Metaphase: Definition and Stages

Metaphase Definition

Metaphase is a stage in eukaryotic cell division in which the chromosomes align on the metaphase plate in the middle of the cell. The stages of prophase and prometaphase come before metaphase. In those stages of cell division, the chromosomes are condensed, the spindle fibers form, and the nuclear envelope is broken down. During metaphase and late prometaphase, the cell performs as series of checkpoints to ensure that the spindle has formed. The microtubules emanating from each side of the cell attach to each chromosome. As the microtubules are retracted, an equal tension is applied from each side of the cell the chromosomes. This moves them to the middle of the cell. After metaphase, the sister chromatids that comprise the chromosomes are divided, and the process of cell division is completed.

At the beginning of eukaryotic cell division, the centriole divides and begins to set up the microtubule network that will move the chromosomes and organelles throughout the process of cell division. These microtubules wind together to form larger fibers, that reach out from the centrosomes. Although the fibers are stable near the centrosomes, as they reach out towards the chromosomes, they are less stable. As the fibers grows towards the chromosome, they both add and subtract pieces at the unstable end. As the fibers grow in this way, 3 steps forward 2 steps back, they wander through the cytoplasm. Eventually the fibers connect with the centromere of a chromosome. Each centromere has a kinetochore where the microtubules can attach.

The most important process to occur before and during metaphase the spindle assembly checkpoint. The spindle assembly checkpoint is a complex series of mechanisms that ensures the proper division of chromosomes. Although the chromosomes align differently during mitosis and meiosis, both go through a spindle assembly checkpoint during metaphase. If these checkpoints are skipped, or do not function properly, the cell will begin anaphase before the chromosomes are properly attached to microtubules and aligned on the metaphase plate. If this is the case, the chromosomes get sorted into the wrong cells. This can produce too many, or too few chromosomes in the resulting daughter cells. In meiosis, this can lead to birth defects and non-viable offspring. If it happens during mitosis, this can lead to cells becoming cancerous.

Metaphase in Mitosis

During mitosis, the chromosomes align in the middle of the cell, with the sister chromatids of each chromosome on either side of the metaphase plate. Before mitosis, during interphase, the cell replicates its DNA. The chromosomes containing the DNA condense before metaphase, so they will not be damaged by the movements that will take place during metaphase. At the start of metaphase, and during late prometaphase, the chromosomes are randomly arranged within the nucleus. The nuclear membrane is dissolved, and microtubules connect to each chromosome.

In mitosis, microtubules from each centrosome connect to each chromosome. The chromosomes consist of two sister chromatids, which are connected by proteins called cohesins. Before the cohesins can be broken apart, the mitotic spindle checkpoint must be met, meaning all chromosomes are attached to microtubules from both sides and are aligned on the metaphase plate. When this checkpoint is passed, a signal is released by the chromosomes, which activates the anaphase-promoting complex. The activation of this complex leads to the end of metaphase in mitosis and the start of anaphase.

The alignment of the chromosomes, with sister chromatids on each side of the metaphase plate ensures the two new cells will be identical. The sister chromatids represent the two new strands of DNA created from one chromosome during the synthesis stage of interphase. By separating all of these copies into new cells, the two new cells created are identical to the starting cell. Mitosis is used in this way to develop new organisms and repair damaged tissues. As will be seen in meiosis, the chromosomes align differently and the cell is divided twice, resulting in a reduction of genetic material in each cell.

Metaphase in Meiosis

Metaphase I

During the first division of meiosis, meiosis I, the homologous chromosomes are divided in a cell. As in mitosis, the DNA has replicated before meiosis, and all the chromosomes exist as sister chromatids. Each chromosome has a homologous pair, which represents the same portion of DNA but with different alleles. Unlike in mitosis, these homologous pairs attach to each other through metaphase I of meiosis. Instead of sister chromatids being aligned on the metaphase plate, in metaphase I the homologous pairs are lined up on the metaphase plate.

A spindle checkpoint must still be passed, this one called the meiotic spindle checkpoint. If all the chromosomes are attached to their homologous pair, and each pair is attached to microtubules from each side, the cell can proceed to anaphase I. During anaphase I, the homologous pairs will be separated. Thus, the ploidy of the cells will be reduced to haploid from diploid because each new cell will have only one copy of the genome, or only one allele per gene. Mishaps during metaphase I can cause cells to have an improper number of each chromosome in each cell. If even one homologous pair doesn’t separate right, the resulting gametes can produce non-viable offspring. If metaphase I is successful, meiosis I can continue, creating two cells, each with two copies of half a full genome.

Metaphase II

After a short break called interkinesis, the cells will begin dividing again. No DNA replication takes place during this break, therefore each cell has two copies of one allele for each gene. The chromosomes are again condensed in prophase II and the nuclear envelope breaks down at the start of metaphase II. This time however there are no homologous pairs present, only sister chromatids. During metaphase II these chromosomes will become aligned on the metaphase plate through the same processes described before. As in mitosis, the sister chromatids will break apart once the meiotic spindle checkpoint is passed. The cells can then continue their division until 4 cells are produced in total. These cells will each have only one allele per gene, and only one copy of each allele.

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