DNA transcription is 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 acts as the basis for the translation of DNA. By controlling the production of mRNA within 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 three subunits joined by phosphodiester bonds. However, as detailed in the table below, there are key differences in the monomer units for each compound.
|Bases||Adenine, guanine, cytosine, thymine||Adenine, guanine, cytosine, uracil|
|Structure||Double stranded helix||Single stranded helix|
Stages of Transcription
The process of DNA transcription can be split into 3 main stages: initiation, elongation & termination. These steps are also involved in DNA replication.
Transcription is catalysed by the enzyme RNA polymerase, which attaches to and moves along the DNA molecule until it recognises a promoter sequence. This area of DNA indicates the starting point of transcription, and there may be multiple promoter sequences within 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 promoter 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 its base sequence is identical to the synthesised mRNA, except for the replacement of thiamine bases with uracil.
RNA polymerase uses incoming ribonucleotides to form the new mRNA strand. It does this by catalysing the formation of phosphodiester bonds between adjacent ribonucleotides, using complementary base pairing (A to U, T to A, C to G and G to C). Bases can only be added to the 3′ (three-prime) end, so the strand elongates in a 5’ to 3’ direction.
Elongation continues 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. This 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 the genetic sequence of a single pre-MRNA to code for many different proteins, conserving genetic material. This process is sequence dependent and occurs within the transcript. It involves:
- Removal of introns (non-coding sequences) via spliceosome excision
- Joining together of exons (coding sequence) by ligation
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, is the open reading frame (ORF). This region 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)
PKU occurs due to a single base pair substitution (G to A) in the enzyme phenylalanine hydroxylase. This results in intron skipping, producing unstable mRNA. PKU is one of several genetic conditions tested for in babies via the newborn blood spot (heel prick) test.
Individuals with phenylketonuria accumulate phenyalanine in their tissues, plasma and urine. Phenylketones are also found in their urine. This results in 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.