Transcription is a process by which cells are able to express their genes. It is how DNA is re-written into RNA (specifically messenger RNA). mRNA may then direct the synthesis of various proteins.
Controlling the production of mRNA in the nucleus allows the regulation of gene expression. In this article we will look at the process of DNA transcription and how mRNA is processed.
RNA Vs DNA
RNA, like DNA, is a polymer of subunits joined by phosphodiester bonds. However there are some key differences in these monomer units and how they are structured.
|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. There may be multiple promoter sequences in a DNA molecule. They indicate transcription start sites.
It then unwinds a portion of the DNA double helix, exposing the bases on each of the singular DNA strands. One of the strands will act as a template to create the new mRNA strand.
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). The RNA polymerase then binds the ribonucleotides together covalent by phosphodiester bonds.
Eukaryotes vs Prokaryotes
Understanding transcription in both eukaryotes and prokaryotes allows you to target their metabolic differences. This allows transcription to be disrupted in prokaryotic bacteria without harming the body’s eukaryotic cells.
|RNA polymerase present||3 types of RNA polymerase present||1 type of RNA polymerase present|
|Examples of promoter sequences||TATA box at -30*||Pribnow box at -10|
|Type of regulation||RNA polymerase requires several transcription factors to initiate transcription. It can be said to have a complex regulation||RNA polymerase only requires one additional protein (s factor). It can be said to have a simple regulation|
*(present 30 base pairs down stream from coding region)
This is the process by which ribonucleotides are added to the template strand, enabling growth of the mRNA transcript. The mRNA transcript is made 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.
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:
Addition of a methylated guanine cap confers protection to the mRNA. This is necessary as RNA is much more unstable than DNA. It involves:
- Addition of a methylated guanine
- Occurs at the 5’ end of the mRNA transcript
- Protects the mRNA from degradation
Addition of a polyA tail confers protection to the mRNA. This is necessary as RNA is much more unstable than DNA. It involves:
- Endonucleases* recognise a specific sequence along the mRNA transcript and cleaves it there
- Several adenine nucleotides are added (approximately 200) to the transcript by the enzyme polyA polymerase
- Occurs at the 3’ end, protecting the mRNA from degradation
* Endonucleases break within the polynucleotide. Exonucleases degrade the polynucleotide from the ends (either the 5’ or 3’ end)
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 (in the middle of) 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.