Difference Between Prophase 1 and 2

Main Difference – Prophase 1 vs 2

Prophase 1 and 2 are two phases in the meiotic division of cells which produce gametes in order to carry out their sexual reproduction. Two stages of meiosis can be identified, meiosis 1 and meiosis 2. Meiosis 1 is followed by meiosis 2. Prophase 1 is the initial phase of meiosis 1 and prophase 2 is the initial phase of meiosis 2. Diploid germ cells undergo the above mentioned two stages of meiosis in order to produce their haploid gametes. The main difference between prophase 1 and 2 is that genetic recombination occurs through crossing overs and the “Chiasmata” formation during prophase 1 whereas no genetic recombination is noticed at the prophase 2.

This article explains,

1. What is Prophase 1
     – Definition, Process, Substages
2. What is Prophase 2
     – Definition, Process, Substages
3. What is the difference between Prophase 1 and 2

What is Prophase 1

Prophase 1 is the initial phase of the meiosis 1. It is considered as the longest phase of the whole meiosis. Chromosomal crossover occurs during prophase 1, leading to genetic variations by recombination. In order to enter a cell into the meiotic division, chromosomes in the vegetative germ cell should be replicated. These replicated chromosomes are called bivalents. These bivalents pairs form tetrads with other homologues during the prophase 1. Homologous chromosome pairing, which is known as synapsis, is a critical step in meiosis, in order to obtain a proper segregation of chromosome sets between two daughter cells. During the synapsis, non-sister chromatids are allowed to cross-over at their chiasmata. A chiasma is a point where the homologous chromosomes are in contact. Meiosis crossover is shown in figure 1. The two homologous chromosomes are separately shown in red and green colors. The cross-over occurs at a chiasma, leading to the exchange of chromosomal parts.

Figure 1: Crossover

A series of prophase substages can be identified depending on the appearance of the chromosomes. They are leptotene, zygotene, pachytene, diplotene, diakinesis and synchronous processes. Throughout these stages, disappearing of the nucleolus, the formation of meiotic spindle between the two centrosomes in the opposite poles in the cytoplasm, disappearing of the nuclear envelope, and allowing the spindle microtubules to invade the nucleus take place sequentially. The prophase 1 consumes 90% of the time taken to complete the whole meiosis.

What is Prophase 2

Prophase 2 is the initial phase of the meiosis 2. Telophase 1 is followed by prophase 2. No interphase can be found between telophase 1 and prophase 2. During meiosis 2, individual bivalent chromosomes are divided into sister chromatids, ultimately known as daughter chromosomes. Meiosis 2 produces haploid gametes from diploid cells which result in meiosis 1.

During prophase 2, the centrosome is duplicated. A centrosome contains two centrioles which are perpendicular to each other. Each centrosome moves to the opposite pole. The nucleoli and the nuclear envelope which are formed at the telophase 1 disappear. The chromatids are condensed into thick, short chromosomes. Theses chromosomes move towards the two opposite poles. Meanwhile, spindle fibers are arranged in the new equatorial plane, which is rotated by 90º relative to the first equatorial plane, arranged at the meiosis 1. The spindle apparatus is formed during the late prophase 2. 

Figure 2: Phases of Meiosis

Difference Between Prophase 1 and 2

Meiosis

Prophase 1: Meiosis 1 begins with prophase 1.

Prophase 2: Meiosis 2 begins with prophase 2.

Interphase

Prophase 1:  Prophase 1 follows a long interphase.

Prophase 2: No interphase takes place before prophase 2. Telophase 1 is followed by prophase 2.

Centrosome Duplication

Prophase 1: Centrosome is duplicated during the interphase, which is a process prior to prophase 1.

Prophase 2: Centrosome is duplicated during the prophase 2 due to the lack of an interphase prior to the prophase 2.

Involvement of chromosomes

Prophase 1: Homologous chromosomes are involved in the prophase 1.

Prophase 2: Individual chromosomes are involved in the prophase 2.

Diploid vs Haploid

Prophase 1: Prophase 1 occurs in diploid cells.

Prophase 2: Prophase 2 occurs in haploid cells.

Plane

Prophase 1: During prophase 1, the spindle apparatus begins to form in the cell equator.

Prophase 2: During prophase 2, the spindle apparatus is arranged in a plane which is rotated by 90º relative to the meiosis 1.

Occurrence of Crossovers

Prophase 1: Occurrence of crossovers and the formation of chiasmata takes place during prophase 1.

Prophase 2: No crossovers and chiasmata formation is identified in prophase 2.

Recombination

Prophase 1: Genetic material is exchanged by crossing over leads to the recombination during the prophase 1.

Prophase 2: No recombination can be identified during prophase 2.

Conclusion

Prophase 1 and 2 are the two initiating phases of the meiosis 1 and meiosis 2, respectively. Prior to the prophase 1, a long interphase can be identified in the cell cycle, synthesizing necessary proteins for the cell division and increasing the number of organelles in the cell. DNA replication takes place in the S phase of the interphase, prior to the prophase 1. This replication results in tetrad chromosomes which appear as homologous chromosome pairs during prophase 1 of the meiosis 1. No interphase can be identified prior to the prophase 2. Telophase 1 is followed by prophase 2. Therefore, no former DNA replication is taken place at the prophase 2. During meiosis 2, the bivalent chromosomes resulted in the meiosis 1 are separated into sister chromatids, producing the gametes containing haploid nuclei. But, during prophase 1, the pairing of homologous chromosomes or the synapsis allows the non-sister chromatids to cross-over at chiasmata, leading to genetic recombination between chromosomes. During prophase 2, synapsis does not occur; hence, no genetic recombination takes place between the chromosomes. Therefore, the main difference between prophase 1 and 2 is the recombination between chromosomes.   

Reference:
1.En.wikipedia.org. N.p., 2017. Web. 9 Mar. 2017.
2.I: Prophase I – Pearson – The Biology Place”. Phschool.com. N.p., 2017. Web. 9 Mar. 2017.
3.”Meiosis II: Prophase II – Pearson – The Biology Place”. Phschool.com. N.p., 2017. Web. 9 Mar. 2017.

Image Courtesy:
1.”Meiosis crossover” By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Meiosis diagram” By Marek Kultys – Own work (CC BY-SA 3.0) via Commons Wikimedia