Which phase of meiosis is being described by the following sentence? Homologous chromosomes line up at the midplate and spindle fibers attach to centromeres. Select one:
a. Metaphase I
b. Metaphase II
c. Anaphase I
d. Anaphase II
e. C and D
The correct answer and explanation is:
The correct answer is a. Metaphase I.
In metaphase I, homologous chromosomes line up at the metaphase plate (the central plane of the cell), and spindle fibers attach to the centromeres of each chromosome. This phase is crucial for ensuring the correct segregation of chromosomes during meiosis.
Meiosis is a type of cell division that reduces the chromosome number by half, resulting in four non-identical daughter cells, which are gametes (sperm or egg cells). It consists of two stages: meiosis I and meiosis II, each with several phases: prophase, metaphase, anaphase, and telophase.
In metaphase I, homologous chromosomes (pairs of chromosomes, one from the mother and one from the father) align at the metaphase plate. The key distinction of this phase is that the chromosomes are still in pairs, not individual chromatids as in metaphase II. The spindle fibers extend from the centrioles and attach to the centromeres of each chromosome.
The importance of this alignment is to ensure that during the subsequent phase (anaphase I), the homologous chromosomes are separated, with one chromosome from each pair being pulled toward opposite poles of the cell. This segregation reduces the chromosome number by half and is critical for maintaining genetic diversity.
Metaphase II is similar in terms of chromosome alignment, but it occurs in the second meiotic division, where individual chromosomes (not homologous pairs) line up at the metaphase plate. Therefore, the description in the question applies to metaphase I, not metapahse II.