WHAT IS PACHYTENE AND WHAT HAPPENS IN IT? - BIOLOGY - 2025

The pachytene or pachynema is the third stage of meiotic prophase I; in it the recombination process is verified. In mitosis there is one prophase, and in meiosis there are two: prophase I and prophase II.

Previously, except for prophase II, the chromosomes were duplicated, each giving rise to a sister chromatid. But only in prophase I do homologues (duplicates) pair up, forming bivalents.

Products of meiosis in which crossover has occurred during pachytene (Prophase I). Taken from commons.wikimedia.org

The term paquiteno comes from the Greek and means “thick threads”. These "thick threads" are the paired homologous chromosomes that, after duplicating, form tetrads. That is to say, four "threads", or strings, that make each chromosome appear thickened.

There are unique aspects of meiotic prophase I that explain the unique characteristics of pachytene. Only in the pachytene of prophase I of meiosis do chromosomes recombine.

To do this, the recognition and matching of homologues is verified. As in mitosis, there must be duplication of the chromatids. But only in the meiosis I pachytene are band exchange complexes formed, which we call chiasmata.

In them occurs what defines the recombinational power of meiosis: the crossover between chromatids of homologous chromosomes.

The entire process of DNA exchange is possible thanks to the previous appearance of the synaptonemic complex. This multiprotein complex allows homologous chromosomes to mate (synapse) and recombine.

The synaptonemic complex during pachytene

The synaptonemic complex (CS) is the protein framework that allows end-to-end bonding between homologous chromosomes. It only occurs during the pachytene of meiosis I, and is the physical foundation of chromosomal pairing. In other words, it is what allows chromosomes to synapse and recombine.

The synaptonemic complex is highly conserved among eukaryotes undergoing meiosis. Therefore, it is evolutionarily very old, and structurally and functionally equivalent in all living things.

It consists of a central axial element and two lateral elements that are repeated like the teeth of a zipper or closure.

The synaptonemic complex is formed from specific points on the chromosomes during zygotene. These sites are collinear with those where DNA breaks occur where synapses and recombination will be experienced in the pachytene.

During the pachytene, therefore, we have a closed zipper. In this conformation, specific points are defined where DNA bands will be exchanged at the end of the stage.

Components of the synaptonemic complex and chiasms

The meiotic synaptonemic complex contains many structural proteins that are also found during mitosis. These include topoisomerase II, condensins, cohesins, as well as cohesin-associated proteins.

In addition to these, proteins that are specific and unique to meiosis are also present, along with proteins of the recombinational complex.

These proteins are part of the recombinosome. This structure groups all the proteins required for recombination. Apparently the recombinosome does not form on the crossover points, but is recruited, already formed, towards them.

Chiasmas

Chiasms are the visible morphological structures on chromosomes where crossovers occur. In other words, the physical manifestation of the exchange of DNA bands between two homologous chromosomes. Chiasms are the distinctive cytomorphological marks of pachytene.

In all meiosis, at least one chiasm per chromosome must occur. This means that every gamete is recombinant. Thanks to this phenomenon, the first genetic maps based on linkage and recombination could be deduced and proposed.

On the other hand, the lack of chiasms, and therefore of crossover, causes distortions at the level of chromosomal segregation. Recombination during pachytene then acts as a quality control of meiotic segregation.

However, evolutionarily speaking, not all organisms undergo recombination (for example, male fruit flies). In these cases, other mechanisms of chromosomal segregation not dependent on recombination operate.

A, diagram showing the central axial element and the lateral elements of two chromosomes in complete synapse. B, chiasmata and crossovers. Taken from wikimedia.org

Pachytene progression

Upon exiting the zygotene, the synaptonemic complex is fully formed. This is complemented by the generation of double-band DNA breaks from which crossovers are verified.

Double DNA breaks force the cell to repair them. In the DNA repair process, the cell recruits the recombinosome. Band exchange is used, and as a result, recombinant cells are obtained.

When the synaptonemic complex is fully formed, the pachytene is said to begin.

The bivalents in synapses in the pachytene interact basically through the axial element of the synaptonemic complex. Each chromatid is organized in a loop organization, the base of which is the central axial element of the synaptonemic complex.

The axial element of each counterpart contacts that of the other through the lateral elements. The sister chromatid axes are highly compacted, and their chromatin loops emerge outward from the central axial element. The spacing between the ties (~ 20 per micron) is evolutionarily conserved across all species.

Towards the termination of the pachytene, crossovers are evident from some of the DNA double-band break sites. The appearance of the crossovers also signals the beginning of the unraveling of the synaptonemic complex.

Homologous chromosomes become more condensed (look more individual) and begin to separate, except in the chiasms. When this happens, the pachytene ends and the diplotene begins.

The association between the recombinosome and the axes of the synaptonemic complex persists throughout the synapse. Particularly in recombinogenic crossovers to the end of the pachytene, or a little beyond.

References

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2. de Massy, ​​B. (2013) Initiation of meiotic recombination: how and where? Conservation and specificities among eukaryotes. Annual Reviews of Genetics 47, doi: 10.1146 / annurev-genet-110711-155423
3. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Philadelphia, PA, USA.
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5. Zickler, D., Kleckner, N. (2015) Recombination, pairing, and synapsis of homologs during meiosis. Cold Spring Harbor Perspectives in Biology, doi: 10.1101 / cshperspect.a016626