Question

Which of the following represents the correct order in Prophase I? (a) Zygotene, diplotene, pachytene, leptotene, diakinesis (b) Diakinesis, diplotene, leptotene, pachytene, zygotene (c) Leptotene, zygotene, pachytene, diplotene, diakinesis (d) Pachytene, leptotene, zygotene, diplotene, diakinesis

Short answer

The correct order of stages in Prophase I is represented by option (c) Leptotene, zygotene, pachytene, diplotene, diakinesis.

Step-by-step solution

Identifying the stages of Prophase I

The five identified stages of Prophase I are: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Comparing the options against the correct order

After identifying these stages, match the correct order with the options provided in the exercise.

Select the correct option

Option (c) Leptotene, zygotene, pachytene, diplotene, diakinesis matches the correct order of the stages in Prophase I of meiosis.

Key concepts

Leptotene

Leptotene is known as the 'thin thread' stage and marks the beginning of Prophase I during meiosis. This phase is characterized by the condensation of chromatin into long, thin structures called chromosomes. It's crucial for students to visualize chromosomes at this stage as they start becoming visible under a microscope but remain unpaired.

In leptotene, each chromosome has not yet interacted with its homologous counterpart. Therefore, understanding this stage is important for comprehending how meiosis leads to genetic variation. The key events here involve the initiation of important processes that set the stage for subsequent steps, such as recombination and the segregation of genetic material.

Zygotene

Zygotene, from the Greek for 'paired threads,' follows leptotene in Prophase I. During this stage, chromosomes begin to search for and align with their corresponding homologous chromosomes. This critical pairing process is called synapsis, facilitated by a protein structure known as the synaptonemal complex.

The synaptonemal complex acts as a scaffold that holds homologs together, and this pairing is necessary for the exchange of genetic information, ensuring diversity in the resulting gametes. Students often struggle with the concept of synapsis, but visualizing it as a zipper that brings the homologous chromosomes into close proximity can simplify understanding.

Pachytene

Pachytene, translating to 'thick thread,' is the stage where things get interesting in terms of genetic recombination. At this point, the paired chromosomes, now called bivalents or tetrads, are fully synapsed. Crossing over, the exchange of genetic material between non-sister chromatids, occurs during pachytene.

This process is crucial for genetic diversity and is one of the key concepts emphasized in genetics education. The chiasma formation, observable under a microscope, is a clear indication of crossing over taking place. It is important to note that the exact points where the chromatids exchange segments are called chiasmata, which will be visible until the next stage.

Diplotene

Following pachytene, diplotene is the stage where the synaptonemal complex starts to break down, and the homologous chromosomes begin to pull apart slightly. However, they remain connected at chiasmata points.

At this juncture, the chromosomes start to 'repel' each other due to the decrease in synaptonemal complex proteins. This stage is essential for understanding how the reduction division in meiosis contributes to genetic variation. This tension created by the repulsion of homologous chromosomes is a hallmark of diplotene and sets the stage for the next phase of separation.

Diakinesis

Diakinesis is the final stage of Prophase I and is a further continuation of chromosome condensation and chiasma maintenance before the cell moves into metaphase I. During diakinesis, the nuclear membrane breaks down, and the spindle apparatus starts to form, preparing the cell for the alignment of chromosomes at the metaphase plate.

It is during diakinesis that the cell prepares to first separate homologous chromosomes. The key takeaway for students is that diakinesis represents the transition from the complex recombination and pairing processes of earlier Prophase I stages to the mechanical separation processes of subsequent meiotic stages.