Mechanism or process of crossing-over includes following stages.
- Synapsis or Syndesis
- Duplication of Chromosomes
- Actual Crossing Over (Break and Exchange)
- Terminalization
Synapsis or Syndesis
Occurs during zygotene of prophase I. Synapsis is the pairing of two homologous chromosomes. The homologous chromosomes, one maternal and one paternal are attracted towards each other. When homologous chromosomes synapse, they come closer together until they are connected by synaptonemal complex. The synaptonemal complex develops only when DNA (single-stranded) successfully carries out the process of “homology searching” to facilitate the segment exchange process.
Synapsis begins at one or more points along the length of chromosome.
- Proterminal synapsis – Pairing of homologous chromosomes starts from the end and continues towards their centromere. Ex. Mammals
- Procentric synapsis – starts from centromere and progresses toward ends.
- Random synapsis (localized pairing) – occurs at various points along the length of homologous chromosomes.
While autosomes undergo synapsis during meiosis sex chromosomes usually remain unpaired.
Duplication of Chromosomes
Occurs during pachytene of prophase I. Formation of two identical sister chromatids takes place by longitudinal splitting of homologous chromosomes of bivalent. Sister chromatids remain held together by centromere. As each bivalent contains four chromatids at this stage, it is known as tetrad.
Actual Crossing Over (Break and Exchange)
Basic principle is breakage and union. During pachytene stage, recombination nodule becomes visible. The two non-sister chromatids first break at corresponding points. Enzyme endonuclease facilitates the process. Segment on one side of each break connect with a segment on the opposite side of the break, such that two chromatids cross each other. Replication of the remaining 0.3% DNA occurs at this stage, to fill the gap. Ligase is the enzyme for fusion process.
Crossing of two chromatids is known as chaisma formation. Crossing over can occur at one or more points, resulting in the formation of several chaismata. The number depends on length of chromosome and is in proportion with. More the physical distance between genes situated on chromosome, greater the probability of chaisma to occur between them. Important to note, in a species, each chromosome has a characteristic number of chaismata.
Terminalization
After crossing over, force of synapsis decreases. Because of this two chromatids start to repel each other. Desynapsis begins during diplotene. Chromosomes detaches from nuclear envelop during diakinesis. Each bivalent can be observed to contain four chromatids, with sister chromatids attached at centromere and non-sister chromatids undergone crossing over linked at chaismata. Chromatid separation progresses from centromere to chaismata and chaismata itself moves towards the end of tetrad in a zipper fashion. This movement of chaismata towards end of chromosome is called as terminalization.