5 Steps to a 5 AP Biology, 2014-2015 Edition

Cell Division ❮ 91

Meiosis I

Prophase I.Each chromosome pairs with its homolog. Crossover (synapsis) occurs in this

phase. The nuclear envelope breaks apart, and spindle apparatus begins to form.

Metaphase I.Chromosomes align along the metaphase plate matched with their homolo-

gous partner. This stage ends with the separation of the homologous pairs.

Anaphase I.Separated homologous pairs move to opposite poles of the cell.

Telophase I.Nuclear membrane reforms; the process of cytoplasmic division begins.

Cytokinesis.After the daughter cells split, the two newly formed cells are haploid (n).

As discussed earlier, meiosis consists of a single synthesis period during which the DNA is

replicated, followed by two acts of cell division. With the completion of the first cell

division, meiosis I, the cells are haploid because they no longer consist of two full setsof

chromosomes. Each cell has one of the duplicated chromatid pairs from each homologous

pair. The cell then enters meiosis II.

Meiosis II

Prophase II.The nuclear envelope breaks apart, and spindle apparatus begins to form.

Metaphase II.Sister chromatids line up along the equator of the cell.

Anaphase II.Sister chromatids split apart and are called chromosomesas they are pulled to

the poles.

Telophase II.The nuclei and the nucleoli for the newly split cells return.

Cytokinesis.Newly formed daughter cells physically divide.

Figure 9.4 is a pictorial representation of the stages of meiosis I and II.

Prophase I Metaphase I Anaphase I Telophase I

Telophase II/Cytokinesis Anaphase II Metaphase II Prophase II

Intermission

Figure 9.4 The stages of meiosis.

KEY IDEA

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