Mitosis fulfills nearly all the requirements for cell division in your body. It generates new cells during growth and replaces old and damaged cells throughout your lifetime. The purpose of mitosis is to create daughter cells that are genetically identical to their parent cells, maintaining the exact same number of chromosomes.
Meiosis, conversely, serves a singular function in the human body: the creation of gametes—reproductive cells, namely sperm and eggs. Its aim is to produce daughter cells that have precisely half the number of chromosomes as the original cell.
In other terms, meiosis in humans is a process of division that transitions us from a diploid cell—containing two sets of chromosomes—to haploid cells—having one set of chromosomes. In humans, the haploid cells produced during meiosis are sperm and ova. When a sperm and an egg unite during fertilization, the two haploid chromosome sets create a full diploid set: a new genome.
Stages of meiosis
In several respects, meiosis resembles mitosis significantly.
- The cell undergoes comparable phases and employs akin methods to arrange and divide chromosomes.
- In meiosis, the cell faces a more intricate challenge. It still requires the separation of sister chromatics (the two parts of a duplicated chromosome), similar to mitosis.
- It must also segregate homologous chromosomes, the analogous yet no identical chromosome pairs that an organism inherits from its two parents.
These objectives are achieved in meiosis through a two-step division mechanism. Homologous pairs divide in the initial stage of cell division, known as meiosis I. Sister Chromatics split in the
subsequent phase, referred to as meiosis II.
Phases of Meiosis
Because meiosis involves two rounds of cell division, a single starting cell can generate four gametes (sperm or eggs). During every division cycle, cells experience four phases: prophase, metaphase, anaphase, and telophase.
Key aspects of Meiosis I
A cell must first complete interphase before entering meiosis I. Similar to mitosis, the cell increases in size during G1 phase, duplicates all of its chromosomes in S phase, and gets ready for division in G2.
- In prophase I, distinctions from mitosis start to emerge. Similar to mitosis, the chromosomes start to condense, but in meiosis I, they also align with each other. Every chromosome meticulously pairs with its homologous partner, ensuring that both align at matching sites throughout their entire lengths.
- In the image below, the letters A, B, and C symbolize genes located at specific positions on the chromosome, where uppercase and lowercase letters denote different versions, or alleles, of each gene. The DNA is cleaved at identical locations on each homologue—specifically, between genes B and C—and joined in a crispr-cross manner, allowing the homologs to swap portions of their DNA.
- A microscope will show crossovers as chiasmata, cross-shaped structures where homologous chromosomes are connected. Each homologous pair needs at least one chiasma to maintain the connection between the homologs, even after the synaptonemal complex disintegrates. Several crossovers are typical for every homologous pair.
- The locations of crossovers are mostly arbitrary, resulting in the creation of new, “remixed” chromosomes with distinct allele combinations.
- After crossing over, the spindle starts to take up chromosomes and transport them to the center of the cell, which is known as the metaphase plate. Although it may resemble mitosis, there is a twist.
- The two homologous chromosomes of a pair bind to microtubules from opposing poles of the spindle, while each chromosome only connects to microtubules from one pole.
Therefore, for separation during metaphase I, homologous pairs—not individual chromosomes—line up at the metaphase plate.
Meiosis II
1) Inter phase:
The DNA in the cell is copied, resulting in two identical full sets of chromosomes.
Outside of the nucleus are two centrosomes, each containing a pair of centrioles; these structures are critical for the process of cell division.
During interphase, microtubules extend from these centrosomes.
2) Prophase I:
The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of two sister chromatids containing identical genetic information.
The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.
The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over.
At the end of Prophase I, the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.
The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.
3) Metaphase I:
The chromosome pairs line up next to each other along the center (equator) of the cell.
The centrioles are now at opposite poles of the cell, with the meiotic spindles extending from them.
The meiotic spindle fibers attach to one chromosome of each pair.
4) Anaphase I:
The pair of chromosomes is then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.
In meiosis I, the sister chromatids stay together. This is different from what happens in mitosis and meiosis II.
5) Telophase I and cytokinesis:
The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell, a full set of chromosomes gathers together.
A membrane forms around each set of chromosomes to create two new nuclei.
The single cell then pinches in the middle to form two separate daughter cells, each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.
