In the last post we discussed the process of crossing over. In the post DNA packaging, we discussed how complex is the packaging mechanism? In the process of crossing over three major happenings takes place first breakage, then movement of these segments (transposition) and fusion by ligase in correct position. In the sea of genetic material, a process like crossing over to take place there should be a framework or a mechanism with all the necessary enzymes to carry out the process effectively. I once again want to tell you when we move deep down into the cell to explain the happenings in this minute system I often think these words are not powerful enough to convey the reality. It is incredibly amazing. Synaptonemal complex is the structure assigned with the duty of carrying out the process “crossing over”.
First look at the figure
Synaptonemal complex is a highly organised structure of filaments seen between synapsed chromosome during zygotene and pachytene stages of prophase I of meiosis I. The structure is complex and we got the detail only with an electron microscope.
Synaptonemal complex consists of three parallel dense lines that are equally spaced or it is a tripartite ribbon like structure. It is a 0.2uM thick protein rich frame work. These lateral lines are flanked by chromatin. The two lateral lines are denser than the central line. Each lateral line is attached to the inner side of the homologous chromosome. The central and lateral elements are joined by LC fibres( L lateral to C cental fibres). The ends of lateral elements are attached to the inner surface of nuclear membrane.
Lateral lines are rich in DNA, RNA and protein whereas central line has RNA and protein with little DNA. Synaptonemal complex is essential for chiasma formation and crossing over. Recombination process is carried out by a large protein assembly of 90nm diameter called Recombination nodule (RNs) seen on the central element of Synaptonemal complex.
The major functions of Synaptonemal complex1. Maintenance of synapsis in fixed state for an extended period for crossing over to occur.
2. To provide a structural framework within which exchange of segments takes place.
3. To segregate recombination DNA from the rest of other chromosomal DNA.