Meiosis I

Meiosis I, also known as reductional division, is the first cell division of meiosis that produces two haploid cells from a diploid cell. Meiosis I is divided into four distinct stages – prophase I, metaphase I, anaphase I, and telophase I.

Prophase I consists of five substages – leptotene, zygotene, pachytene, diplotene, and diakinesis.

In leptotene, the chromosomes begin to condense, appearing as a thread-like, beaded structure where each chromosome consists of a pair of sister chromatids held together by  cohesin complexes.

At the onset of zygotene, homologous chromosomes pair to form tetrads.

The pairing, also known as synapsis, occurs lengthwise where each gene segment of paternal chromosomes pairs with its counterpart on the maternal chromosome. 

A protein based ladder-like structure called the synaptonemal complex forms between the paired chromatids.  This complex aids pairing and recombination of the chromosomes.

In pachytene, the exchange of equal chromosome segments or crossing over occurs between two homologous chromosomes. The chromosomes connect at the point of the crossover called chiasma.

During diplotene, the synaptonemal complex breaks down and the paired chromosomes begin to dissociate, with the chiasma becoming visible.

Diakinesis, the final step of prophase, is characterized by further condensation of the chromosomes, dissolution of the nuclear envelope and the formation of a meiotic spindle.

In metaphase I,  the paired homologous chromosomes are randomly positioned at the equator and the centromere of each chromosome in a tetrad attaches to the opposite poles of the spindle.  The distribution of the chromosomes into two different cells is arbitrary and independent of other chromosomes. This is also known as an independent assortment of the chromosomes, and it helps to produce unique gametes.

In anaphase I, the centromere holding the duplicated chromosomes together does not get divided, and therefore, the whole chromosome with a pair of chromatids moves to the opposite poles of the cell, creating two haploid sets of the genome.

In the  final stage, telophase I, chromosomes decondense and a  nuclear membrane forms around each haploid genome.

During cytokinesis in animal cells, the cell membrane pinches in forming two haploid daughter cells.

Meiosis I plays a vital role in creating genetic diversity in the species through genetic recombination and independent assortment of chromosomes.