Table of Contents
In this RNA processing events after transcription in eukaryotes post we have briefly explained about Overview of pre-mRNA processing events in eukaryotes, 5′ cap and poly-A tail, RNA splicing and Alternative splicing.
RNA Processing in Eukaryotes
In this RNA processing in eukaryotes article, we’ll look at the cap, tail, and splicing modifications that occur in RNA processing in eukaryotes, how they’re carried out, and why they’re important for getting the right protein from our RNA.
The newly made RNA, also known as the primary transcript (the product of transcription is known as a transcript) is further processed before it is functional. Both prokaryotes and eukaryotes process their ribosomal and transfer RNAs.
There is an major difference in RNA processing events between prokaryotes and eukaryotes, however, is in the processing of messenger RNAs. In this discussion, we will concentrate on the RNA processing in eukaryotes mRNAs.
You may recall that mRNA is translated directly as it exits the DNA template in bacterial cells. RNA synthesis, which occurs in the nucleus, is distinct from protein synthesis, which occurs in the cytoplasm in eukaryotic cells.
Furthermore, eukaryotic genes contain introns, which are noncoding regions that interrupt the gene’s coding sequence. As a result, mRNA copied from genes with introns will contain regions that interrupt the gene’s information.
Before the mRNA is sent out of the nucleus to direct protein synthesis, these regions must be removed. Splicing is the process of removing introns and re-joining the coding sections or exons of an mRNA.
After the mRNA has been capped, spliced, and given a polyA tail, it is transferred from the nucleus to the cytoplasm for translation.
The initial product of transcription of a protein coding gene is called the pre-mRNA (or primary transcript). After it has been processed and is ready to be exported from the nucleus, it is called the mature mRNA or processed mRNA.
What are the processing steps for messenger RNAs?
In eukaryotic cells, pre-mRNAs undergo three main processing steps:
Capping at the 5′ end
Addition of a poly-A tail at the 3′ end
Splicing to remove introns
5' cap and poly-A tail
Chemical groups are added to both ends of a pre-mRNA to modify it. The group at the start (5′ end) is known as a cap, while the group at the end (3′ end) is known as a tail.
Both the cap and the tail protect the transcript and help it get exported from the nucleus and translated on the ribosomes (protein-making “machines”) found in the cytosol.
During transcription, the 5′ cap is added to the first nucleotide of the transcript. The cap is a guanine (G) nucleotide that protects the transcript from degradation. It also assists the ribosome in attaching to the mRNA and beginning to read it in order to make a protein.
The 3′ end of the RNA forms in kind of a bizarre way. When a sequence called a polyadenylation signal shows up in an RNA molecule during transcription, an enzyme chops the RNA in two at that site. Another enzyme adds about 100 – 200 adenine (A) nucleotides to the cut end, forming a poly-A tail. The tail makes the transcript more stable and helps it get exported from the nucleus to the cytosol.
RNA splicing is the third major RNA processing in eukaryotes event that occurs in your cells. In RNA splicing, specific parts of the pre-mRNA, called introns are recognized and removed by a protein-and-RNA complex called the spliceosome. Introns can be thought of as “junk” sequences that must be removed in order for the “good parts version” of the RNA molecule to be assembled.
The pieces of the RNA that are not chopped out are called exons. The exons are pasted together by the spliceosome to make the final, mature mRNA that is shipped out of the nucleus.
A key point to remember here is that only the exons of a gene encode a protein. Not only do introns not carry information for protein construction, but they must be removed in order for the mRNA to encode a protein with the correct sequence. If the spliceosome fails to remove an intron, an mRNA with extra “junk” in it is produced, and an incorrect protein is produced during translation.
We don’t know why splicing exists, and it appears to be a wasteful system in some ways. However, splicing allows for a process known as alternative splicing, in which multiple mRNAs can be produced from the same gene. We (and other eukaryotes) can encode more different proteins than we have genes in our DNA by using alternative splicing.
One pre-mRNA can be spliced in either of two (or sometimes many more!) different ways in alternative splicing. In the diagram below, for example, the same pre-mRNA can be spliced in three different ways depending on which exons are managed to retain. As a result, three distinct mature mRNAs are produced, each of which translates into a protein with a distinct structure.