If you were to examine the mature mRNA that comes from a certain human gene, and then line it up to the actual DNA sequence of that same gene, you would see many differences. What would these differences be and where do they come from?

The differences between mature mRNA and DNA sequence of a gene are primarily due to a process called RNA processing or post-transcriptional modification. Let's break down the differences and understand where they come from:

1. Introns: In eukaryotic organisms (such as humans), the genes contain regions called introns, which do not encode any functional protein. During RNA processing, these introns are removed from the primary transcript (pre-mRNA) to create a mature mRNA molecule. Therefore, one major difference is that mature mRNA lacks the intronic sequences found in the DNA.

2. Exons: The remaining sequences in the pre-mRNA, called exons, are rearranged and joined together to form the final mRNA molecule. This process is called splicing. The order and arrangement of exons in mature mRNA may differ from the original DNA sequence due to alternative splicing. Alternative splicing refers to the phenomenon where different combinations of exons can be included or excluded in the final mRNA, resulting in multiple isoforms of the protein.

3. 5' Cap and 3' Poly-A Tail: In mRNA, a modified guanine nucleotide is added to the 5' end, forming a structure called the 5' cap. Additionally, a long chain of adenine nucleotides, known as the poly-A tail, is added to the 3' end. These modifications at the ends of mRNA molecules are absent in the corresponding DNA sequence.

4. RNA Editing: In some cases, specific nucleotides in mRNA may undergo editing, leading to changes in the mRNA sequence. This type of modification can occur through a process known as RNA editing, where particular bases are modified by enzymes.

In summary, the differences between mature mRNA and DNA sequence of a gene arise from RNA processing, which involves removal of introns, rearrangement of exons through splicing, addition of a 5' cap and a 3' poly-A tail, as well as potential RNA editing. These modifications are essential for the appropriate expression and functioning of genes in complex organisms like humans.