Meiosis

The process of meiosis takes place in two stages, meiosis I and meiosis II. Combined, both stages involve reduction, that is, converting the chromosome number from diploid to haploid, producing four gametes, each with a complete set of chromosomes, and genetic variation. There are four main steps in meiosis: prophase, metaphase, anaphase, and telophase. Before meiosis I (the first meiotic division), an interphase takes place in which all the chromosomes are replicated. In between interphase and the second meiotic division is meiosis I, which undergoes the following steps.

The First Meiotic Division

Prophase I

Early prophase I

Following interphase, the chromatin begins coiling and compacting.

Middle prophase Ia

In this stage, the chromosomes progressively shorten and a process called synapsis aligns homologous chromosomes. Also, the centrosomes begin making their way to the opposite poles of the nucleus.

Middle prophase Ib

During the second half of middle prophase I, the centrosomes form spindle fibers as they finally reach the opposite poles. Sister chromatids are linked by the centromeres and the process of crossing over occurs at the chiasmata, during which genetic material is exchanged between homologs.

Late prophase I

Late in prophase I, the chromosomes continue to shorten and coil. In addition, the nuclear envelope disappears.

Metaphase I

During metaphase, chromosomes are aligned on the equatorial plate and the kinetochores of sister chromatid pairs face the same pole.

Anaphase I

As the spindle fibers interact with the kinetochore fibers, the homologous chromosomes are pulled toward opposite poles. Sister chromatids remain attached at their centromeres.

Telophase I

Homologous chromosomes are still pulled toward opposite poles and eventually reach the poles. Each of the poles has a haploid set of chromosomes and each set begins to form its own nucleus. This follows with cytokinesis (division of the cytoplasm) in which a cleavage furrow begins to divide the cell into two separate cells, each with its own nucleus.

This ends the first meiotic division, and the cell then prepares for the second meiotic division.