Mitosis
Mitosis is part of the eukaryotic cell cycle and results in the production of two daughter cells each with a copy of the genome. The cell cycle itself is comprised of an interphase (made up of three stages G1, S, and G2) and the M (mitotic) phase. Cell growth, active transcription and translation, and DNA replication occur during interphase. During M phase duplicated DNA (chromatin) condense into sister chromatids (prophase); the nuclear envelop breaks down, kinetochore microtubles attach to the chromosomes and centrosomes are pushed to the poles of the growing spindle (prometaphase); the chromosomes are lined up on the metaphase plate (metaphase); sister chromatids are pulled to spindle poles at opposite ends of the cell (anaphase); the nuclear envelop is reformed and the chromatids decondense to chromatin (telophase); and the cell is cleaved into two by a contractile ring and the resolution of a cleavage furrow (cytokinesis). In some variant cell cycles nuclear division may not be followed by cell division, or G1 and G2 phases may be absent.
Trans-splicing
Trans-splicing is an RNA processing event that fuses together sections of two different pre-mRNA sequences. In C. elegans, ~70% of mRNAs are trans-spliced to one of two 22 nucleotide spliced leaders, SL1 or SL2, with more than half of all transcripts undergoing SL1 splicing. During SL1 splicing, the 5' ends of pre-mRNAs are removed and replaced with SL1 sequence in a process very closely related to cis-splicing (intron/exon processing). SL1 sequence is ~100nt and is donated by small nuclear ribonucleoprotein particles (snRNPs). The remaining genes are trans-spliced by SL2. These genes are all downstream genes in closely spaced gene clusters similar to bacterial operons. They are transcribed from a promoter at the 5' end of the cluster of between 2 and 8 genes. This transcription makes a polycistronic pre-mRNA that is co-transcriptionally processed by cleavage and polyadenylation at the 3' end of each gene, and this event is closely coupled to the SL2 trans-splicing event that occurs only ~100 nt further downstream. Recent studies on the mechanism of SL2 trans-splicing have revealed that one of the 3' end formation proteins, CstF, interacts with the only protein known to be specific to the SL2 snRNP.