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RNA processing refers to a series of modifications that occur to a newly synthesized RNA molecule, particularly in eukaryotes, before it becomes a functional molecule. These processes are crucial for ensuring the stability, transport, and proper function of RNA.

The primary type of RNA processing occurs for messenger RNA (mRNA). Transcription in eukaryotes produces a precursor mRNA (pre-mRNA) molecule that contains both coding regions (exons) and non-coding regions (introns). The first major processing step is splicing, where introns are removed and exons are joined together. This is carried out by a complex molecular machine called the spliceosome.

Another critical modification for eukaryotic mRNA is the addition of a 5' cap. This cap is a modified guanine nucleotide that is added to the 5' end of the pre-mRNA. The 5' cap protects the mRNA from degradation by exonucleases, aids in its export from the nucleus, and is recognized by ribosomes for the initiation of translation.

The final processing step for mRNA is the addition of a poly-A tail to the 3' end. This tail consists of a string of adenine nucleotides that are added after cleavage of the pre-mRNA. The poly-A tail also contributes to mRNA stability and influences its translation efficiency and cellular localization.

Other RNA molecules, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), also undergo processing, which can involve cleavage, chemical modifications, and folding to achieve their mature, functional forms. These processing events are highly regulated and essential for gene expression and cellular function.