Prophase I of Meiosis – Five Stages, Key Events and Significance

Prophase I is the first phase of meiosis I, during which the pairing of homologous chromosomes and genetic recombination take place. It consists of five substages, namely leptotene, zygotene, pachytene, diplotene, and diakinesis, which involve different changes in chromosome organisation, homologous chromosome pairing, and recombination.

Before Prophase I, DNA replication has already occurred, and each chromosome consists of two sister chromatids. During this phase, the homologous chromosomes come together and pair closely by a process called synapsis. The non-sister chromatids exchange corresponding segments of DNA. This is referred to as crossing over, which produces new combinations of genetic material.

As the phase progresses, homologous chromosomes begin to separate along their length but remain connected at certain points called chiasmata (sites of crossing over). These connections hold the homologous pairs together and help in their proper alignment before the separation of homologous chromosomes during meiosis I.

Five Stages of Prophase I

Prophase I is divided into five successive stages depending on the changes in chromosome structure, pairing of homologous chromosomes, and genetic recombination. These stages are as follows-

1. Leptotene

Leptotene is the initial stage of Prophase I. The already replicated chromosomes begin to undergo condensation and appear as long, thin thread-like structures. Each chromosome has two sister chromatids. However, the chromatids are closely associated with each other and cannot be distinguished clearly at this stage.

2. Zygotene

In this stage, the homologous chromosomes come together and begin to pair along their entire length. The pairing is referred to as synapsis.

During this process, a protein structure called the synaptonemal complex is formed between the homologous chromosomes, which holds them together. The paired homologous chromosomes form a bivalent. It consists of four chromatids and is also called a tetrad.

3. Pachytene

Pachytene is the stage where the pairing of homologous chromosomes is completed. The chromosomes become shorter and thicker. Four chromatids of each bivalent can now be distinguished.

During pachytene, crossing over takes place between the non-sister chromatids of homologous chromosomes. In this process, the chromatids exchange corresponding segments of genetic material, resulting in the formation of new genetic combinations through genetic recombination.

4. Diplotene

The synaptonemal complex begins to disappear during diplotene. As a result, the homologous chromosomes start moving apart from each other, but remain attached at certain points where crossing over has occurred.

These points are called chiasmata (singular: chiasma). They become clearly visible during this stage.

5. Diakinesis

It is the last stage of Prophase I, during which the chromosomes undergo further condensation and become highly condensed. The chiasmata appear to move towards the ends of the chromosomes. This process is referred to as terminalisation of chiasmata.

The nucleolus disappears during this stage. Towards the end of Prophase I, the nuclear envelope breaks down, and the meiotic spindle begins to form for the subsequent separation of homologous chromosomes.

Synapsis and Tetrad Formation

During the zygotene stage of Prophase I, homologous chromosomes begin to come together and pair along their entire length. This pairing is called synapsis.

Homologous chromosomes consist of one maternal and one paternal chromosome, having the same genes at corresponding positions, but the forms of these genes may differ. Each chromosome has already undergone replication and contains two sister chromatids joined at the centromere. The sister chromatids belong to the same replicated chromosome, while the homologous pair contains two separate chromosomes.

During synapsis, a protein structure called the synaptonemal complex develops between the pairing chromosomes. It has a ladder-like structure, made up of two lateral elements present along each homologous chromosome and a central element connected to them by transverse filaments. The synaptonemal complex holds the homologous chromosomes in close association. Pairing is completed in the pachytene stage.

The two homologous chromosomes after pairing form a bivalent. Both chromosomes are replicated and contain two sister chromatids each. There are four chromatids in a single bivalent, which is also referred to as a tetrad.

Crossing Over in Prophase I

• Crossing over is the exchange of corresponding genetic material between nonsister chromatids of homologous chromosomes. The two chromatids are from different homologues (one maternal and one paternal). DNA is broken and rejoined during this process. Each chromatid receives the corresponding DNA segment from the other.
• The homologous chromosomes come together, bringing their corresponding regions into alignment. Their close pairing is referred to as “synapsis”. Each homologue has two sister chromatids, making four chromatids in the paired unit (tetrad). Only two nonsister chromatids take part in a single crossover.
• Pairing and DNA recombination are closely associated. In many organisms, early recombination helps in bringing the homologues into alignment. Recombination usually starts in leptotene and continues through zygotene into pachytene.
• During pachytene, the homologues are fully synapsed. Crossover products are formed while the chromosomes remain closely paired, so crossing over is conventionally associated with this stage. Some recombination events are completed without reciprocal exchange of chromosome arms. These do not form crossovers.
• In diplotene, the synaptonemal complex breaks down. The homologues start moving apart but remain connected at the crossover sites. These connections now become visible as chiasmata (singular, chiasma). They commonly appear X-shaped.
• Crossing over is the actual genetic exchange, whereas chiasmata are the visible chromosome connections associated with the exchange. Sister chromatid cohesion helps to maintain these connections.
• For example, the maternal homologue carries AB and the paternal homologue carries ab, at corresponding positions. Their four chromatids initially carry AB, AB, ab, ab.A single crossover between the A and B positions exchanges the corresponding segments of two nonsister chromatids. Ab and aB are formed. The two chromatids which did not take part in this exchange remain AB and ab.

Significance of Prophase I

• Chromosome Pairing- Pairing and synapsis bring the corresponding regions of homologous chromosomes close together for DNA recombination between nonsister chromatids.
• Genetic Variation- During crossing over, the maternal and paternal homologues exchange corresponding DNA segments. New combinations of alleles are formed. The chromosomes containing these combinations are later passed into the gametes.
• Chromosome Separation- Homologues remain connected even after synapsis breaks down. They are held together by the connections produced through crossing over, along with sister chromatid cohesion. These connections are visible as “chiasmata”. Chiasmata help in orienting the homologues towards opposite spindle poles for separation during meiosis I.
• Meiotic Checkpoints- In many organisms, chromosome synapsis and DNA repair are monitored during prophase I. When defects are present, further meiotic progression can be delayed or stopped. Cells may undergo cell death if the defects persist.

Prophase I at a Glance

References

1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Meiosis. In Molecular biology of the cell (4th ed.). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK26840/
2. Clark, M. A., Douglas, M., & Choi, J. (2018). 11.1 The process of meiosis. In Biology 2e. OpenStax. https://openstax.org/books/biology-2e/pages/11-1-the-process-of-meiosis
3. Cooper, G. M. (2000). Meiosis and fertilization. In The cell: A molecular approach (2nd ed.). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK9901/