Meiosis I

Meiosis, a nuclear division that forms haploid cells, consists of two main stages: meiosis I 1 and meiosis II.

Prophase I1

Early in prophase I1, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pairs close to each other. Recall that, in mitosis, homologous chromosomes do not pair together.

The phenomenon of crossing over happens in prophase I1.

Prometaphase I1

The key event in prometaphase I 1 is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres.

Metaphase I1

During metaphase I1, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles.

Anaphase I1

In anaphase I1, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere.

The first image at the top, called prophase one, presents a transparent sphere with purple, thick, rod-like structures, randomly spread inside the sphere. They are surrounded by blue mesh. The second image, called metaphase one, presents the same sphere, but now the purple rods are all horizontally aligned in the center, and the blue mesh surrounding them is barely visible. The last image, called anaphase one, presents the same sphere, but now the purple structures are divided in half, each half being pulled towards the opposite side of the sphere. The blue mesh is clearly visible, some threads are attached to the purple rods, being responsible for their division.

Figure 1. Phases of meiosis one.

Telophase I 1and Cytokinesis

In telophase, the separated chromosomes arrive at opposite poles.

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Therefore, only one full set of chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.