Скачать или смотреть Meiosis lec no 5 part 2 cell cycle

Prophase I of meiosis is the first stage of meiosis I, during which several important events occur to prepare the cell for the reduction division that will follow. This phase can be further broken down into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. Here’s a breakdown of what happens during prophase I:

1. Leptotene:

Chromosomes begin to condense, becoming visible under a microscope.

Each chromosome is in the form of a long, thin thread.

Homologous chromosomes (chromosomes from the mother and father) are present but not yet paired.



2. Zygotene:

Homologous chromosomes begin to pair up through a process called synapsis.

The paired chromosomes are aligned gene by gene along their lengths.

A structure called the synaptonemal complex forms between the homologous chromosomes, holding them together.



3. Pachytene:

The chromosomes become fully aligned and are closely attached to one another.

This is the stage where crossing over occurs. In crossing over, homologous chromosomes exchange genetic material at points called chiasmata. This creates genetic diversity in the offspring.

Each chromosome consists of two sister chromatids, which are held together by the centromere.



4. Diplotene:

The synaptonemal complex begins to break down, and homologous chromosomes start to separate slightly, though they remain connected at the chiasmata (where crossing over occurred).

The chromatids are now visible as distinct structures.



5. Diakinesis:

The chromosomes further condense and become more visible.

The chiasmata shift towards the ends of the chromosomes (called terminalization).

The nuclear envelope begins to break down, and the spindle apparatus starts to form, preparing for the next stages of meiosis.