Synaptonemal complex
Encyclopedia
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
structure that forms between homologous chromosomes (two pairs of sister chromatids
Sister chromatids
Sister chromatids are two identical copies of a chromatid connected by a centromere. Compare sister chromatids to homologous chromosomes, which are the two different copies of the same chromosome that diploid organisms inherit, one from each parent...
) 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....
and that is thought to mediate chromosome pairing, synapsis
Synapsis
Synapsis is the pairing of two homologous chromosomes 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...
, and 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...
(crossing-over
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...
). It is now evident that the synaptonemal complex is not required for genetic recombination. Research has shown that not only does it form after genetic recombination but mutant yeast cells unable to assemble a synaptonemal complex can still engage in the exchange of genetic information. It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities. The synaptonemal complex is a tripartite structure consisting of two parallel lateral regions and a central element.
Three specific components of the synaptonemal complex have been characterized: SC protein-1 (SYCP1), SC protein-2 (SYCP2), and SC protein-3 (SYCP3). In humans, the SYCP1 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
is on chromosome 1p13; the SYCP2 gene is on chromosome 20q13.33; and the gene for SYCP3 is on chromosome 12q.
The synaptonemal complex (SC) was described by Montrose J. Moses in 1956 in primary spermatocytes of crayfish and by D. Fawcett in spermatocytes of pigeon, cat and man. As seen with the electron microscope, the synaptonemal complex is formed by two "lateral elements", mainly formed by SYCP3 and secondarily by SYCP2, a "central element" that contains at least two additional proteins and the amino terminal region of SYCP1, and a "central region" spanned between the two lateral elements, that contains the "transverse filaments" composed mainly by the protein SYCP1.
The SCs can be seen with the light microscope using silver staining or with immunofluorescence
techniques that label the proteins SYCP3 or SYCP2.
This "tripartite structure" is seen during the pachytene stage of the first meiotic prophase, both in males and in females during gametogenesis. Previous to the pachytene stage, during leptonema, the lateral elements begin to form and they initiate and complete their pairing during the zygotene stage. After pachynema ends, the SC usually becomes disassembled and can no longer be identified.
Formation of the SC usually reflects the pairing or "synapsis
Synapsis
Synapsis is the pairing of two homologous chromosomes 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...
" of homologous chromosomes and may be used to probe the presence of pairing abnormalities in individuals carrying chromosomal abnormalities, either in number or in the chromosomal structure.
The sex chromosomes in male mammals show only "partial synapsis" as they usually form only a short SC in the XY pair.
The SC shows very little structural variability among eukaryotic organisms despite some significant protein differences.
In many organisms the SC carries one or several "recombination nodules" associated to its central space. These nodules are thought to correspond to mature genetic recombination events or "crossovers".
In cell development the synaptonemal complex disappears during the late prophase of meiosis I.
External links
- http://www.olympusbioscapes.com/gallery/2009/2.html - Synaptonemal complex
by 3D-Structured Illumination, photograph by Dr. Chung-Ju Rachel Wang
University of California Berkeley, Department of Molecular and Cell Biology
Berkeley, CA, USA, second place winner of the 2009 Olympus Bioscapes Digital Imaging Competition.