The synaptonemal complex (SC) is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

structure that forms between homologous chromosomes (two pairs of

sister chromatids A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...

) during

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

and is thought to mediate synapsis and recombination during meiosis I in

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

s. It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their

crossover Crossover may refer to: Entertainment Albums and songs * ''Cross Over'' (Dan Peek album) * ''Crossover'' (Dirty Rotten Imbeciles album), 1987 * ''Crossover'' (Intrigue album) * ''Crossover'' (Hitomi Shimatani album) * ''Crossover'' (Yoshino ...

activities.

Composition

The synaptonemal complex is a tripartite structure consisting of two parallel lateral regions and a central element. This "tripartite structure" is seen during the pachytene stage of the first meiotic

prophase Prophase () is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin ret ...

, both in males and in females during

gametogenesis Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic di ...

. 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. In humans, three specific components of the synaptonemal complex have been characterized: SC protein-1 (SYCP1), SC protein-2 (SYCP2), and SC protein-3 (

SYCP3 Synaptonemal complex protein 3 is a protein that in humans is encoded by the ''SYCP3'' gene. It is a component of the synaptonemal complex formed between homologous chromosomes during the prophase of meiosis Meiosis (; , since it is a reduct ...

). The SYCP1

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

is on chromosome 1p13; the SYCP2 gene is on chromosome 20q13.33; and the gene for SYCP3 is on chromosome 12q. The synaptonemal complex 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.

Assembly and Disassembly

Formation of the SC usually reflects the pairing or " synapsis" of homologous

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

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 Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

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 male mice, gamma irradiation increases meiotic crossovers in SCs. This indicates that exogenously caused DNA damages are likely repaired by crossover recombination in SCs. The finding of an interaction between a SC structural component ynaptonemal central element protein 2 (SYCE2)and recombinational repair protein

RAD51 DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to th ...

also suggests a role of the SC in DNA repair. In cell development the synaptonemal complex disappears during the late prophase of meiosis I.It is formed during zygotene

Necessity in Eukaryotes

It is now evident that the synaptonemal complex is not required for genetic recombination in some organisms. For instance, in

protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

s such as ''

Tetrahymena ''Tetrahymena'', a unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrahymena cells can recog ...

thermophila'' and ''

Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...

tetraurelia'' genetic crossover does not appear to require synaptonemal complex formation. Research has shown that not only does the SC form after genetic recombination but mutant yeast cells unable to assemble a synaptonemal complex can still engage in the exchange of genetic information. However, in other organisms like the ''C. elegans'' nematode, formation of chiasmata require the formation of the synaptonemal complex.

External links

- 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.

* Kounetsova A. et al, ''Meiosis in Mice without a Synaptonemal Complex'' PLOS ONE (2011)

References

{{DEFAULTSORT:Synaptonemal Complex Molecular genetics