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Transcription creates RNA strands from specific sections of DNA called transcription units. These units contain both exons and introns, but only exons code for proteins. After transcription, introns are removed through splicing, and alternative splicing can result in different final mRNA sequences that can code for multiple proteins.
Transcription is a process that occurs in all living cells. During transcription, RNA strands are created based on the DNA found inside the cells. When a strand of messenger RNA (mRNA) is created, it is then used to make proteins during translation. An entire strand of DNA is not usually transcribed into mRNA, but specific sections of the DNA, called transcription units, are instead.
Throughout the DNA strands found within cells, there are nucleotide sequences that code for starts and stops. Transcription enzymes use these codes to start and stop the production of RNA strands. A transcription unit is the chain of nucleotides in a DNA strand that starts in an initial code and continues to the final 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 occurred because research has shown that the DNA that is being transcribed contains many extra nucleotides that are not used to form the protein. The sections of DNA that are not used to code for a protein are called introns, and the sections that code for the protein are called exons.
Often, there are many more noncoding introns found within a DNA strand than there are exons. A gene is the term used to describe the DNA nucleotide sequence that codes for a protein. It was thought to be continuous, but research has shown that genes are not continuous strands but are divided into exons throughout the DNA transcription unit.
The transcription unit contains both introns and exons, which are encoded in RNA by the enzyme RNA polymerase. After the mRNA strand has been made and before it is translated into a protein, introns are removed from the RNA. Splicing cuts the introns from the mRNA strand 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 occurs may not always be the same. Alternative splicing refers to the fact that introns themselves can be removed from an mRNA strand, but exons can join in different ways. This means that a single transcription unit can in fact code for several proteins, since the final sequence of the mRNA strand can be different based on which exons are joined and in what order.
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