Difference Between Meiosis I and Meiosis II

The key difference between meiosis I and meiosis II is that meiosis I is the first cell division of meiosis that produces two haploid cells from a diploid cell while meiosis II is the second cell division that completes the meiosis by producing four haploid cells.

Meiosis is a complex cellular and biochemical process that reduces the chromosome number into half during the formation of gametes in an organism. In the end, this process produces four daughter cells each having a haploid number of chromosomes from one diploid cell. Meiosis occurs only during the sex cell formation in spermatogenesis and oogenesis. It consists of two nuclear divisions, namely meiosis I and meiosis II. Accordingly, meiosis I and meiosis II have four subphases in each. As soon the cells finished undergoing meiosis I, they start undergoing meiosis II. Moreover, there is no interphase between these two phases.

CONTENTS

1. Overview and Key Difference
2. What is Meiosis I
3. What is Meiosis II
4. Similarities Between Meiosis I and Meiosis II
5. Side by Side Comparison – Meiosis I vs Meiosis II in Tabular Form
6. Summary

What is Meiosis I?

Meiosis I is the first cell division of meiosis. There is interphase before meiosis I. It runs for a longer time. Meiosis I consists of four sub-phases namely Prophase I, Metaphase I, Anaphase I, and Telophase I. During prophase I, chromosomes condense and pair up and align with the homologous chromosomes. Then these homologous chromosome pairs, exchange their genetic materials between them by forming chiasmata. Herein, the exchange of homologous parts between homologous chromosomes is known as crossing over, and it is responsible for the genetic variation.

After crossing over, these pairs move to the metaphase plate and arrange beside it during the metaphase I. Spindle from each pole begin to attach with the centromeres of the chromosomes. Each chromosome attaches with one spindle coming from one pole. Hence, two homologous chromosomes attach with the spindles coming from opposite poles. When anaphase I starts, spindles become shorten and pull homologous chromosomes apart to opposite poles. Once, the chromosomes reach two poles of cells, telophase I begins by forming a nuclear membrane and enclosing the chromosomes. At this stage, the haploid set of chromosomes are present in each nucleus. Then the chromosomes condense again, and two cells appear. That completes the meiosis I.

What is Meiosis II?

Meiosis II is the second phase of meiosis, in which longitudinal division of the duplicated chromatids and further cell division take place. During meiosis II, daughter cells produced by meiosis I continue their further division so that each daughter cell coming from meiosis I produces two gametes. Similar to meiosis I, meiosis II also has four subphases namely Prophase II, Metaphase II, Anaphase II and Telophase II. These phases are very much similar to the sub-phases of meiosis I. Meiosis II resembles the mitotic cell division. Furthermore, meiosis II is shorter than the meiosis I.

During the prophase II, chromosomes condense and nuclear membranes break. Chromosomes move apart. Furthermore, spindles develop from each pole. Chromosomes line up at the metaphase plate individually. During the metaphase II, two spindles; one from each pole attach with the centromere of each chromosome. Then the anaphase II begins. Spindles become shorten. Hence, centromeres split and sister chromatids separate from each other. Sister chromatids are pulled towards the opposite poles. During the telophase II, nuclear membranes reform and enclose the haploid sets of chromosomes creating four haploid cells. That is the end of meiosis II.

What are the Similarities Between Meiosis I and Meiosis II?

• Meiosis I and II are major nuclear divisions of meiosis.
• Both processes have four subphases.
• Also, each meiosis produces haploid cells.
• Besides, these processes occur during the sex cell formation.
• Hence, they are important in sexual reproduction.

What is the Difference Between Meiosis I and Meiosis II?

Meiosis I is the first phase of gamete production while meiosis II is the second phase of it. Thus, this is the key difference between meiosis I and meiosis II. Furthermore, the sub-phases of meiosis I are Prophase I, Metaphase I, Anaphase I, and Telophase I, whereas that of meiosis II are Prophase II, Metaphase II, Anaphase II, and Telophase II.

Furthermore, the separation of homologous chromosomes called synapsis occurs only during the meiosis I. Also, cross-over only happens during the meiosis I. Thus, these two features also highlight the difference between meiosis I and meiosis II. Besides, one more difference between meiosis I and meiosis II is that meiosis-I starts with a diploid parent cell and ends with two haploid cells while meiosis II starts with the two haploid cells and ends with four haploid cells.

Moreover, meiosis I separates homologous chromosomes while meiosis II separates sister chromatids. Hence, this is also a difference between meiosis I and meiosis II. Most importantly, genetic recombination occurs in meiosis I while it does not happen in meiosis II. Therefore, this is an important difference between meiosis I and meiosis II.

Below infographic on the difference between meiosis I and meiosis II summarizes these differences with more facts.

Summary – Meiosis I vs Meiosis II

Meiosis is one of the two major cell divisions. It occurs via two major phases; meiosis I and meiosis II. Each meiosis has four subphases. Meiosis I produces two haploid cells while meiosis II produces four haploid cells. Furthermore, in meiosis I, crossing over between homologous chromosomes takes place and it causes genetic variation. But, in meiosis II, crossing over and genetic variation do not occur. Also, meiosis I is a heterotypic division while meiosis II is a homotypic division. Thus, this summarizes the difference between meiosis I and meiosis II.

Reference:

1. “Meiosis.” Khan Academy, Khan Academy. Available here  
2. Nature News, Nature Publishing Group. Available here  

Image Courtesy:

1.”Meiosis1″By Boumphreyfr – Own work, (CC BY-SA 3.0) via Commons Wikimedia  
2.”Meiosis2″By Boumphreyfr – Own work, (CC BY-SA 3.0) via Commons Wikimedia