Biology Made Easy - Guide to Transcription

Transcription is the process by which DNA is converted to its equivalent RNA sequence. The RNA sequence can then be used in the process of translation to form a protein. The strand of RNA formed by transcription is called messenger RNA, or mRNA, because it carries the sequence needed to make a protein.

The initiation of transcription requires a promoter sequence in the DNA. The most common promoter sequence in eukaryotes is the TATA box. RNA polymerase is the enzyme that attaches to the DNA and creates the corresponding RNA strand. In eukaryotic transcription, proteins called transcription factors are needed to help the RNA polymerase bind to the promoter sequence.

The double-stranded DNA is split so that complementary RNA molecules can bind to the DNA bases. When the transcript is first forming, there is a tendency for the RNA polymerase to slip off, creating aborted transcripts. After the transcript is about 23 nucleotides long, it is no longer prone to slipping. After this point, it is in the process of elongation.

Elongation is simply creating an RNA strand by using one strand of DNA as a template. Transcription occurs in the 5' to 3' direction, where the 5' end is the end of DNA terminating in a phosphate group, and the 3' end terminates in a hydroxyl group. The mRNA molecule created differs from DNA in two different ways. First, RNA molecules use the nucleotide uracil instead of thymine. Second, RNA uses the sugar ribose, whereas DNA uses deoxyribose.

In bacteria, transcription is halted when the RNA molecule forms a hairpin loop, causing the RNA polymerase to fall off. In eukaryotes, termination occurs and adenines are added to the end of the RNA molecule.

The addition of a poly-adenine tail at the 3' end of the mRNA is just one aspect of post-transcriptional modification. About 200 adenines are added to the tail, protecting the end of the molecule from degradation by ribonucleases.

At the 5' end of the molecule, the phosphate is removed and a 7-methylguanosine is put in its place. This is commonly called the 5' cap. Like the poly-A tail, this cap protects the end of the molecule from degradation.

The final post-transcription modification is called splicing. This process removes introns, regions of RNA that do not code for protein, from the molecule. The exons that remain are connected, and together with the 5' cap and poly-A tail, the mature mRNA is formed.