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

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.

Role in Sexual Reproduction

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.

Phases of Meiosis

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

Meiosis I – Reductional Division

Meiosis I is a reductional cell division that halves the number of chromosomes. It changes a diploid to a haploid state so that gametes can then combine during reproduction.

Meiosis II – Equational Division

Meiosis II refers to the second division in the process of meiosis. In contrast to meiosis I that halved the number of chromosomes, in meiosis II equational division occurs, very much like a typical mitotic division. The sister chromatids of each chromosome are separated into two different cells. In this way, each of the four resulting haploid cells has a single set of chromosomes.

Stages of 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.

Anaphase I

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

• The separated homologous chromosomes move to opposite poles

• 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 consists 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.

Stages of 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.

Errors During Meiosis

Errors in meiosis can lead to genetic disorders in which the number of chromosomes is incorrect.

Nondisjunction

Nondisjunction is an error during meiosis in which homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II) fail to separate properly. This results in gametes with abnormal numbers of chromosomes, leading to chromosomal abnormalities. Such abnormalities can cause developmental and genetic disorders in offspring.

Chromosomal Disorders (Down Syndrome, Turner Syndrome)

The most common chromosomal disorder includes aneuploidy, having more or fewer than 46 chromosomes. Chromosomal disorders can result in a range of developmental and health problems. For example, having an extra copy of chromosome 21 causes Down syndrome. Turner syndrome is a condition in which a female has only one X chromosome instead of two.

Significance of Meiosis

The meiosis has significance in various fields such as:

Genetic Diversity

Meiosis is essential in providing genetic variation. Through crossing over processes during prophase I and independent assortment in metaphase I, leads to specific sets of genetic materials that form the gametes. Such genetic diversity is quite important for the survival as well as adaptation of species.

Maintenance of Chromosome Number

Meiosis also provides for the decreased number of chromosomes from diploid to haploid. So that when such gametes fuse during fertilisation, the correct number of chromosomes of the species is present in the zygote.

Role in Evolution and Reproduction

Meiosis causes genetic diversity and evolution 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 i.e., the haploid number of chromosomes. Errors in meiosis like nondisjunction, can lead to chromosomal disorders.

Meiosis NEET MCQs (With Answers & Explanations)

This topic carries a significant weightage in NEET exam. Important topics are:

• Stages of Meiosis I

• Stages of Prophase I

• Significance in sexual reproduction

Practice Questions for NEET

Q1. Which stage of meiotic prophase shows terminalization of chiasmata as its distinctive feature?

1. Leptotene

2. Zygotene

3. Diakinesis

4. Pachytene

Correct answer: 3) Diakinesis

Explanation:

Diakinesis is the stage in which chromosomes are fully prepared for alignment at the metaphase plate. The terminalization of chiasmata reaches completion in diakinesis, distinguishing it as a critical step before homologous chromosomes are separated during metaphase I.

Hence, the correct answer is option 3) Diakinesis.

Q2. The significance of meiosis lies in

1. Reduction of the diploid number of chromosomes to haploid

2. Maintaining constancy in the number of diploid chromosomes during sexual reproduction

3. Production of genetic variability in the population of a species

4. All of these

Correct answer: 4) All of these

Explanation:

Meiosis II occurs in haploid cells where each chromosome still consists of duplicated sister chromatids, which are separated during this phase, resulting in haploid cells with single chromatids. The significance of meiosis lies in reducing the diploid chromosome number to haploid, ensuring constancy of chromosome numbers across generations during sexual reproduction, and generating genetic variability through crossing over and independent assortment. This variability plays a key role in evolution, while meiosis also enables the formation of haploid gametes (sperms and ova), essential for sexual reproduction in organisms.

Hence the correct answer is option 4) All of these.

Q3. During anaphase I, the _____ separate while during anaphase II _____ separate.

1. Chromatids, Homologous chromosomes

2. Homologous chromosomes, chromosomes

3. Homologous chromosomes, chromatids

4. None of these

Correct answer: 3) Homologous chromosomes, chromatids

Explanation:

To ensure that each daughter cell obtains one chromosome from each pair, homologous chromosomes are driven apart to opposing poles during anaphase I of meiosis. At their centromeres, the sister chromatids stay together. In anaphase II, analogous to mitosis, the sister chromatids are divided as the centromeres divide, with chromatids traveling to opposite poles. Four haploid cells with a single pair of chromosomes are produced as a result.

Hence, the correct option is 3) Homologous chromosomes, chromatids

Also Read:

• MCQs Stages of Meiosis

• Cell Cycle and Cell Division

• Cell Division

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