A newly made mRNA copy of a gene (pre-mRNA), must undergo three major modifications before it can leave the nucleus and be translated into a protein:

1) Capping - a special nucleotide is attached at the 5' end of the mRNA. It is believed that this modification is necessary for efficient initiation of protein synthesis and serves as stabilisation.

2) Poly(A)-tail - a special enzyme attaches a chain of 150-200 adenine nucleotides to the 3' end of the pre-mRNA directly after transcription. This is primarily believed to increase the stability and therefore prolong the lifetime of an mRNA molecule.

3) Splicing - The removal of noncoding sequences, introns, from the pre-mRNA to create mRNA, containing only the coding sequences for a protein. This process is carried out by the splicosome. The splicosome is a special complex made up by proteins and a catalytic kind of small nuclear RNA molecules, the snRNAs..

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