Study of stages of meiosis using permanent slides.
What is Meiosis?
Meiosis is a type of cell division in which the number of chromosomes is halved (from diploid to haploid) in the daughter cells, i.e., the gametes. The meiosis is completed in two phases.
Events During Meiosis
Meiosis I and Meiosis II
- Meiosis I is known as reductive division, as the cells are reduced from being diploid cells to being haploid cells or in which the chromosomes of homologous pairs separate from each other.
- Meiosis I occurs after Interphase I, where proteins are grown in G phase and chromosomes are replicated in S phase. Following this, four phases occur they are:
- Prophase 1
- Metaphase 1
- Anaphase 1
- Telophase 1
- Meiosis II is equational division resulting in the formation of four daughter cells.
- Meiosis II may begin with interkinesis or interphase II. This differs from interphase I in that no S phase occurs, as the DNA has already been replicated. Thus only a G phase occurs.
- Stages of meiosis can be observed in a cytological preparation of the cells of testis or in the pollen mother cells of an anthers of flower buds.
Stages of Meiosis I
- In this stage, the chromosomes condense and move towards the centre of the cell. It consists of five different sub-phases.
- It is the longest phase of meiosis I. In this stage, the chromosomes condense and move toward the center of the cell. It consists of five different sub-phases.
- Leptotene is the first substage of prophase I.
- At leptotene the homologous chromosomes replicates.
- In this phase, the chromatin threads get condensed and appear shortened and thick.
- The homologous chromosomes form a pair called as synapsis.
- After the pairing of the homologous chromosomes, they can be seen as paired chromatin threads which are called tetrad or bivalent.
- In pachytene the bivalents, exchange the gene segments between the non-sister chromatids of the homologous chromosomes and this process is called the crossing over.
- The place where the exchange of chromosomes or crossing over takes place is called chiasma.
- In diplotene, the bivalent chromosomes unlock themselves resulting in the separation of the two homologous chromosomes.
- The two homologous chromosomes migrate apart and disintegrate between the chromosomal arms.
- The homologous chromosomes separate from each other and regain their individual identity.
- Also, the nuclear membrane and the nucleolus completely disappear at this stage and the centrosome moves to the equator.
- In this stage, the homologous pairs align on the equator of the cell with a random orientation.
- This arrangement forms a metaphase plate and is a result of independent assortment.
- The homologous pairs separate from each other and move toward the opposite poles and the separated chromosomes are pulled toward the centrioles on either side of the cell.
- Microtubules begin to shorten, this pulling of one chromosome of each homologous pair to opposite poles in a process known as disjunction.
- The sister chromatids of each chromosome stay connected. The cell begins to elongate in preparation for cytokinesis.
- Meiosis I end when the chromosomes of each homologous pair arrive at opposing poles of the cell. In telophase 1 the microtubules start to disintegrate, and the chromosomes are completely pulled apart and a new nuclear envelope forms around each haploid set of chromosomes.
- The chromosomes get uncoiled, forming the chromatin again, and the cytokinesis occurs, which results in the forming of two non-identical daughter cells.
- These daughter cells are haploid and are formed from the diploid parent cells. However, Chromosomes are still attached to the sister chromatids. So, in order to separate the sister chromatids from each other. Each haploid cell undergoes another round of division.
- Before moving ahead the haploid cells enter a small resting phase called the interkinesis or interphase II. It’s a short phase with no activities involved.
Stages of Meiosis II
Meiosis II may begin with interkinesis or interphase II. This differs from interphase I in that no S phase occurs, as the DNA has already been replicated. Thus, only a G phase occurs. Meiosis II is known as equational division, as the cells begin as haploid cells and end as haploid cells.
There are again four phases in meiosis II: these differ slightly from those in meiosis I.
- In this stage, the chromosomes start condensing and become short and compact. Whereas, The nuclear envelope and nucleolus disintegrate.
- The Centrioles and spindle fibers begin to appear.
- No crossing over occurs in this stage.
- In metaphase II the chromosomes align at the equatorial plane forming the metaphase plate.
- Spindle fibers connect to the kinetochore of each sister chromatid.
- One sister chromatid faces each pole, with the arms divergent.
- In anaphase II the sister chromatids separate and are known as sister chromosomes.
- The spindle fibers connected to each sister chromatid gets shorten, pulling one sister chromatid to each pole.
- The cell divides into two and a new nuclear envelope surrounds the chromosomes.
- Meiosis II ends when the sister chromosomes have reached opposing poles.
- The spindle disintegrates, and the chromosomes recoil, forming chromatin.
- This nuclear envelope forms around each haploid chromosome set, before cytokinesis occurs, forming two daughter cells from each parent cell, or four haploid daughter cells in total.
- Students can understand the concept of meiosis.
- Students can understand different stages of meiosis.
- Students can learn the theory of crossing over and cell division.