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Understanding the structure of DNA and mRNA is key to solving why processed mRNA is shorter. DNA is a double-stranded molecule that holds our genetic code. It consists of coding regions called exons and non-coding regions called introns. When a gene is transcribed, the produced mRNA initially includes both these exons and introns. However, raw mRNA (also called pre-mRNA) must undergo further processing to become a mature mRNA molecule that cells can use for protein synthesis. This structure sets the stage for understanding the changes mRNA undergoes before it can be effectively translated into proteins.

One of the crucial steps in mRNA processing is splicing. This process removes the non-coding introns from the pre-mRNA. Specialized enzymes called spliceosomes carry out this task. The spliceosome cuts out the introns and stitches the exons together, creating a shorter, cohesive mRNA strand. This mature mRNA contains only the genetic sequences required to produce proteins. Without splicing, the mRNA would retain introns and result in a non-functional protein. Thus, mRNA splicing is essential for proper gene expression and protein creation.

Introns and exons are the two main components within a gene's DNA sequence. Introns are the non-coding sections that do not contribute to the final protein structure. Conversely, exons are the coding sections that remain in the mature mRNA and are translated into proteins. During gene expression, the entire DNA sequence of a gene, including both introns and exons, is transcribed into pre-mRNA. The next step is splicing, where introns are removed, and exons are joined together to form the mature mRNA. This spliced mRNA is then shorter than the original DNA sequence, consisting only of exons. Understanding the roles of introns and exons is vital for comprehending how mRNA processing affects the final length of the mRNA.