Question

1. Can you explain why eukaryotic mRNA should undergo processing first before leaving the nucleus?

Answer 1

Question.

Answer: Eukaryotic cells consists of a nucleus bound by a membrane and their genes possess alternating introns and exons. Thus, the immature pre-mRNA should be processed before it leaves the nucleus. Eukaryotic cells carry out mRNA processing, it includes 3 steps include 5′ capping, 3′ cleavage/polyadenylation, and RNA splicing. mRNA processing occurs in the eukaryotic nucleus and the functional mRNA produced is transported to the cytoplasm. Extensive processing was carried out in eukaryotic pre-mRNA before it is ready to be translated. Compared with prokaryotic mRNA, additional steps involved in eukaryotic mRNA maturation develop a molecule with a longer half-life. At first, the eukaryotic pre-mRNA receives a 5′ cap and a 3′ poly (A) tail before introns are removed and the mRNA is ready for translation. In this process, a 7-methylguanosine cap is added to the 5′ end of the pre-mRNA while elongation is under progress. This 5′ cap protects the nascent mRNA from degradation and assists in ribosome binding during translation. During this process, a poly (A) tail is added to the 3′ end of the pre-mRNA once elongation is finished. The poly (A) tail protects the mRNA from degradation and helps in the transport of mature mRNA to the cytoplasm, and it helps in binding proteins involved in initiating translation. Before mRNA is exported to the cytoplasm introns must be removed completely or precisely from the pre-mRNA before protein synthesis. If the process aids by a single nucleotide the reading frame of the rejoined exons can shift and the resulting protein would be dysfunctional. The process of removing non-coding introns and joining of exons by spliceosomes is called splicing. The process was carried out in the nucleus. Splicing was performed with the help of sequence-specific mechanism that ensures introns are removed and exons rejoined with the accuracy and precision of a single nucleotide. The splicing of pre-mRNAs is aided by protein complexes and RNA molecules called spliceosomes. Each spliceosome is composed of five subunits called small nuclear ribonucleoparticles (snRNPs). snRNP is a protein complex and a found only in the nucleus called snRNAs (small nuclear RNAs). This spliceosomes recognize sequences at the 5′ end of the intron because introns always start with the nucleotides GU and they recognize sequences at the 3′ end of the intron because they always end with the nucleotides AG. The spliceosome cleaves the pre-mRNA’s sugar phosphate backbone at the G that starts the intron and then covalently attaches that G to an internal A nucleotide within the intron. Then the spliceosme connects the 3′ end of the first exon to the 5′ end of the following exon, cleaving the 3′ end of the intron in the process.

In short, pre- mRNA processing consists of 3 steps such as addition of a 5' cap to the beginning of the RNA, addition of a poly-A tail (tail of A nucleotides) to the end of the RNA and removal of introns and joining of exons.Once it's completed these steps, the RNA is a mature mRNA. It can travel out of the nucleus and be used to make a protein.