Synapsis
Synapsis is the formation of a structure termed a bivalent (a pairing of homologous chromosomes) during meiotic division. The chromotids of each of the homologous chromosomes align, essentially gene for gene to form a synaptonemal complex that allows crossover (exchange of genetic material).
In contrast to mitosis, meiosis results in the reduction of the cellular genome to a haploid state. This reduction of chromosome number is achieved by a single round of DNA replication followed by two sequential rounds of nuclear and cellular division (meiosis I and meiosis II). Like mitosis, meiosis is initiated after the synthetic phase is completed and the parental chromosomes have replicated to produce identical sister chromatids. Once the cellular DNA has replicated, the chromosomes condense into daughter chromatids of the duplicated chromosome. However, the patterning of chromosomal segregation during meiosis I differs from that of mitosis in that during meiosis I homologous chromosomes first pair up with each other and then segregate to different daughter cells.
Synapsis is the specific pairing up of these chromatids with their homologous chromosomes along the midline of the cell. Not only is the pairing of homologous after DNA replication a key event in the meiotic segregation of chromosomes, but also this lining up allows a process called crossing over to occur. Crossing over involves the symmetrical breaking of one maternal and one paternal chromosome and allows the reciprocal exchange of genetic information by breaking followed by the crosswise rejoining of the chromosomes.
As a result of synapsis and crossing over the daughter cells contain a mixture of parental types called the recombination form.
Synapsis takes place during an extended prophase of meiosis I that is further divided into five stages. These five stages are named leptotene, zygotene, pachytene, diplotene, and diakinesis (this naming is based on the physical appearance of the chromosomes during these various stages). It is thought that the initial association of homologous chromosomes is most likely the result of base pairing between complementary DNA strands but true synapsis occurs during the zygotene stage. A base-for-base zipper-like protein structure called the synaptomenal complex is formed along the length of the chromosome. This complex keeps the chromosomes aligned with one another and tightly associated through the pachytene stage where crossing over occurs.
Some congenital defects are associated with synaptic misalignment that can cause chromosomal anomalies to occur. One of these anomalies is called chromosomal nondisjunction and it results in aneuploidy. Aneuploidy refers to any condition where the normal number of chromosomes is not present, either too few to too many. In humans, Down syndrome is an example of aneuploidy. Other types of chromosomal anomalies are translocations and inversions.
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