Difference Between Leptotene and Zygotene

The key difference between leptotene and zygotene is that leptotene is the first substage of prophase I during which nuclear chromatin condenses to form long thin strands of individual chromosomes while zygotene is the second substage of prophase I during which chromosomes recognize and align with each other as homologous chromosome pairs in order to form synaptonemal complexes.

Prophase I is the longest and crucial phase in meiosis. Homologous chromosomes form tetrads, and crossing over takes place between non-sister chromatids. Genetic material is exchanged between mother and father chromosomes. It leads to the formation of genetically different gametes. Therefore, meiosis is the main event that creates genetically different organisms. There are five substages in prophase I. They are leptotene, zygotene, pachytene, diplotene and diakinesis. Leptotene is the first substage, and it is followed by zygotene. During leptotene, replicated chromosomes condense, and individual chromosomes become visible as thread-like structures. During zygotene, homologous chromosomes line up with each other, and synapsis occurs.

CONTENTS

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

What is Leptotene?

Leptotene is the first subphase of prophase I. During the leptotene phase, nuclear chromatins begin to condense and form a species-specific number of chromosomes. Moreover, each chromosome duplicates and two sister chromatids become distinguishable.

Figure 01: Synaptonemal Complex at Different Stages during Prophase I

Individual chromosomes appear as single, long thread-like structures. Furthermore, centriole is duplicated, and daughter centrioles migrate towards two opposite poles of the cell.

What is Zygotene?

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

Figure 02: Homologous Chromosome Pair

During the synapsis, the corresponding regions of genetic information of each homologous chromosome align with each other, allowing the genetic recombination to occur during the next substage, which is pachytene.

What are the Similarities Between Leptotene and Zygotene?

• Leptotene and zygotene are two substages of prophase I of meiosis I.
• Both phases belong to the longest and most complex phase of meiosis.
• Chromosomes appear as thread-like structures in both phases.
• Both phases occur inside the nucleus.
• Events occurring in both stages are very important for the formation of genetically different gametes.
• In the end, both phases contribute to the genetic variation between organisms.

What is the Difference Between Leptotene and Zygotene?

During the leptotene, chromatin is arranged into long and thin strands. During the zygotene, homologous chromosomes pair with each other, and synapsis takes place in order to facilitate homologous recombination. So, this is the key difference between leptotene and zygotene. Moreover, leptotene is the first substage of prophase I, and it is followed by zygotene. Zygotene is the second substage of prophase I, and it is followed by pachytene. Thus, this is another difference between leptotene and zygotene.

Below infographic lists more differences between leptotene and zygotene in tabular form.

Summary – Leptotene vs Zygotene

Leptotene is the first substage of prophase I. During leptotene, chromosomes begin to condense, and two sister chromatids become visible. Zygotene is the second substage of prophase I. During zygotene, chromosomes recognize each other, align as homologous chromosome pairs, and synapsis takes place. Thus, this summarizes the difference between leptotene and zygotene. Both stages contribute to making genetically different organisms.

Reference:

1. “Leptotene Stage.” ScienceDirect Topics, Available here.
2. Knapp, Sarah. “Prophase I.” Biology Dictionary, 28 Oct. 2020, Available here.

Image Courtesy:

1. “Synaptonemal Complex” By Daniel Wells – Delineation of meiotic gene expression in male mice" (2020) PhD Thesis (CC BY-SA 4.0) via Commons Wikimedia
2. “0330 Homologous Pair of Chromosomes” By OpenStax – (CC BY 4.0) via Commons Wikimedia

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