Transcription describes the process by which the genetic information contained within DNA is re-written into messenger RNA (mRNA) by RNA polymerase. This mRNA then exits the nucleus, where it provides the basis for the translation of DNA. By controlling the production of mRNA in the nucleus, the cell regulates the rate of gene expression.
In this article we will look at the process of DNA transcription, including the post-transcriptional modification of mRNA and its importance.
RNA Vs DNA
RNA, like DNA, is a polymer of subunits joined by phosphodiester bonds. However, as detailed below, there are key differences in the monomer units for each compound.
|Bases present||Adenine, guanine, cytosine, thymine||Adenine, guanine, cytosine, uracil|
|Structure||Double stranded helix||Single stranded helix|
Stages of Transcription
The process of transcription can be broadly categorised into 3 main stages: initiation, elongation & termination.
Transcription is catalysed by the enzyme RNA polymerase. It attaches to and moves along the DNA molecule until it recognises a promoter sequence, which indicates the starting point of transcription. There may be multiple promoter sequences in a DNA molecule. Transcription factors are proteins that control the rate of transcription. They too bind to the promoter sequences with RNA polymerase.
Once bound to the promotor sequence, RNA polymerase unwinds a portion of the DNA double helix, exposing the bases on each of the two DNA strands.
One DNA strand (the template strand) is read in a 3′ to 5′ direction and so provides the template for the new mRNA molecule. The other DNA strand is referred to as the coding strand. This is because the base sequence of the new mRNA is identical to it, except for the replacement of thiamine bases with uracil.
Incoming ribonucleotides are used by RNA polymerase to form the mRNA strand. It does this using complementary base pairing (A to U, T to A, C to G and G to C). RNA polymerase then catalyses the formation of phosphodiester bonds between adjacent ribonucleotides. Bases can only be added to the 3′ (three-prime) end, so the strand elongates in a 5’ to 3’ direction.
Elongation will continue until the RNA polymerase encounters a stop sequence. At this point, transcription stops and the RNA polymerase releases the DNA template.
Pre-translational mRNA processing
The mRNA which has been transcribed up to this point is referred to as pre-mRNA. Processing must occur to convert this into mature mRNA. The processing includes:
Capping describes the addition of a methylated guanine cap to the 5′ end of mRNA. Its presence is vital for the recognition of the molecule by ribosomes, and to protect the immature molecule from degredation by RNAases.
Polyadenylation describes the addition of a poly(A) tail to the 3′ end of mRNA. The poly(A) tail consists of multiple molecules of adenosine monophosphate. This stabilises RNA, which is necessary as RNA is much more unstable than DNA.
Splicing allows one genetic sequence to code for different proteins. This conserves genetic material. It involves:
- Introns (non-coding sequences) are removed via spliceosome excision
- Exons (coding sequence) are then joined together by ligation
- It is sequence dependent and occurs within the transcript
This allows many proteins to be made from a single pre-mRNA.
By the end of transcription, mature mRNA has been made. This acts as the messaging system to allow translation and protein synthesis to occur.
Within the mature mRNA, you have the open reading frame (ORF). This is the region that will be translated into protein. It is translated in blocks of three nucleotides, called codons.
At the 5’ and 3’ ends, there are also untranslated regions (UTRs). These are not translated during protein synthesis.
Clinical Relevance – Phenylketonuria (PKU)
This condition occurs due to a single base pair substitution (G to A) in the enzyme phenylalanine hydroxylase. This results in intron skipping, producing unstable mRNA. It is a genetic condition that is tested for in newborn babies via the heel prick test.
Individuals with phenylketonuria accumulate phenyalanine in their tissues, plasma and urine. Phenylketones are also found in their urine.
Symptoms of PKU include intellectual disability, developmental delay, microcephaly, seizures and hypopigmentation.
Treatment includes consuming diets low in phenylalanine and avoiding high protein foods such as meat, milk and eggs.