19.2: Meiosis I
Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
Prophase I is the most extended and complex step of meiosis I characterized by synapsis, chromosome pairing, and recombination of the homologous chromosomes. This process is facilitated by a proteinaceous structure called the synaptonemal complex. The assembly of the synaptonemal complex starts with the formation of the scaffold between two homologous chromatids. These scaffolds are known as the axial element, and they extend along the entire length of the chromosomes. This is followed by pairing and stabilizing the chromosomes along the axial elements with the help of transverse filaments and the middle central elements.
In most organisms, synapsis requires the formation of the double-strand breaks in the DNA created by the dedicated enzyme called Spo11. Two homologous chromosomes may cross over during the repair of these double-strand breaks. Each chromosome undergoes at least one cross-over during the prophase I stage.
In metaphase I, the paired homologous chromosomes are aligned on the equator and then start moving to the opposite poles of the cell during anaphase I. In a process called nondisjunction, the paired chromosomes fail to segregate during anaphase I, leading to the production of daughter cells with an abnormal number of chromosomes. Abnormalities in chromosome numbers can cause genetic diseases such as Down syndrome. Down syndrome is the most common disease caused by nondisjunction of chromosome number 21 during maternal meiosis. This results in an impediment in the physical and cognitive growth of the affected individuals.