Meiosis is the specialized form of cell division used by sexually reproducing organisms to create reproductive cells, or gametes. This process is often called reduction division because it reduces the number of chromosomes in a cell by half. For example, in humans, a cell with 46 chromosomes divides to produce sperm or egg cells, each containing only 23 chromosomes. This reduction occurs through two successive rounds of division: Meiosis I and Meiosis II. The goal is to ensure that when gametes fuse during fertilization, the resulting zygote contains the correct total number of chromosomes.
The State of Cells Entering Meiosis II
The cells entering Meiosis II are fundamentally different from the original cell that began the meiotic process. Meiosis I, which precedes the second division, separates the homologous pairs of chromosomes (pairs inherited, one from each parent).
The outcome is two haploid daughter cells, containing only one chromosome from each original homologous pair. Despite being haploid, the chromosomes are still duplicated. Each chromosome remains composed of two closely joined sister chromatids.
These sister chromatids are identical copies created during DNA replication before Meiosis I, tethered together at the centromere. The purpose of Meiosis II is to separate these sister chromatids to produce true haploid gametes with single, non-duplicated chromosomes.
Aligning the Duplicated Chromosomes for Separation
Metaphase II is the stage where the cell organizes the duplicated chromosomes for separation. The chromosomes, still consisting of two sister chromatids, move toward the cell’s center and align along the equatorial plane, called the metaphase plate.
A key distinction from Metaphase I is that the chromosomes line up individually in a single file along this central plate. This arrangement resembles the alignment seen in mitosis.
The spindle apparatus, made of microtubules, plays a role in this alignment. Spindle fibers extend from opposite poles of the cell and attach to protein structures located at the centromere of each chromatid, known as kinetochores.
During Metaphase II, spindle fibers attach to the kinetochores in a bipolar fashion. One sister chromatid attaches to the spindle pole at one end, while the other attaches to the opposite end. This attachment creates tension, ensuring the two chromatids are oriented correctly to be pulled apart in Anaphase II.
Ensuring Accurate Segregation and Genetic Diversity
The precise alignment of chromosomes in Metaphase II ensures the genetic viability of the resulting gametes. Before separation, the Spindle Assembly Checkpoint (SAC) is active. This checkpoint confirms that every chromosome is properly aligned and that the kinetochores of both sister chromatids are under tension from spindle fibers pulling from opposing poles.
If attachment is incorrect or a chromosome is misaligned, the checkpoint halts cell cycle progression. This safeguard prevents aneuploidy, a condition where gametes have an abnormal number of chromosomes. Errors at this stage can have consequences for embryonic development and contribute to genetic disorders.
Metaphase II also contributes to genetic diversity. While major genetic shuffling, such as crossing over, occurred earlier in Meiosis I, the final alignment is significant. The random orientation of sister chromatids on the metaphase plate ensures that each of the four final gametes receives a unique combination of maternal and paternal genetic material.