What Happens During Each Stage of Meiosis II?
Meiosis is a crucial process in sexually reproducing organisms because it creates haploid gametes, preventing the continuous doubling of chromosomes across generations. After completing Meiosis 1, cells enter Meiosis 2, which ensures the separation of sister chromatids and results in four distinct haploid cells. In this article, we will explain what happens in meiosis 2, describe the meiosis 2 stages in detail, provide an overview, and discuss the importance of cell division.
What is Meiosis 2 Also Known As?
Meiosis 2 is often called the “equational division” because, unlike Meiosis 1 (which reduces the chromosome number by half), the chromosome number in each daughter cell remains the same during Meiosis 2. Each cell that enters Meiosis 2 retains the haploid number of chromosomes, but the sister chromatids split into separate chromosomes, much like in a mitotic division.
What Happens in Meiosis 2?
Meiosis 2 - Schematic Representation
Although this article focuses on descriptions, you can refer to a simple meiosis 2 diagram that typically shows:
One haploid cell with duplicated chromosomes enters Prophase II.
Chromosomes line up at the centre in Metaphase 2.
Sister chromatids separate in Anaphase 2.
Four haploid cells result at the end of Telophase 2 meiosis.
Such a diagram visually illustrates what happens in meiosis 2 and clarifies how each of the meiosis 2 stages progresses.
Meiosis 2 resembles mitosis in many ways. Each haploid cell produced after Meiosis 1 undergoes another round of division to separate the sister chromatids. The result is four haploid cells, each genetically unique. Let us delve deeper into the four meiosis 2 stages:
Prophase II
The nuclear membrane, if reformed after Meiosis 1, disappears once again.
Chromosomes condense and become visible under the microscope.
The centrosomes migrate to opposite poles.
The cell prepares its spindle apparatus for the next stage.
Metaphase II
During metaphase 2, the condensed chromosomes align at the equatorial plane of the cell (the metaphase plate).
Spindle fibres connect the centromeres to both poles.
Each chromosome’s sister chromatids are set to move to opposite poles.
Anaphase II
Anaphase 2 begins when the centromeres split, allowing the sister chromatids to separate.
The separated chromatids (now individual chromosomes) are pulled towards opposite poles of the cell by the spindle fibres.
This ensures that each new daughter cell will receive one copy of each chromatid.
Telophase II
In telophase 2 meiosis, the chromosomes reach the opposite ends of the cell.
Nuclear membranes reform around each set of chromosomes.
Chromosomes begin to decondense, turning back into a diffuse chromatin state.
Cytokinesis follows, dividing the cytoplasm and resulting in four distinct haploid daughter cells.
These four meiosis 2 stages (Prophase II, Metaphase 2, Anaphase 2, and Telophase 2 meiosis) are crucial for ensuring that each gamete has the correct set of genetic information.
Significance of Meiosis II
Ensures Haploid Gametes: By splitting sister chromatids, Meiosis 2 finalises the process started in Meiosis 1, guaranteeing that each gamete retains only a single copy of each chromosome.
Genetic Variability: While the main event of genetic recombination occurs during Prophase I of Meiosis 1, the distribution of chromatids in Meiosis 2 contributes to the overall assortment of chromosomes. This assortment, combined with fertilisation, increases genetic diversity in offspring.
Prevents Chromosome Doubling: Without both meiosis 1 and 2, the chromosome number would double each generation, eventually making normal development impossible.
Differences Between Meiosis 1 and 2
Chromosome Pairing
In Meiosis 1, homologous chromosomes pair up and may exchange genetic material. In Meiosis 2, there is no pairing of homologues; sister chromatids simply separate.
Division Type
Meiosis 1 is called reductional division because it reduces the chromosome number from diploid to haploid. Meiosis 2 is called equational division because the chromosome number remains haploid in the daughter cells.
Genetic Recombinations
Crossing over typically occurs in Prophase I of Meiosis 1. There is no crossing over in Meiosis 2.
Outcome
Meiosis 1 produces two haploid cells, each with duplicated chromosomes. Meiosis 2 produces four haploid cells, each with a single copy of each chromosome.
Understanding the key contrasts in meiosis 1 and 2 helps explain how organisms maintain stable chromosome numbers across generations and introduce genetic variation simultaneously.
Quick Quiz (With Answers)
Test your understanding of Meiosis 2 with these simple questions:
Which phase of Meiosis 2 involves the separation of sister chromatids?
Answer: Anaphase 2.
Name the stage in Meiosis 2 when chromosomes line up at the equator of the cell.
Answer: Metaphase 2.
How many haploid cells are produced at the end of Meiosis 2?
Answer: Four haploid cells.
What is Meiosis 2 also known as?
Answer: Equational division.
True or False: Meiosis 2 begins with diploid cells.
Answer: False. It begins with haploid cells (though each chromosome is duplicated).
Related Topics
FAQs on Meiosis II Cell Division: Complete Guide, Diagrams & Importance
1. What is Meiosis II and why is it called an equational division?
Meiosis II is the second phase of meiotic cell division where the two haploid cells from Meiosis I divide to produce four genetically unique haploid cells. It is called an equational division because, similar to mitosis, it separates sister chromatids, and the chromosome number of the cells at the beginning of the phase is equal to the number in the daughter cells at the end.
2. What are the key stages of Meiosis II?
Meiosis II proceeds through four main stages to ensure the separation of sister chromatids. These stages are:
Prophase II: The nuclear envelope breaks down, and the spindle fibres begin to form.
Metaphase II: The chromosomes, each consisting of two sister chromatids, align individually along the metaphase plate (the cell's equator).
Anaphase II: The centromeres split, and the sister chromatids are pulled to opposite poles of the cell. Each chromatid is now considered an individual chromosome.
Telophase II: The chromosomes arrive at the poles, and nuclear envelopes reform around them, followed by cytokinesis, which divides the cytoplasm to form the final four haploid cells.
3. What is the main difference between Meiosis II and Mitosis?
The most significant difference is the ploidy of the starting cell. Meiosis II begins with haploid cells (n) that were produced in Meiosis I. In contrast, mitosis typically begins with diploid cells (2n). Consequently, Meiosis II produces four haploid cells, while mitosis produces two diploid cells that are genetically identical to the parent cell.
4. Why is Meiosis II necessary if Meiosis I already halves the chromosome number?
Meiosis I separates homologous chromosomes, but it does not separate the sister chromatids. This means that after Meiosis I, each chromosome is still composed of two identical strands (sister chromatids). Meiosis II is essential to separate these sister chromatids, ensuring that the final gametes (sperm or egg cells) contain only one copy of each chromosome, making them truly haploid (n).
5. How does Prophase II differ from Prophase I?
The key difference is that crossing over, the exchange of genetic segments between homologous chromosomes, occurs only during Prophase I. This event is a major source of genetic variation and does not happen in Prophase II. Additionally, Prophase II begins with a haploid set of chromosomes, whereas Prophase I starts with a diploid set.
6. What is the most important event that occurs during Anaphase II?
The most critical event during Anaphase II is the splitting of the centromeres. This action allows the sister chromatids, which were joined together, to finally separate and be pulled to opposite poles. This separation is what ensures each daughter cell receives the correct number of individual chromosomes.
7. What is the end product of Meiosis II?
The end product of Meiosis II is the formation of four genetically distinct haploid cells from the two haploid cells that entered the division. In animals, these cells develop into gametes (e.g., sperm cells in males). Each of these cells contains half the number of chromosomes as the original parent cell.
8. What happens if an error occurs during Meiosis II?
An error in Meiosis II, such as the failure of sister chromatids to separate during Anaphase II (an event called nondisjunction), can lead to aneuploidy. This means the resulting gametes will have an incorrect number of chromosomes (e.g., n+1 or n-1). If such a gamete is involved in fertilization, it can cause genetic disorders like Down syndrome or Turner syndrome in the offspring.
