulled toward each pole.

Telophase I

The nuclear envelope may form again.

Cytokinesis

Two daughter cells are formed.

Meiosis II

Prophase II

Nuclear envelope disperses

Metaphase II

Chromosomes align between poles

Anaphase II

Chromatids segregate.

Telophase II

After cytokinesis, four daughter cells are formed.

Egg Development

When eggs develop, often one daughter cell gets all the cytoplasm and organelles; the other gets only nuclear material, this becomes a polar body. The polar bodies formed by this process are not viable and they degenerate. This is a mechanism that results in an increase in the egg’s size. An acentric spindle is essential to this process.

This page was produced by Barbara Krumhardt, Ph.D., Science Group Leader, Biology Instructor, Urban Campus, Des Moines Area Community College, Des Moines, Iowa, USA 50314

Back to Urban’s Biology Home Page