Passage VII

Figure 1 depicts meiosis. Meiosis consists of 2 cell divisions, Meiosis I and Meiosis II. During Meiosis I, 1 cell (Cell A) divides into 2 new cells (Cells AA and AB). Then each new cell divides during Meiosis II.

A cell entering meiosis contains 2N chromosomes (N homologous pairs). After meiosis, the number of chromosomes in each cell is N. Table 1 lists N and 2N for various animals.

Two types of chromosomes are shown in Figure 2: chromosomes that contain 2 double-stranded DNA (dsDNA) molecules (referred to here as double chromosomes) and chromosomes that contain 1 dsDNA molecule (referred to here as single chromosomes). When 1 double chromosome splits, 2 single chromosomes are formed.

Figure 2 also shows that as a result of meiosis, each of the 4 cells produced during Meiosis II contains 1 of the single chromosomes from each homologous pair of double chromosomes. (The cells depicted in Figure 2 represent cells from an organism that has 1 pair of homologous chromosomes.)

Two students present opposing models predicting when each double chromosome splits during meiosis.

Student 1

During Meiosis I, the 2 double chromosomes from each homologous pair segregate into different cells. During Meiosis II, each double chromosome splits into 2 single chromosomes, which then segregate into different cells (see Figure 3).

Student 2

During Meiosis I, each double chromosome splits into 2 single chromosomes, which then segregate into different cells. During Meiosis II, in each cell, the 2 single chromosomes from each homologous pair segregate into different cells (see Figure 4).