Meiosis Phases

Definition: Meiosis

A single cell divides twice to create four daughter cells during a process called meiosis. As haploid cells, these offspring cells consist of half the chromosomes (DNA) as the parent cell. The gametes, or sex cells, are formed during meiosis (eggs- females and sperm- males). Flowering plants exhibits meiosis. The ovaries of the flower produce megaspore cells during meiosis. In the stamens, it produces microspore cells.

Meiosis I and II are the two phases that complete the meiotic process. The initial cell divides into two cells by the time Meiosis I is complete. DNA is present in both cells, although not completely. The original DNA is duplicated twice in each cell. The two complete cells will later divide into four half-cell gametes during Meiosis II. Half of the original DNA is replicated once in each gamete.

Stages of Meiosis

Each of the two meiosis has four steps: Prophase, Metaphase, Anaphase, and Telophase. Before the start of meiosis, the cells undergo a growth period called interphase which includes the G1, S, and G2 phases.

• G1 (gap) phase − The phase where an increase in cell mass takes place to prepare for cell division.

• S (synthesis) phase − DNA synthesis takes place during this phase.

• G2 phase − Phase that comes after DNA is synthesized but just before the start of prophase. Here the cells keep getting bigger while synthesizing proteins.

At this stage, the cell's doubled chromosomes are still in the form of chromatin and are surrounded by the nuclear envelope. The interphase stage ends and the cells move on to the meiosis I prophase stage.

Meiosis I

Prophase I

Chromosome starts to condense and travel to the cell's center during prophase I. In this stage, the nuclear shield deteriorates.

Prophase I consists of five sub-phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

Leptotene

• A tiny chromatin fiber thread can be seen at this stage.

• The nuclear membrane is unharmed and the centriole stays at one pole.

• Condensation of chromatin fibers begins and forms a beaded structure.

• The thread is linked to the nuclear membrane at one end and additional membranous structures remain the same.

Zygotene

• The nuclear membrane remains the same.

• Synapsis begins, where the homologous chromosome pairs take place.

• Condensation continues to occur.

• Chromosome pairs formed at this stage are referred to as dyads. These chromatids divide vertically to create tetrads/ bivalents and are held together by a synaptonemal complex.

Pachytene

• The nuclear membranes are unharmed. Centrioles remain in their original location.

• Each chromosome's two sister chromatids split from one another.

• Throughout pachytene, localized DNA breakage takes place around non-sister chromatids, followed by DNA transfer between them referred to as crossing over which in turn creates chiasma.

Diplotene

• In this stage, synaptonemal complex disintegrates, and recombined homologous chromosomes start to segregate from one another.

• This marks the start of terminalization where the chiasmata move toward the end of the chromatids.

Diakinesis

• The nuclear membrane breaks and centrioles separate and the chromosome completes condensation.

• Spindle fibers that bind to the chromosomes become apparent to prepare the homologous chromosomes for separation.

• The cell is then prepared to enter Metaphase I.

Metaphase I − At the center of the cell/ metaphase plate, the homologous chromosomes align themselves. The meiotic spindles are discovered to be expanded from the centrioles, which are now shifting at the opposite poles of the cell and starting to pull chromosomes to the poles.

Anaphase I − The homologous chromosome pair splits apart and moves in the direction of its antipode. The sister chromatids continue to exist as single, fully replicated chromosomes. Disjunction, the separation of homologous chromosomes takes place which aids in dividing DNA equally between 2 cells.

Telophase I − In telophase I, the nuclear membrane resurfaces, and the spindle fibres decompose followed by Cytokinesis I.

Cytokinesis I − Two daughter cells are formed during cytokinesis, which is the squeezing or formation of cell walls in animal and plant cells creating two new cells.

Meiosis II

Prophase II − Prophase II marks the beginning of meiosis II with the 2 daughter cells formed during meiosis I. The chromosomes condense, centrosomes are reproduced, and head toward the poles as the nuclear envelopes start to degrade.

Metaphase II − Chromosomes align at the metaphase plate and each centromere's 2 kinetochores attach to spindle fiber microtubules from either poles.

Anaphase II − Sister chromatids of each chromosome split and migrate in opposite directions due to the shortening of the microtubules.

Telophase II − In telophase II, the chromosomes that have reached the opposite poles uncoil and the nuclear envelope develops.

Followed by cytokinesis II where a tetrad of cells, or four haploid daughter cells, are produced.

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Importance of Meiosis

• It keeps the number of chromosomes in sexually reproducing organisms constant. A diploid cell produces haploid gametes, which combine during fertilization to form a diploid cell.

• Gametes or sex cells, which are needed for sexual reproduction are created during meiosis.

• During the recombination process, it creates genetic variations and corrects some genetic flaws.

• Meiosis is a kind of cell division in which genetic material crosses across to make novel combinations in the developing daughter cells.

• Meiotic cell division abnormalities lead to a genetic mutation. Natural selection perpetuates advantageous mutations.

Conclusion

Meiosis is the process in which the splitting of a single cell twice produces 4 progeny cells where each cell contains half of the genetic material as the original cell. The basic components of sexual reproduction, known as gametes or sex cells, are produced during meiosis. Meiosis is broken down into two stages: Meiosis I and II which are then further divided into prophase, metaphase, anaphase, telophase, and cytokinesis, respectively.

FAQs

Q1. What are synaptonemal complexes (SC)?

Ans. During meiotic prophase, the SC, a highly organized proteinaceous structure, forms at the junction of aligned homologous chromosomes and aids in the stabilization of homolog pairing as well as the development of crossovers.

Q2. What are interkinesis?

Ans. A relaxation stage known as interkinesis, or interphase II, is experienced by the cells of some species between meiosis I and II, where DNA replication doesn't take place and it is the phase where the cell is getting ready to start the second meiotic division.

Q3. When does genetic recombination take place during meiosis?

Ans. In the Pachytene phase of prophase I of meiosis I, genetic recombination takes place, where information is transferred from the parents to the progeny.

Q4. What kind of cells are involved in mitosis and meiosis?

Ans. Somatic cells or the body cells are formed during mitosis through asexual reproduction and germ cells or the cells that are capable of producing gametes are formed during meiosis and sexual reproduction.

Q5. What are kinetochores?

Ans. The protein patch on the centromere, where the chromosomes (each chromatid) are linked is known as a kinetochore. It tightly grips the centromere and aids in the movement of chromosomes during cell division by adhering to spindle fibers.