Here's how it works:

1. Transcription: DNA is transcribed into pre-mRNA, which contains both introns (non-coding regions) and exons (coding regions).

2. Splicing: Introns are removed from the pre-mRNA by a process called splicing. This is carried out by a complex called the spliceosome.

3. Ligation: After splicing, the exons are joined together by RNA ligase. This enzyme catalyzes the formation of phosphodiester bonds between the exons, creating a mature mRNA molecule.

Important Note: While RNA ligase is responsible for joining exons, it's important to remember that the splicing process itself is much more complex and involves a variety of proteins and RNA molecules.