Synapsis
Encyclopedia
Synapsis is the pairing of two homologous chromosome
s that occurs during meiosis
. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover
between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton
, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex
. While autosome
s undergo synapsis during meiosis, sex chromosomes often remain unpaired.
When the non-sister chromatids intertwine, segments of chromatids with similar sequence may break apart and be exchanged in a process known as genetic recombination
or "crossing-over". This exchange produces a chiasma
, a region that is shaped like an X, where the two chromosomes are physically joined. At least one chiasma per chromosome appears to be necessary to stabilise bivalents along the metaphase
plate during separation. The crossover of genetic material also provides a possible defence against 'chromosome killer' mechanisms, by removing the distinction between 'self' and 'non-self' through which such a mechanism could operate. A further consequence of recombinant synapsis is to increase genetic variability
within both offspring and the wider population. Repeated recombination also has the general effect of allowing genes to move independently of each other through the generations, allowing for the independent concentration of beneficial genes and the purging of the detrimental.
The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis, but the chiasma formation that takes place within synapsis has consequences at many levels, from cellular mechanics through to wide-ranging impacts upon evolution itself.
UC Berkeley video of chromosome end migration and match assessment during prophase
Homologous chromosome
Homologous chromosomes are chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father...
s that occurs during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex
Synaptonemal complex
The synaptonemal complex is a protein structure that forms between homologous chromosomes during meiosis and that is thought to mediate chromosome pairing, synapsis, and recombination . It is now evident that the synaptonemal complex is not required for genetic recombination...
. While autosome
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
s undergo synapsis during meiosis, sex chromosomes often remain unpaired.
When the non-sister chromatids intertwine, segments of chromatids with similar sequence may break apart and be exchanged in a process known as genetic recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
or "crossing-over". This exchange produces a chiasma
Chiasma (genetics)
A chiasma , in genetics, is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis...
, a region that is shaped like an X, where the two chromosomes are physically joined. At least one chiasma per chromosome appears to be necessary to stabilise bivalents along the metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
plate during separation. The crossover of genetic material also provides a possible defence against 'chromosome killer' mechanisms, by removing the distinction between 'self' and 'non-self' through which such a mechanism could operate. A further consequence of recombinant synapsis is to increase genetic variability
Genetic variability
Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. Variability is different from genetic diversity, which is the amount of variation seen in a particular population. The variability of a trait describes how much that trait tends to...
within both offspring and the wider population. Repeated recombination also has the general effect of allowing genes to move independently of each other through the generations, allowing for the independent concentration of beneficial genes and the purging of the detrimental.
The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis, but the chiasma formation that takes place within synapsis has consequences at many levels, from cellular mechanics through to wide-ranging impacts upon evolution itself.
UC Berkeley video of chromosome end migration and match assessment during prophase