In eukaryotic cell division, metaphase occurs when the chromosomes align on the metaphase plate. Metaphase occurs after prophase and prometaphase. During these stages of cell division, the chromosomes are condensed, the spindle fibers are formed, and the nuclear envelope is broken down. A series of checkpoints ensure that the spindle has formed during metaphase and late prometaphase.
Each chromosome is attached to a microtubule emanating from each side of the cell. From each side of the cell, equal tension is applied to chromosomes as the microtubules retract. As a result, they are moved to the center of the cell. As a result of metaphase, the sister chromatids that make up the chromosomes are divided, and the process of cell division is completed.
During eukaryotic cell division, the centriole divides and sets up the microtubule network that moves the chromosomes and organelles. The microtubules wind together to form larger fibers that extend out from the centrosomes. While the fibers are stable near the centrosomes, as they reach out towards the chromosomes, they become less stable. At the unstable end, fibers add and subtract pieces as they grow towards the chromosome.
Fibers wander through the cytoplasm as they grow in this way, three steps forward, two steps back. A chromosome’s centromere is eventually connected to the fibers. There are kinetochores at each centromere where microtubules can attach.
The spindle assembly checkpoint is one of the most important processes before and during metaphase. Chromosome division is ensured by the spindle assembly checkpoint, a complex series of mechanisms. The chromosomes align differently during mitosis and meiosis, but both go through a spindle assembly checkpoint during metaphase.
Cells will begin anaphase before their chromosomes are properly attached to microtubules and aligned on the metaphase plate if these checkpoints are skipped or do not function properly. In this case, the chromosomes are sorted into the wrong cells. In the resulting daughter cells, too many or too few chromosomes may be produced. Birth defects and non-viable offspring can result from meiosis. This can lead to cancerous cells if it occurs during mitosis.
Metaphase in Mitosis
Chromosomes align in the middle of the cell during mitosis, with sister chromatids on either side of the metaphase plate. During interphase, the cell replicates its DNA before mitosis. DNA-containing chromosomes condense before metaphase, so the movements that occur during metaphase will not damage them.
Chromosomes are randomly arranged within the nucleus at the start of metaphase and during late prometaphase. Chromosomes are connected by microtubules after the nuclear membrane is dissolved.
Each chromosome is connected to its centrosome by microtubules during mitosis. Chromosomes are composed of two sister chromatids that are connected by proteins called cohesins. In order for cohesins to be broken apart, all chromosomes must be attached to microtubules from both sides and aligned on the metaphase plate by the mitotic spindle checkpoint.
The anaphase-promoting complex is activated when this checkpoint is passed by the chromosomes. Activation of this complex marks the end of metaphase in mitosis and the beginning of anaphase.
With sister chromatids on each side of the metaphase plate, the chromosomes align to create identical cells. In interphase, sister chromatids represent the two new strands of DNA created from a chromosome. When all of these copies are separated into two new cells, the two new cells are identical to the starting cell.
The process of mitosis is used in this way for the development of new organisms and the repair of damaged tissue. Chromosome alignment changes and the cell is divided twice in meiosis, resulting in less genetic material in each cell.
Metaphase in Meiosis
Homologous chromosomes are divided in a cell during the first division of meiosis, meiosis I. As in mitosis, all chromosomes exist as sister chromatids before meiosis. Each chromosome has a homologous pair, which represents the same DNA, but with different alleles. Through metaphase I of meiosis, these homologous pairs attach to each other instead of in mitosis. In metaphase I, homologous pairs line up on the metaphase plate instead of sister chromatids.
This spindle checkpoint is called the meiotic spindle checkpoint. A cell can proceed to anaphase I if all its chromosomes are attached to their homologous pairs, and each pair is attached to microtubules from both sides. Separation of homologous pairs will occur during anaphase I. As a result, each new cell will contain only one copy of the genome, or only one allele per gene, so the ploidy of the cells will be reduced to haploid from diploid.
A mishap during metaphase I can result in cells having an unbalanced number of chromosomes. The resulting gametes can be non-viable if even one homologous pair doesn’t separate correctly. Meiosis I can continue if metaphase I is successful, creating two cells with half a genome each.
A short break called interkinesis will be followed by the cells dividing again. During this break, DNA replication does not occur, so each cell has two copies of one allele. At the beginning of metaphase II, the nuclear envelope breaks down and the chromosomes condense again.
There are no homologous pairs present this time, only sister chromatids. In metaphase II, these chromosomes will align on the metaphase plate through the same processes described previously. After the meiotic spindle checkpoint is passed, the sister chromatids will break apart. Afterwards, the cells can divide until four cells have been produced. Each cell will have one allele per gene, and only one copy of each allele.
Related Biology Terms
- Prometaphase – The stage immediately preceding metaphase, in which the nuclear membrane breaks down.
- Anaphase – The stage immediately after metaphase, when the chromosomes are separated.
- Metaphase Plate – The imaginary line in the middle of the cell that chromosomes align on during metaphase.
- Eukaryote – An organism with a membrane-bound nucleus and organelles.
Metaphase is a stage of cell division, specifically mitosis and meiosis, during which the chromosomes align along the equator of the cell.
Metaphase is an important stage in cell division because it ensures that the chromosomes are properly aligned before they are separated during anaphase. Proper alignment is important to prevent errors in the distribution of genetic material to the daughter cells.
Metaphase is different from other stages of cell division, such as prophase and anaphase, because it is characterized by the alignment of the chromosomes along the equator of the cell. In prophase, the chromosomes condense and the nuclear envelope breaks down, while in anaphase the chromosomes are pulled apart and moved to opposite poles of the cell.
Metaphase is regulated by several checkpoints in the cell cycle to ensure proper chromosome alignment and separation. These checkpoints are controlled by a variety of proteins and enzymes that monitor the progress of the cell cycle and halt the process if necessary.
Abnormalities in Metaphase can lead to chromosomal abnormalities and genetic disorders such as Down syndrome. These abnormalities can occur due to errors in chromosome alignment or segregation during cell division. Understanding Metaphase and its regulation is important in preventing and treating genetic disorders.