Meiosis is how sex cells reproduce, dividing their chromosomes by half to create four haploid cells that are distinct from the original cells that birthed them. Meiosis has two different phases: meiosis one (meiosis I) and meiosis two (meiosis II). These phases are further divided into sub-phases, and metaphase one is a sub-phase of meiosis one.
“Don’t let your relationship meiosis and reduce, rather mitosis and multiply.” — Satyakam Saha
During metaphase one, the tetrads of the cell congregate on the metaphase plate located halfway between the opposing ends of the cell. After this, the spindle fibers attach themselves to the chromosomes by grabbing onto the centromeres of each chromosome. The kinetochores of the respective sister chromatids are repositioned and moved to face the same pole of the cell. Due to this, both of the kinetochores become linked to the spindle fibers from their respective opposing pole. This is one of the primary differences between mitosis and meiosis. The two chromosomes in each of the chromosome pairs are separated from each other during anaphase one, the phase following metaphase one.
In a nutshell, that’s what occurs in metaphase one of meiosis. Yet to better understand the purpose of metaphase one, it must be placed in context as part of meiosis overall.
What Is Meiosis?
The cells in your body divide in order to replace themselves when old ones get worn out. This cellular division is referred to as mitosis, but meiosis is different from mitosis because the goal is to produce sex cells – gametes, or sperm and eggs – that will be combined with the sex cells of another individual and pass on the genetic code to the next generation. With meiosis, the daughter cells have half as many chromosomes as the parent cell. Instead of a diploid cell, one that has two chromosomes sets, haploid cells have only a single chromosome set. The haploid cells that humans have are eggs and sperm, and when they joined together they create a complete new diploid set.
The Phases Of Meiosis
The meiosis process. Photo: Rdbickel via Wikimedia Commons, CC-BY-SA 4.0
Meiosis is similar to mitosis in different ways, there are some key differences between the two processes, however. In general, meiosis uses similar strategies and mechanisms to both organize in separate chromosomes, but meiosis doesn’t just need to separate sister chromatids as mitosis does. Meiosis must also separate out the non-identical chromosomes that an individual organism will receive from its parents, the homologous chromosomes. This is accomplished through two different division processes, with the first cycle of division being referred to as meiosis one and the second cycle of division being called meiosis two.
Because the cellular division process happens twice, the parent cell produces four gametes instead of two. Both rounds of division go through four different phases: prophase, metaphase, anaphase, and telophase.
Meiosis I
Interphase And Prophase
A sex cell must first go through interphase before it enters meiosis one. The cell grows during interphase and begins making copies of its chromosomes, in preparation for cellular division. When prophase one of meiosis begins, the differences from mitosis begins to become clear. Chromosomes begin to condense during meiosis one, and they pair up, with each chromosome aligning with its homologous partner. The chromosomes align with one another perfectly, so that the respective genes on each of the chromosomes are parallel with one another.
After the chromosomes have aligned, they undergo a process known as crossing over. This is when the genes break apart, cross to the other chromosome and are then reconnected. This process of exchange between the chromosomes mixes the DNA of the two chromosomes, and it is facilitated by protein structures called the synaptonemal complex. The areas where crossing over occurs are basically random, which helps promote genetic diversity.
Metaphase I
Metaphase of meiosis one begins after crossing over occurs. During this phase, the mitotic spindle grabs the chromosomes and begins pulling on them, moving them towards the metaphase plate in the center of the cell. This is very similar to the positioning of the chromosomes in mitosis, but unlike mitosis, the chromosomes are linked to the microtubules from only one pole of the spindle. As a result, each half of the chromosome is bound to microtubules from one pole, and thus the homologous chromosome pairs are prepared for separation, instead of individual chromosomes. The orientation of the homologous chromosome pairs is essentially random, much like crossing over.
Anaphase I
During anaphase one, the homologs that lined up on the metaphase plate in the previous phase are pulled apart by the mitotic spindle. After being separated by the spindle, they are pulled to opposite poles of the cell. The sister chromatids are not separated by the spindle, staying together.
Telophase I
During telophase, the chromosomes that were pulled apart complete their journey to opposite ends of the cell. Whether or not the nuclear membrane (which degraded during the previous steps) reforms during telophase one is dependent upon the type of cell in question, as some organisms de-condense the chromosomes and reform the membrane during telophase Ione, while other organisms don’t. It isn’t strictly necessary to reform the membrane because the cells will proceed through another round of division in meiosis II. Cytokinesis, the process that divides one cell into two cells, typically starts during this phase and creates two haploid cells.
How the chromosomes and chromatides are distributed throughout meiosis I and meiosis II. Photo: By Qniemiec – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=22799112
Meiosis II
After the cell completes meiosis one, it readies for another round of division – meiosis two. The transition from meiosis one to meiosis two occurs without copying the DNA in the cell. Compared to meiosis one, meiosis two is a shorter, simpler process. It also has more in common with mitosis than meiosis one, and meiosis two can be thought of as mitosis but for haploid cells.
The haploid cells that were created in meiosis one begin meiosis two having a single chromosome from each of the original homolog pairs. Despite this, the chromosomes within the haploid cells are still made out of two sister chromatids. The sister chromatids will separate during the process of meiosis two, with the final result being haploid cells with non-duplicate chromosomes.
During prophase of meiosis two, the nuclear envelope will break apart if required, and the chromosomes will condense. The mitotic spindle is formed as the centrosomes move to opposite ends of the cell. The microtubules of the mitotic spindle will begin to capture the chromosomes in prophase and continue this action into metaphase. (The latter half of prophase is sometimes referred to as prometaphase.)
“Therefore, I reasoned that study of the cell cycle responsible for the reproduction of cells was important and might even be illuminating about the nature of life.” — Paul Nurse
During metaphase two, the sister chromatids that have been captured by the microtubules are shuffled around by the mitotic spindle. The chromosomes will be lined up on the metaphase plate, much as in metaphase one. Anaphase of meiosis two sees the cell ready for cytokinesis, with the sister chromatids being separated and moved towards the opposing ends of the cell.
Telophase two has the nuclear membranes in the cell begin to reform, and then develop their respective sets of chromosomes. The chromosomes de-condense now, and cytokinesis finishes, dividing the two cells into four cells. These four cells have chromosomes in them that are made out of a single chromatid. These single chromatid cells are now considered sex cells – eggs or sperm.
Why Is Metaphase Important In Meiosis One?
During metaphase of meiosis one, the homolog pairs are oriented on the metaphase plate. This orientation is necessary because, without it, the homolog pairs would have substantially less genetic diversity. The random orientation that happens means that there’s a massive amount of different possible arrangements for the homologous chromosomes. Thanks to this random orientation, for human cells there are over 8 million possible different gametes that can be created. When these possibilities are multiplied with the genetic exchange that happens in processing over, the number of different possible gametes that can be made is functionally infinite.
How Does Metaphase One Differ Between Meiosis And Mitosis?
The process of mitosis is similar to meiosis, though there is only one cycle. Photo: By Kelvinsong – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=22965076
Metaphase in mitosis is extremely similar to metaphase one of meiosis, with few important differences. Much like in metaphase one of meiosis, the chromosomes line up on the metaphase plate for cellular division, moved by the mitotic spindle. The key differences in metaphase one of mitosis and meiosis are as follows:
“A band of bacterial brothers, Swigging ATP with some others; In a jocular fit, they laughed till they split, Now they’re all microbial mothers.” — Richard Cowen
In mitosis single chromosomes line up on the metaphase plate, these chromosomes have two chromatids each. In meiosis, tetrads or paired chromosomes that have four chromatids line up on the metaphase plate.
The chromatids can extend freely as they lie on the equator of the spindle in mitosis, whereas in meiosis the paired chromosomes are properly oriented by being directed towards the poles.
In mitosis, centromeres are linked with spindle fibers from both poles, while in meiosis the spindle fibers extend from only one of the poles.
Finally, mitotic chromosomes are genetically identical while meiotic chromosomes are dissimilar because of the effects of crossing over.