Genes.
The role of mRNA is to carry coded instructions from genes (sections of DNA) in the cell's nucleus to other parts of the cell. The coded instructions are essential for manufacturing proteins vital to cell function. Each of the cell's genes has three components. One is called the "promoter" and it acts as a sort of switch. Then there is a "coding region," a section of DNA that contains the code with the instructions for a specific protein synthesis. The third component, the "termination sequence," is that part of the gene that signals the end of a particular section of code
Enzyme.
Also present in the cell's nucleus is a crucial enzyme called RNA polymerase. (An enzyme is a kind of catalyst and works by facilitating a reaction without being changed or used up itself.) In the case of RNA polymerase, it reads the code sequence from the gene's DNA and translates it into an mRNA sequence. To initiate this, the RNA polymerase enzyme receives a signal to begin coding from the promoter on the gene. The enzyme then attaches to a section of the DNA molecule and begins to unravel it..
DNA
It may help to think of the DNA molecule as a twisted ladder. Each rung in the ladder would have a coupling in the center allowing the two halves to be split apart. Another analogy would be that of a zipper (just think of the zipper as being twisted). So imagine a twisted zipper that is securely zipped from bottom to top. Now imagine a small device that would attach to one section of the zipper somewhere in the middle and separate the two halves along that section without unzipping the entire length. That small device in an actual cell is the RNA polymerase enzyme.
To continue the analogy, all the little teeth that make up the zipper are of five different kinds and they're arranged in sequences that represent a code, a sort of blueprint for how to build something. (In the DNA molecule the "zipper teeth" are actually individual molecules called nucleotides.) The code exists in the intact zipper (DNA) but is of little functional value unless the coded instructions can be extracted from the zipper without destroying it. This is where mRNA comes in
Reading the Code..
The RNA polymerase enzyme attaches to the zipper (DNA) and opens up a section of it. It then begins reading the code from the teeth on one side of the zipper (DNA). From a pool of separate little zipper teeth (nucleotides) that are available nearby, the RNA polymerase draws these individual teeth in. It zips them to the sequence that it's reading from the one side of the full zipper. In doing this it makes a partial replica of that section of the main zipper (the gene's DNA). This process is called transcription.
When the RNA polymerase reaches the terminator sequence, transcription comes to a halt. A new strand of zipper teeth---a strand of mRNA--- is now a separate entity from the main zipper and is a carrier of the code from that key section of the zipper. After the RNA polymerase molecule and the new messenger RNA strand separate from the gene's DNA, this mRNA strand folds over onto itself. The two halves of the DNA zipper or ladder reconnect so that the zipper (DNA) is once again closed with its original code still intact.
Now that the mRNA molecule is assembled it is ready to move out into other regions of the cell and provide the code it received from the DNA. This code it carries is essential for the cell to manufacture proteins. The mRNA binds to structures in the cell called ribosomes. The ribosomes are where proteins are assembled from subunits called amino acids. The mRNA forms the backbone of this process, supplying the all important code it derived from the genetic DNA in the cell's nucleus.