Final answer:
In eukaryotic cells, mRNA contains non-coding sequences called introns, which are spliced out to form mature mRNA composed of coding sequences called exons that go on to produce functional proteins.
Step-by-step explanation:
mRNA molecules in eukaryotic cells have additional sequences that are not translated into protein. These non-coding sequences are known as introns. Introns are removed from the pre-mRNA during a process called splicing, and the remaining coding sequences, known as exons, are joined together to form the mature mRNA that is then translated into protein. Eukaryotic genes are typically composed of both exons and introns, where exons correspond to the protein-coding sequences and introns serve various roles including gene regulation and mRNA transport. The accurate removal and splicing of these introns is crucial for the synthesis of functional proteins.