Question

What is meant by capping and tailing?

Answer 1

MRNA goes under two additional processes known as capping and tailing. In capping an unusual nucleotide called methyl guanosine triphosphate is added to the 5- end of MRNA. In tailing adenine is added to the 3-end of MRNA process of capping help recognising mRNA by the ribosomes.

Complete answer:
Capping- capping enzyme is an enzyme which catalyses the attachment of the 5' cap to messenger RNA molecule. The addition of the cap occurs after the growing RNA molecule contains 25 nucleotides. The enzymatic reaction is catalysed by the phosphorylated carboxyl-terminal domain of RNA polymerase II. The 5' cap is specific to RNAs synthesised by this polymerase rather than those synthesised by RNA polymerase III. Pre-mRNA undergoes a series of modifications - 5' capping, splicing and 3' polyadenylation before it becomes a mature mRNA which exits the nucleus so that it can be translated into functional proteins. Three enzymes, RNA triphosphatase, guanylyltransferase and methyltransferase, help in the addition of the methylated 5' cap to the mRNA.

Tailing- tailing is the addition of a poly (A) tail to messenger RNA. The poly (A) tail consists of multiple adenosine monophosphates. In eukaryotes, polyadenylation is part of the process which helps in producing mature messenger RNA translation. In bacteria, the poly (A) tail helps in degradation of the mRNA. The process of polyadenylation begins when the transcription of a gene stops. The 3′-most segment of the newly made pre-mRNA is cleaved off by proteins; these proteins then synthesize the poly (A) tail. These proteins add a poly (A) tail at several possible sites. It can produce more than one transcript from a single gene.

Note: The 5′ cap protects mRNA from degradation and helps in ribosome binding during translation. The poly (A) tail protects the mRNA from degradation, helps in the export of the mature mRNA to the cytoplasm, and also helps in binding proteins involved in initiating translation.