Transcription is a biological process by which information stored on a gene is copied to a short molecule called messenger RNA.
A gene can be divided into two sections:
– a Promoter region, which includes a sequence (TATA box), that can be recognised by the active site of the enzyme RNA Polymerase
– a Coding region, which stores the information needed to make a polypeptide
Figure 1: Organisation of a Gene (Image copyright: doodlesforscience)
Transcription happens in the nucleus.
The enzyme RNA Polymerase is responsible for Transcription. Proteins called Transcription Factors are also used in transcription.
Transcription is divided into three steps: Initiation, Elongation and Termination
Initiation
RNA Polymerase recognises the start sequence of the gene, called the promoter. It binds to the promoter region, along with transcription factors.
Fig 2: Transcription – the Initiation Step (Image copyright: doodlesforscience)
Elongation
On binding to the promoter, the RNA Polymerase unzips the gene by breaking the hydrogen bonds. This results in two single strands – the template (antisense), and the coding (sense) strand.
Complementary RNA nucleotides bind to the template strand via Hydrogen bonds.
Fig 3: Transcription – unzipping of DNA, and binding of complementary RNA nucleotides (Image copyright: doodlesforscience)
RNA Polymerase starts synthesis of messenger RNA, by adding new RNA nucleotides to the 3′ end of the chain (the end with a free -OH group). This is a condensation reaction, creating a phosphodiester bond.
RNA Polymerase moves along the template strand, extending the mRNA chain. Transcription Factors are released from the gene very early on in this process.
This process repeats, and many thousand mRNA are made from a gene.
The message encoded in mRNA, in the form of nucleotides, is used to construct a polypeptide chain during Translation.
HINT: Think of DNA as a ‘recipe’ book, made of multiple recipes (genes) for different products (proteins). There is only one copy of DNA per cell, and it is stored safely in a vault (nucleus). You can make multiple copies of the recipe (gene).
A mRNA is a copy of a recipe – each gene will be transcribed multiple times, making multiple copies of mRNA. mRNA then transports all the information needed to make the product (protein), from the vault (nucleus), to the processing centre/kitchen of the cell (RER).
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