Transcription is the first stage of protein synthesis. It transcribes a DNA base sequence into a messenger RNA (mRNA) base sequence. In eukaryotic cells, DNA is found in the nucleus so that is where transcription occurs. In prokaryotic cells it happens in the cytoplasm because the DNA is not contained in a nucleus. In this article will we look at the steps involved in the process, then take a look at how mRNA can be edited before it leaves the nucleus in eukaryotic cells.
Transcription
The first step of transcription is the binding of the enzyme RNA polymerase to DNA at the start of a gene. The two DNA strands separate because the hydrogen bonds between complementary bases are broken. The section of DNA uncoils, leaving bases exposed on the template strand. The template strand will be used as a template for synthesis of the mRNA molecule.
In the nucleus, there are free RNA nucleotides available. These complementary base pair to the exposed DNA bases (don’t forget that RNA has uracil instead of thymine). For the mRNA molecule to be completed, the RNA nucleotides must be joined with phosphodiester bonds to form the sugar-phosphate backbone. RNA polymerase catalyses the condensation reactions which form phosphodiester bonds.
RNA polymerase continues moving down the DNA molecule, gradually extending the new mRNA molecule, until it reaches a specific base sequence which tells it to stop. At that point, it detaches from the DNA molecule and releases the newly completed mRNA molecule. The DNA recoils into the double-helix shape behind it.
Technically, in eukaryotic cells the mRNA produced at this point is called pre-mRNA. The next section looks at how this is edited to form mRNA.
Splicing
In eukaryotic cells, the initial molecule of mRNA produced by transcription (called pre-mRNA) contains both introns and exons. Remember from the last article that introns are regions of DNA that don’t code for amino acids, so these base sequences need removing from pre-mRNA. The introns are removed in a process called splicing – the introns are chopped out and the exons are joined together. Sometimes, certain exons might get chopped out too. This means that more than one polypeptide can be produced from the same pre-mRNA, and therefore the same gene. Let’s look at a specific example.
Say we had a pre-mRNA molecule with 4 exons. If exon 2 was removed during splicing, the final mRNA molecule would contain exons 1, 3, and 4.
Alternatively, if exon 3 was removed the final mRNA molecule would contain exons 1, 2 and 4. This means there are different possible polypeptides that could be synthesised at the ribosomes.
Summary
- Transcription produces mRNA which is complementary to a DNA base sequence (a gene).
- RNA polymerase joins RNA nucleotides with phosphodiester bonds.
- In eukaryotic cells, pre-mRNA can be edited by splicing to remove introns.
- Splicing sometimes removes certain exons, meaning more than one mRNA can be produced from one gene.
