What is ‘capping’ in DNA translation?

Answer

Dear Anouk,

Capping is a process that takes place after transcription (ie after mRNA has been transcribed from DNA) and not during translation (translation is the conversion of the genetic code contained in the mRNA into amino acids that form proteins). You explain splicing correctly, but it also happens during transcription and not during translation. Don’t let these two terms confuse you!

What’s that capping now?
When mRNA molecules are formed, on one side of the chain (we call this side the 5′ side, the other side we call the 3′ side) there is a triphosphate group. In the figure in the link I attached here, you can see that there are three phosphate groups on the 5′ side of the nucleotide. This 5′ end is also the nucleotide from which the growing mRNA chain begins. As soon as a number of nucleotides are attached to each other, there is an enzyme complex that comes into action: the Capping Enzyme Complex (CEC). This complex hangs an extra nucleotide “backwards” from that first nucleotide.

What do I mean by “backwards”?
Well, normally the 3′ end of one nucleotide will be joined by the 5′ end of the next nucleotide, so that all the nucleotides hang together in the same direction. You can compare it with a polonaise procession. There all people look in the same direction, and there is always a free shoulder at the back and a free pair of arms at the front. Additional dancers join at the back. What happens in capping is that the 5′ end of a nucleotide is attached to the 5′ end of the growing chain. Compare it to someone who takes the first dancer in the polonaise necklace by the hand with his arms, who also hangs “backwards” from the chain. The nucleotide that hangs in the reverse direction from the mRNA chain is called the 5′ cap.

But why does the cell do that?
That cap has several functions. For example, it helps the ribosomes to start translation. Another function is to aid the transport from the nucleus to the cytoplasm of the cell. And another very important role is to prevent mRNA degradation by protecting the 5′ end of the mRNA.

Then there are two 3′ ends?
With the cap at the beginning of the chain, we have a chain that has a 3′ end in both the front and back. However, there is an additional caveat: After the nucleotide is attached “back to front” to the chain, this nucleotide undergoes an additional chemical change, a methylation. This tells the cellular machinery that this is the cap and not the 3′ end.

Conclusion:
Capping is a process in which the cell attaches a nucleotide “backwards” to the 5′ end of a growing mRNA chain and is then methylated.

Regards,
Benjamin