Splicing allows the genetic sequence of a single pre-MRNA to code for many different proteins, conserving genetic material. This stabilises RNA, which is necessary as RNA is much more unstable than DNA. The poly(A) tail consists of multiple molecules of adenosine monophosphate. Polyadenylation describes the addition of a poly(A) tail to the 3′ 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. This includes: 5′ CappingĬapping describes the addition of a methylated guanine cap to the 5′ end of mRNA. Processing must occur to convert this into mature mRNA.
The mRNA which has been transcribed up to this point is referred to as pre-mRNA. At this point, transcription stops and the RNA polymerase releases the DNA template. TerminationĮlongation continues until the RNA polymerase encounters a stop sequence. Bases can only be added to the 3′ (three-prime) end, so the strand elongates in a 5’ to 3’ direction. 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). RNA polymerase uses incoming ribonucleotides to form the new mRNA strand. This is because its base sequence is identical to the synthesised mRNA, except for the replacement of thiamine bases with uracil. The other DNA strand is referred to as the coding strand. One DNA strand (the template strand) is read in a 3′ to 5′ direction, and so provides the template for the new mRNA molecule. 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. Transcription factors are proteins that control the rate of transcription they too bind to the promoter sequences with RNA polymerase. This area of DNA indicates the starting point of transcription, and there may be multiple promoter sequences within a DNA molecule. Transcription is catalysed by the enzyme RNA polymerase, which attaches to and moves along the DNA molecule until it recognises a promoter sequence. These steps are also involved in DNA replication. The process of DNA transcription can be split into 3 main stages: initiation, elongation & termination.