Question

Why is meiosis called reduction division? Describe the various stages with relevant diagrams. Add a note on the significance of meiosis.

Answer 1

Hint: It is a phenomenon that occurs in sexually reproducing organisms in which shuffling of genetic information takes place.

Complete answer:
Meiosis is a type of cell division in which the chromosome number in the cells reduces by half that resulted in the production of haploid daughter cells, hence this cell division is known as a reduction division. It involves two rounds of nuclear and cell division known as meiosis I and meiosis II. Before meiosis begins, the DNA of each chromosome is replicated in the S-phase of the cell cycle. The meiotic event includes various stages to complete the cell division.

Meiosis I
Meiosis I segregate the homologous chromosome and produce two haploid cells. It reduces the ploidy from diploid to haploid hence referred to as the reductional division. It consists of different stages:
Prophase I: Prophase I is the longest and more complex phase of cell division that has been further divided into the five stages on the basis of chromosomal appearance: Leptotene, Zygotene, Pachytene, Diplotene and, Diakinesis. The leptotene stage starts with the compaction of chromosomes and followed by the zygotene. This phase starts with the pairing of the chromosomes together and the process is known as synapsis. The paired chromosomes are called homologous chromosomes and form a complex structure known as the synaptonemal complex. In the pachytene stage, bivalent chromosomes clearly appear as tetrads, and crossing over takes place between non-sister chromatids of the homologous chromosomes. The diplotene stage is recognized by the X-shaped chiasmata where recombined homologous chromosomes of the bivalents separate from each other. The final stage of meiosis I is diakinesis that involves termination of chiasmata. By the end of diakinesis, the nucleolus disappears, chromosomes fully condensed and the nuclear envelope starts to disappear.
Metaphase I: In this stage, bivalent chromosomes align on the equatorial plate and microtubules from the one poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I: In anaphase I, homologous chromosomes are pulled and move apart to opposite ends of the pole but sister chromatids remain associated at their centromeres.
Telophase I: In this phase the nucleus and nucleolus start reappear and the chromosomes arrive at opposite poles of the cell. At the same time, telophase is followed by cytokinesis that forms two cells.

Meiosis II
Meiosis II is a shorter and simpler process than meiosis I and it leads to formation of four haploid cells at the end.
Prophase II: In this phase nuclear membrane starts to disappear and chromosomes get condensed. This phase resembles a mitosis cell division.
Metaphase II: In metaphase II, chromosomes line up individually along the equator and the microtubules get attached to the kinetochores of sister chromatids from opposite poles of the spindle.
Anaphase II: This phase starts with the separation of the centromere of each chromosome that allows them to move toward opposite poles of the cell.
Telophase II: Meiosis II ends with the telophase II in which the chromosomes again arrive at opposite poles of the cell. The cytokinesis starts in this stage that divides the chromosomes into new cells i.e. four haploid daughter cells.

Note: The significance of meiosis includes the formation of gamete cells which is responsible for sexual reproduction.
- Meiosis maintains the constant number of chromosomes by reducing it into half.
- Meiosis increases the genetic variability in the population of organisms from one generation to the next due to crossing over and this genetic variation is essential for the process of evolution.