Difference Between Pachytene and Zygotene

The key difference between pachytene and zygotene is that pachytene is the third sub-stage of prophase 1 in which the homologous recombination or chromosomal crossover takes place between non-sister chromatids. Meanwhile, zygotene is the second sub-stage of prophase 1 in which maternal and paternal chromosomes line up with each other into homologous chromosome pairs.

Meiosis is one of the two types of cell division. Meiosis takes place during sexual reproduction in order to produce sex cells or gametes. There are two successive cell divisions in meiosis as meiosis 1 and meiosis 2. Meiosis 1 and meiosis 2 are again divided into different stages as prophase, metaphase, anaphase, and telophase. Prophase 1 of meiosis 1 is the longest phase of meiotic cell division. It is the phase which is responsible for genetic variation due to crossing over between homologous chromosomes and recombination. Moreover, there are five sub-stages of prophase 1: leptotene, zygotene, pachytene, diplotene, and diakinesis.

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

What is Pachytene?

Pachytene is the third sub-stage of prophase 1 of meiosis 1. During pachytene, crossing over takes place between bivalents formed at the end of the zygotene. The crossing over between non-sister chromatids results in genetic recombination between mother and father’s genetic materials. This phase is critical and very important since it is responsible for the genetic variation between organisms.

Figure 01: Chromosomal Cross Over

What is Zygotene?

Zygotene is the second sub-phase of the prophase 1 of meiosis 1. During this phase, maternal and paternal homologous chromosomes meet up and make a pair. Then, the homologous pair undergoes synapsis by forming a synaptonemal complex called bivalents or tetrads.

Difference Between Pachytene and Zygotene

Figure 02: Pairing of Homologous Chromosomes

During synapsis, the corresponding regions of genetic information of each homologous chromosome align with each other.

What are the Similarities Between Pachytene and Zygotene?

Pachytene and zygotene are two stages of prophase 1 of meiosis.
Both stages belong to meiosis 1.
These stages occur during the longest phase of the meiosis.
Both phases are responsible for the mixing of genetic material between the homologous chromosomes.
As a result, genetic variation between organisms takes place.

What is the Difference Between Pachytene and Zygotene?

Pachytene is the phase in which exchange of genetic material or crossing over takes place between non-sister chromatids of bivalents. Zygotene, on the other hand, is the phase in which the pairing of homologous chromosomes takes place forming synaptonemal complexes. So, this is the key difference between pachytene and zygotene.

Moreover, pachytene is the third sub-stage, while zygotene is the second sub-stage of prophase 1. Furthermore, another difference between pachytene and zygotene is that pachytene is followed by diplotene, while zygotene is followed by pachytene.

Below infographic tabulates more comparisons on the difference between pachytene and zygotene.

Summary – Pachytene vs Zygotene

Pachytene and zygotene are two sub-stages of prophase 1 in meiotic cell division. Zygotene is the second substage, and during this stage, the homologous chromosomes of maternal and paternal origin come closer and make a pair. Then, they form synaptonemal complexes. Zygotene is followed by pachytene, which is the third sub-stage. During pachytene, crossing over between non-sister chromatids takes place, resulting in the exchange of genetic material between them. Thus, this causes genetic variation between organisms. So, in summary, this is the difference between pachytene and zygotene.

Reference:

1. “Meiosis Prophase 1.” Meiosis Prophase 1 – The School of Biomedical Sciences Wiki, Available here.
2. “Meiosis.” Wikipedia, Wikimedia Foundation, 26 Feb. 2020, Available here.

Image Courtesy:

1. “Morgan crossover 1” (Public Domain) via Commons Wikimedia
2. “0330 Homologous Pair of Chromosomes” By OpenStax – (CC BY 4.0) via Commons Wikimedia