Cell Division: Budding, Sporulation, and Meiosis

Special Types of Cell Division

• Budding: Casts asymmetric cytoplasmic material; the daughter cell emerges as a bud from one side of the mother, such as in yeast or sponges.
• Sporulation: Several successive mitoses occur without cytokinesis; multinucleate cells are formed, for example, in fungi and protozoa.

Meiosis

To condense, look better.

Meiosis I

Stages of Prophase I

It is the most complex stage, with five sub-phases:

1. Leptotene: Chromosomes condense to become visible by light microscopy. Each is composed of two chromatids.
2. Zygotene: Homologous chromosomes pair up until they are completely aligned along their length. This pairing is called synapsis and occurs through a protein structure called the synaptonemal complex.
3. Pachytene: Crossover (chromatid exchange of material between chromatids of homologous chromosomes) or genetic recombination occurs.
4. Diplotene: Homologous chromosomes begin their separation but remain united by chiasmata.
5. Diakinesis: The chromosomes are condensed to the maximum, and their two chromatids are already visible. Each pair of sister chromatids is attached at the centromere, while each pair of chromosomes is held together by chiasmata that occur between non-sister chromatids. The nucleolus disappears, the nuclear membrane breaks down, and the spindle fibers begin to form at the kinetochore.

Metaphase I

In the equatorial plate, tetrads linked by chiasmata are arranged. The centromeres are arranged on opposite sides of the plate, but the kinetochores of the chromatids from the same chromosome are fused and are oriented toward the same pole.

Anaphase I

As the two kinetochores have merged, chromatids do not separate, but whole chromosomes do. Each chromosome of a pair goes to one pole of the cell.

Telophase I

The nuclear membrane and nucleolus reappear. The small chromosomes undergo decondensation.

Two daughter cells are produced, with half the chromosomes that the stem cell had and with two chromatids each chromosome.

Meiosis II

Before Meiosis II, there is a short interphase in which no DNA synthesis occurs.

Prophase II

The nuclear membrane disappears, the chromosomes condense, and the spindle forms.

Metaphase II

Chromosomes are in the equatorial plate. Each is composed of two chromatids joined at the centromere, and each has an associated kinetochore.

Anaphase II

Centromeres are separated, and each chromatid migrates to opposite poles.

Telophase II

The nuclear membrane is formed, and four daughter cells are obtained. These are haploid cells with half of the chromosomes (half the chromosomes of stem cells) and are genetically different.