Meiosis I : Reductional Cell Division: Stages, Overview & Importance

What is Meiosis I?

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 without excessive chromosome numbers. The reduction is accomplished in two successive cell divisions: meiosis I and meiosis II.

Meiosis I produce two cells that differ from each other. After prophase I, homologous chromosomes pair up and in anaphase I, the paired chromosomes separate. This allows more genetic diversity through crossing over. Hence, all the resulting gametes will have a unique combination of genetic material, being thus a driver for evolutionary adaptation.

Stages of Meiosis I

Meiosis I is studied under Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

• It is the longest phase of meiosis I.

• It is divided into five substages: leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene

• The chromatin begins to condense to form chromosomes.

• Chromosomes appear as thin and long threads.

• By the end of this phase, chromosomes become visible under a microscope.

Zygotene

• The pairing of the homologous chromosomes initiates in this phase, known as synapsis.

• The synapsed homologous chromosomes appear in the form of bivalent chromosomes or tetrad of chromatids.

• In the tetrad, two similar chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes are termed non-sister chromatids.

• A filamentous ladder-like nucleoprotein complex, called a synaptonemal complex appears between the homologous chromosomes. It holds the homologous chromosomes together.

Pachytene

• The exchange of parts between non-sister chromatids occurs during this phase which is called the crossing over.

• Crossing over occurs through breakage and reunion of chromatid segments.

• Breakage is called nicking and it is assisted by an enzyme endonuclease.

• Reunion is termed annealing and aided by an enzyme ligase.

Diplotene

• This phase involves pulling away the synapsed homologous chromosomes.

• The point of attachment of the homologous chromosomes where crossing over occurs is called chiasma.

• Homologous chromosomes remain attached only at chiasma.

• There can be more than one chiasmata.

Diakinesis

• It marks the terminalization of chiasma.

• The nuclear membrane and nucleolus degenerate.

• Chromosome recondenses and tetrad moves to the metaphase plate.

• Spindle fibres begin to form.

• When the diakinesis of prophase-I is completed then the cells enter into metaphase-I.

Metaphase I

• During this phase, bivalents arrange themselves on the metaphase plate.

• Hence, a fully formed spindle and equatorial alignment of the chromosomes are seen during this phase.

• The alignment of homologous chromosomes is independent of each other.

• This is responsible for generating genetic variability.

Anaphase I

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

• The separated homologous chromosomes move to opposite poles

• Therefore, in this phase, the chromosomes separate and not the chromatids.

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

• Hence, anaphase I involves a reduction in the number of chromosomes.

Telophase I

• Two daughter nuclei are formed but the chromosome number is half 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 and Interkinesis

• Cytokinesis may or may not follow the telophase I. When it occurs, it forms the dyad of cells.

• Following cytokinesis I, the cells enter interkinesis. It is also known as intermeiotic interphase.

• During this phase, there is no duplication or replication of DNA as the chromosomes are already duplicated.

Difference Between Mitosis and Meiosis I

The difference between mitosis and meiosis I are:

Significance of Meiosis I

The significance of Meiosis I includes:

Genetic Diversity

Meiosis I is essential in providing ways in which genetic diversity can take place. Through crossing over processes during prophase I and independent assortment that takes place 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 I 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 characteristic of the species is present in the zygote.

Role in Evolution and Reproduction

Meiosis allows for greater variability in the offspring. Thus, it is recognised as a process of great importance in maintaining genetic integrity and therefore in promoting evolutionary success among sexually reproducing organisms.

Meiosis I NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Stages of Meiosis I

• Stages of Prophase I

• Significance in Genetic Diversity

Practice Questions for NEET

Q1. During mitosis, ER and nucleolus begin to disappear at:

1. Late prophase

2. Early metaphase

3. Late metaphase

4. Early prophase

Correct answer: 4) Early prophase

Explanation:

The disintegration of ER and nucleolus occurs during early prophase. The ER, responsible for protein and lipid synthesis, starts breaking down to facilitate the redistribution of cellular components and the assembly of the mitotic apparatus.

Hence, the correct answer is option 4) Early prophase.

Q2. The complex formed by a pair of synapsed homologous chromosomes is called

1. Equatorial plate

2. Kinetochore

3. Bivalent

4. Axoneme

Correct answer: 3) Bivalent

Explanation:

The complex, bivalent, is formed by synapsed homologous chromosomes. Prophase I of meiosis involves homologous chromosomes pairing and aligning closely, with each bivalent comprising two homologous chromosomes and four sister chromatids. Crossing over enhances genetic diversity. It's crucial for precise chromosome segregation, guaranteeing daughter cells inherit the correct chromosome count.

Hence, the correct answer is option 3) Bivalent.

Q3. Which one is the longer phase of meiosis- I?

1. Metaphase-I

2. Prophase

3. Prophase-I

4. Anaphase-I

Correct answer: 3) Prophase-I

Explanation:

The longest phase of meiosis is called prophase I. Important mechanisms that contribute to the prolonged duration of this phase include synapsis (the pairing of homologous chromosomes), crossing over (the exchange of genetic material between homologous chromosomes), and the creation of the meiotic spindle.

Compared to Prophase-I, metaphase is rather brief in both mitosis and meiosis.

Since prophase in mitosis is a component of the mitotic division, which is different from meiosis, prophase-I of the mitotic phase does not occur.

Because chromosomes are drawn apart and toward opposing poles, anaphase is also a brief phase.

Thus, Prophase-I is the longest phase in meiosis I.

Hence, the correct answer is option 3)Prophase-I of the meiotic phase.

Also Read:

• MCQs Stages of Meiosis

• Cell Cycle and Cell Division

• Cell Division

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