Stages Of Meiosis: Definition, Cell Division, Stages, Diagram, Process

Meiosis can be termed as the type of cell division in which the DNA of the chromosomes replicates only once but the cell divides twice. Meiosis is a topic of the chapter Cell Cycle and Cell Division in Biology.

What is Meiosis?

Meiosis is defined as a special kind of cell division that reduces the number of chromosomes. It is the process by which one diploid cell gets differentiated into four genetically different haploid cells. This process has been considered to be of prime importance in the sexual reproduction of the eukaryotic organisms.

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1. What is Meiosis?
2. Stages of Meiosis
3. Significance of Meiosis
4. Recommended video for Meiosis

Indeed, meiosis forms the very basis of sexual reproduction in that it allows any given offspring to inherit a unique combination of genetic material from both parents. Genetic variation is basic for evolution and adaptation as environments change. By generating haploid gametes-sperm and eggs-meiosis conserves an organism's chromosome number across generations.

Stages of Meiosis

The process of meiotic division is broadly divided into two parts:

Meiosis I

Various stages of meiosis I includes:

Prophase I

• The chromosomes condense further and each becomes visible under a microscope as individual distinct structures.

• Each chromosome has already replicated so each consists of two sister chromatids.

• Homologous chromosomes, each consisting of two sister chromatids, pair closely in a process known as synapsis and form structures called tetrads, or bivalents.

• Each tetrad consists of four chromatids.

• At synapsis, homologous chromosomes sometimes exchange segments like buttons on a string.

• This is crossing over because the genetic material is exchanged where the chromosomes overlap in a very specific location.

Metaphase I

• The tetrads line up along the metaphase plate, a plane set between the poles of the cell equidistant from them.

• The result is that the homologous pair members are oriented toward opposite poles.

• Spindle fibres are now attached to the kinetochores of the homologous chromosomes.

• Each chromosome of one tetrad is attached to one pole's spindle fibres and the other pole's spindle fibres.

• This will eventually have the effect of placing the homologues in different cells in the next stage.

Anaphase I

• The homologous chromosomes of each bivalent separate from each other.

• The separated homologous chromosomes move to opposite poles

• Hence, this is for the separation of the chromosomes and not the chromatids in this phase.

• So, each chromosome will still have two sister chromatids.

• Therefore, anaphase I consist of a decrease in the number of chromosomes.

Telophase I

• Two daughter nuclei are formed but the chromosome number is half of the chromosome number of the mother cell.

• This phase is not necessarily complete wholly.

• The spindle disappears, but new nuclear envelopes need not form before the onset of meiosis II.

Cytokinesis I

• It may or may not follow the telophase I.

• When it occurs, it forms the dyad of cells.

Meiosis II

The cells that enter meiosis II contain the haploid number of chromosomes.

Prophase II

• If nuclear envelopes are formed, they fragment into vesicles.

• The centrosomes that are duplicated during interkinesis move apart from each other toward opposite poles and form new spindles.

Metaphase II

• The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

• The sister chromatids are pulled apart and move toward opposite poles.

• The separation of the chromatids of the chromosomes occurs in this phase.

Telophase II and Cytokinesis

• The chromosomes reach opposite poles and decondense.

• Nuclear envelopes form around the chromosomes.

• Cytokinesis splits the two cells into four different haploid cells.

Also Read:

• MCQ Practice on Stages of Meiosis
• Cell Cycle and Cell Division
• Cell Division
• Mitosis- Equational division

Significance of Meiosis

It is in meiosis that genetic diversity and evolution take place through the creation of genetically diverse haploid gametes. This creates diversity so that offspring may inherit and adopt unique combinations of genotypes from their parents. This is crucial to adapt to changing environments, thereby propelling evolutionary processes. Additionally, meiosis is required for sexual reproduction since it creates gametes with half the number; that is, the haploid number of good chromosomes. Meiosis gone wrong is incredibly strong evidence of the necessity of accurate meiosis in producing healthy offspring. Meiosis errors, such as nondisjunction, can lead to chromosomal disorders, for example, Down syndrome. It shows how integral meiosis is to proper genetic inheritance.

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