Transcription is a process that occurs in all living cells. During transcription, strands of RNA are created based on the DNA found within the cells. When a strand of messenger RNA (mRNA) is created, it is then used to produce proteins during translation. A whole strand of DNA is not usually transcribed into mRNA, but instead specific sections of the DNA are, which are called transcription units.
Throughout the strands of DNA found within cells, there are sequences of nucleotides that code for starts and stops. The enzymes that carry out transcription use these codes to begin and end producing RNA strands. A transcription unit is the chain of nucleotides in a strand of DNA that begins at a start code and continues to the end code.
When referring to the stretch of DNA that is transcribed into RNA, many scientists use the term "transcription unit" rather than "gene." This change in terminology has come about because research has shown that the DNA that is transcribed contains a lot of extra nucleotides that are not used to form the protein. The sections of DNA that are not used to code a protein are called introns and the sections that do code for the protein are called exons.
Often, there are many more non-coding introns found within a strand of DNA than there are exons. A gene is the term used to describe the sequence of DNA nucleotides that codes for a protein. It was thought that it was continuous, but research has shown that genes are not continuous strands but are broken up into exons throughout the DNA transcription unit.
The transcription unit contains both introns and exons, which are coded into RNA by the enzyme RNA polymerase. After the mRNA strand has been produced, and before it is translated into a protein, the introns are removed from the RNA. Splicing cuts the introns from the strand of mRNA and then joins the remaining pieces together to form the final strand that will be used for translation.
The final strand of mRNA that is created after splicing has occurred may not always be the same. Alternative splicing refers to the fact that the same introns can be removed from a strand of mRNA, but the exons can join together in different ways. This means that a single transcription unit can in fact code for different proteins, due to the fact that the final sequence of the mRNA strand can be different based on what exons are joined together and in what order.