Chromosomes line up in the middle in metaphase I only.

Chromosomes line up in the middle in metaphase I only. Chromosomes condense in metaphase I only. A cell has half the number of chromosomes in metaphase II. There are twice the number of cells in metaphase II. Spindle fibers exist in metaphase II only.
Which statements describe differences between metaphase I and metaphase II? Check all that apply. Chromosomes line up in the middle in metaphase I only. Chromosomes condense in metaphase I only. A cell has half the number of chromosomes in metaphase II. There are twice the number of cells in metaphase II. Spindle fibers exist in metaphase II only.

The Correct Answer and Explanation is:

The correct statements describing differences between metaphase I and metaphase II are:

1. A cell has half the number of chromosomes in metaphase II.
2. There are twice the number of cells in metaphase II.

Explanation:

Metaphase I and metaphase II are distinct stages in the process of meiosis, which involves two divisions: meiosis I and meiosis II. Here’s a breakdown of the differences:

1. Chromosomes line up in the middle in metaphase I only:
   This statement is incorrect because chromosomes line up in the middle (on the metaphase plate) in both metaphase I and metaphase II. In metaphase I, homologous chromosome pairs line up, while in metaphase II, individual chromosomes (each consisting of two chromatids) align. The key difference lies in what is lining up — homologous chromosomes in metaphase I and individual chromosomes in metaphase II.
2. Chromosomes condense in metaphase I only:
   This is also incorrect. Chromosomes condense during prophase I and prophase II as part of the preparation for chromosome segregation. They are already condensed when they align in metaphase I and II. The key distinction is that condensation of chromosomes happens earlier in the cycle, not specifically in metaphase I.
3. A cell has half the number of chromosomes in metaphase II:
   This statement is true. In meiosis I, homologous chromosomes are separated, reducing the chromosome number by half. In meiosis II, the number of chromosomes remains halved as the chromatids of each chromosome are separated. Thus, the chromosome number in the cell at metaphase II is half of what it was at metaphase I.
4. There are twice the number of cells in metaphase II:
   This statement is true. After meiosis I, the cell divides into two daughter cells. During meiosis II, both of these daughter cells undergo division again, resulting in four cells in total. Therefore, there are twice as many cells in metaphase II as there were in metaphase I.
5. Spindle fibers exist in metaphase II only:
   This statement is incorrect because spindle fibers are present during both metaphase I and metaphase II. They are responsible for aligning chromosomes and chromatids during cell division. The presence of spindle fibers is a feature of both stages, although their role in pulling apart homologous chromosomes in metaphase I and chromatids in metaphase II differs.

In summary, the main differences between metaphase I and metaphase II concern the chromosome number (halved in metaphase II) and the number of cells (twice as many in metaphase II).