Usually, cells divide by mitosis, which results in two daughter cells – exact copies of the mother cell. However, this division is not suitable for sexual reproduction, the main task of which is to mix maternal and paternal DNA. In order for this to happen, cells are needed that carry one single set of chromosomes, whereas, in a standard cell, each chromosome is duplicated. Such special cells with only one set of chromosomes are formed with the help of another division – meiosis.
Sexual reproduction and genetic diversity
The main role of sexual reproduction is quite obvious – it is the mixing of two different genotypes to form a common daughter organism. A greater variety of genes makes it possible to "improve" the genome – for example, to compensate for the "non-working" gene of the father with its "working" counterpart of the mother.
Sexual reproduction facilitates the "mixing" of genotypes, forming new organisms that are not genetical copies of each other. This not only provides a way for evolution since mutations can thus be fixed and passed on to offspring but also promotes genetic diversity. Genetic diversity is a very important characteristic of a population and a species as a whole since it is precisely this that serves as the basis for both variability and population stability. The more diverse the population is, the more different types of certain genes it contains, the more stable the population is, and the more likely it is that with some serious changes in the environment, part of the population will survive.
Since most often in eukaryotic cells the set of chromosomes is diploid (the gene has two copies each), then only one should come from the "mom" and "dad" to create a new organism. Ordinary diploid cells are not suitable for reproduction. That is why meiosis appeared – a process in which special gamete cells (eggs and sperm) are formed that carry only one set of DNA. Meiosis is also called reduction division because it divides the chromosome set into two.
Crossing over
One of the most important events that occur during meiosis is crossing over – the exchange of chromosomes by homologous regions (those regions that carry the same genes). This is a complex process that ensures the genetic diversity of future offspring.
Each chromosome in a cell is represented by a pair of chromosomes, they are called homologous. During crossing over, chromosomes exchange regions that are also homologous to each other (fig. 2). A special protein complex makes a cut in the DNA chain, this happens in special recombination points. After that, the chromosomes exchange sections, after which the breaks are "sewn up" using repair systems and polymerases.
It is crossing over that allows the offspring to be not a copy of one of the parents, but to have "random" genes that allow creating a more advantageous genotype in terms of survival.
Meiosis I
Meiosis is a special way of dividing eukaryotic cells, as a result of which four daughter cells are formed from one mother cell with a set of chromosomes reduced by 2 times. At the same time, structurally, it looks like two successive divisions, they are called meiosis I and meiosis II. In this case, replication – DNA doubling – precedes only meiosis I, while DNA does not double before the second division. The stages of meiosis are similar to the stages of mitosis: they also distinguish between prophase, metaphase, anaphase, and telophase, after which, depending on the stage of meiosis, we put the number I or II.
Prophase I
Chromatin of the nucleus condenses, and chromosomes consisting of two strands become distinguishable. This state is described by the formula 2n4c, which means that the cell has two chromosomes and four strands of chromatin. In this phase, crossing over occurs.
In this stage, spindle fission is also formed and the nuclear envelope disintegrates.
Metaphase I
At this stage, the fission spindle is finally formed. Chromosomes are located at the equator of the cell and form a metaphase plate. Spindle threads, coming from opposite poles of the cell, are attached to the centromeres of chromosomes.
Anaphase I
The spindle of division pulls the chromosomes to opposite poles of the cell. This is one of the differences between meiosis and mitosis – during mitosis, chromatids are pulled apart to the poles, and not whole chromosomes. The formula at this stage is 1n2c, that is, there is one chromosome at the poles, consisting of two strands.
Telophase I
This is the final stage of meiosis I. The division spindle is destroyed. There is a formation of two daughter cells and nuclei inside them. Each cell is haploid, that is, it has the formula 1n2c. After that, the cells are ready to enter meiosis II. Sometimes there is a short break between the two stages of meiosis, sometimes there is none at all.
Meiosis II
Meiosis II does not require DNA replication; accordingly, cells with a set of 1n2c enter it.
Prophase II
Chromatin spiralization occurs again and two-chromatid chromosomes are formed. The spindle of division is formed. After the breakdown of the nuclear membrane, the chromosomes are released into the cytoplasm of the cell.
Metaphase II
The formation of the fission spindle is completed. The chromosomes line up in the metaphase plate.
Anaphase II
The division of the centromere occurs. Unlike anaphase I, sister chromatids are stretched to the poles of the cell at this stage.
Telophase II
Chromosomes are despiralized, nuclei are formed, and the final daughter cells are formed. Each of them has the formula 1n1c – one chromosome and one thread. These cells are ready for sexual reproduction.
Thus, four cells are formed from one cell during meiosis, potentially ready to "mix" with another haploid cell to form the first cell of a new organism. The new cell will already be diploid and will be able to divide via mitosis.
Conclusion
Meiosis is an important evolutionary "invention" that allows the formation of four haploid cells from one diploid cell, for further sexual reproduction. It is through meiosis that gametes are formed. Each meiosis consists of two stages – meiosis I and meiosis II, the processes in which – prophase, metaphase, anaphase, and telophase – are very similar.