What is the Difference Between Zygotene and Pachytene

The main difference between zygotene and pachytene is that synapsis begins in the homologous chromosomes in zygotene, whereas crossing-over occurs between the non-sister chromatids in pachytene. 

In brief, zygotene and pachytene are two substages of prophase 1. Prophase 1 and prophase 2 are two stages of meiosis.

Key Areas Covered

1. What is Zygotene
– Definition, Facts, Features
2. What is Pachytene
– Definition, Facts, Features
3. Similarities Between Zygotene and Pachytene
– Outline of Common Features
4. Difference Between Zygotene and Pachytene
– Comparison of Key Differences
5. FAQ: Zygotene and Pachytene
– Answers to frequently asked questions

Key Terms

Pachytene, Zygotene, Zygonema

What is Zygotene

Zygotene is the second sub-stage of prophase 1 that occurs in meiosis 1. Zygonema is another name for zygotene. It contains paired threads. A much more stable pairing of homologous chromosomes occurs in this stage. They occur ~100 nm closer. Further, the synapsis of homologous chromosomes occurs in a zipper-like fashion. Also, it starts from a recombination nodule that forms the synaptonemal complex. It aligns the corresponding regions of genetic information of the maternal and paternal chromosomes. And it allows the formation of bivalents or tetrads. However, sex chromosomes do not fully align due to the less homozygosity.

Figure 1: Prophase

What is Pachytene

Pachytene is the third substage of prophase 1. Also, it occurs in meiosis 1. It begins with the completion of synapsis. Moreover, a significant feature of pachytene substage is the chromatin condensation. Also, the recombination nodules allow the genetic exchange between non-sister chromatids. The process of exchanging genetic information is called either crossing over or genetic recombination. Besides, multiple recombination events can occur in chromosomes. Usually, 2-3 recombination events occur per chromosome.

Figure 2: Prophase  

Similarities Between Zygotene and Pachytene

• Zygotene and pachytene are two sub-stages of prophase 1.
• Crossing over is the main event that occurs in prophase 1.
• Therefore, chromosomes can be resolved under the microscope. 

Difference Between Zygotene and Pachytene

Definition

Zygotene refers to the second stage of the prophase of meiosis, following leptotene, during which homologous chromosomes begin to pair. In contrast, pachytene refers to the stage in which the paired chromosomes shorten and thicken, the two chromatids of each separate, and the exchange of segments between chromatids may occur.

Order

Zygotene is the second stage of prophase 1, while pachytene is the third stage of prophase 1.

Events

Synapsis begins in the homologous chromosomes in zygotene, while crossing-over occurs between the non-sister chromatids in pachytene. 

Chromatin Condensation

Chromatin condensation does not occur in zygotene, while chromatin condensation occurs in pachytene.  

FAQ: Zygotene and Pachytene

What is the difference between Diplotene and pachytene? 

Non-sister chromatids of homologous chromosomes undergo crossing over in the pachytene stage. In comparison, in the diplotene stage, synaptonemal complexes dissolve, and homologous chromosomes separate except at the crossing-over sites.   

How is Diplotene different from Diakinesis?

The separation of homologous chromosomes occurs in the diplotene stages by dissolving the synaptonemal complex. In comparison, further condensation of chromosomes occurs in the diakinesis stage. The four parts of the tetrads are visible in the diakinesis stage.

What happens in diplotene of prophase 1?

The late stage of prophase 1 meiosis 1 is the diplotene stage. The dissociation of homologous chromosome pairs occurs in this stage. 

Conclusion

In brief, zygotene and pachytene are two substages of prophase 1. Zygotene is the second stage of prophase 1. Synapsis in the homologous chromosomes occurs in zygotene. In comparison, pachytene is the third stage of prophase 1. Crossing over between the non-sister chromatids occurs in pachytene. Chromatin condensation occurs in this stage. Therefore, the main difference between zygotene and pachytene is the events that occur in each sub-stage.  

References:

1. Stages of Prophase. BioNinja. (n.d.).

Image Courtesy:

1. “Prophase eukaryotic mitosis” By LadyofHats – Own Work (Public Domain) via Commons Wikimedia
2. “3D-SIM-3 Prophase 3 color” By Lothar Schermelleh – Own Work (CC-BY SA 3.0) via Commons Wikimedia