In order to define synapsis, the process of creating sperm or egg gametes (called gametogenesis) must be described first. Imagine two friends making necklaces with beads. Jun has two sets of blue beads labeled A to Z, and Miyu has two sets of purple beads labeled A to Z. Both Jun and Miyu make two chains each with the letters in alphabetical order.
However, Jun and Miyu decide that solid color necklaces look quite dull. So, they trade some beads to make their respective necklaces more colorful. Jun gives Miyu blue PQRST beads from one chain, and blue ABCDE beads from the other. Miyu gives Jun purple PQRST beads from one chain and purple ABCDE beads from the other. Now each of them has a multicolored blue and purple necklace in alphabetical order. This jewelry making analogy is a simplified explanation of how maternal and paternal genes are swapped and mixed during gametogenesis.
How do Jun and Miyu's necklaces relate to each other? Both necklaces were 26 beads long with alphabetized letters in the same order. But Jun's beads were blue, while Miyu's were purple. The necklaces were otherwise identical. In this manner, it could be said that the necklaces were alike or hom*ologous. Likewise, chromosome pairs that carry the same genes in the same order but have variations in how each gene is coded are called hom*ologous chromosomes.
So, what is synapsis and what role does it play in this story? During gametogenesis, there is a point where hom*ologous chromosomes trade genes, called crossing over, similar to how Jun and Miyu traded necklace beads. Synapsis describes the process where hom*ologous chromosomes pair up lengthwise and overlap prior to crossing over.
Synapsis of hom*ologous Chromosomes
In human somatic cells, there are 46 chromosomes comprising 23 hom*ologous pairs. Among those hom*ologous pairs, 23 were inherited maternally, and 23 were inherited paternally. Each chromosome codes for a specific set of genes and is completely different from the next chromosome. The genes on chromosome 1, for instance, are different from the genes on chromosome 2. But a person has two hom*ologous pairs of every chromosome (except the sex chromosome XY, which is not hom*ologous). For example, a person has two hom*ologous pairs of chromosome 1, one paternal and one maternal. Paternal chromosome 1 and maternal chromosome 1 both have the same types of genes, but they are not identical due to genetic differences in how those genes code for their respective proteins.
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When gametogenesis begins, hom*ologous chromosomes meet up during chromosome synapsis in meiosis. For instance, all human beings have a chromosome 1, chromosome 2, and so forth. Maternal chromosome 1 will synapse with hom*ologous paternal chromosome 1. The remaining hom*ologs will do the same. Maternal chromosome 1 will never synapse with paternal chromosomes 2, or any other chromosome that is not paternal chromosome 1, because they are not hom*ologous. The goal of the synapsis of hom*ologous chromosomes is to line up so gene exchange between hom*ologs can create genetic variation in the gametes that are produced.
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When Does Synapsis Occur?
Synapsis facilitates genetic diversity during the creation of gametes in sex cell division, which is called meiotic division. Meiosis is a cellular division process unique to sex cells and is comprised of two stages (meiosis I and meiosis II) that have four phases each.
Stages of Meiosis I
- Prophase I: hom*ologous chromosomes are paired and trade genes; this phase is when the crossing over and synapsis meiosis I processes occur.
- Metaphase I: hom*ologous chromosomes prepare to split as they are held by spindle fibers and centrally align along the metaphase plate.
- Anaphase I: hom*ologous chromosome pairs are pulled apart.
- Telophase I: the cell prepares to split into two haploid daughter cells during cytokinesis.
Stages of Meiosis II
- Prophase II: sister chromatids from the two daughter cells from meiosis I migrate towards the center of the cell with help from the mitotic spindle.
- Metaphase II: sister chromatids prepare to split as they are held by spindle fibers and centrally align along the metaphase plate.
- Anaphase II: sister chromatid pairs are pulled apart.
- Telophase II: the cell prepares to split into four haploid gametes during cytokinesis.
So, when does synapsis occur? Synapsis is unique to meiosis I and occurs during the pachytene phase of prophase I in meiosis I. When meiosis starts, hom*ologous chromosomes are unorganized and unpaired. Synapsis sorts and stabilizes the locations of hom*ologous chromosomes within the cell, so they are lined up with their correct pair to trade genetic information and properly separate during anaphase.
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