Spliceosomes Notes

Spliceosomes are large ribonucleoprotein complexes found in eukaryotic nuclei which catalyse the splicing of pre-mRNA or hnRNA, thereby removing introns from the transcript.

mRNA Synthesis and Processing

mRNA, or messenger RNA, is a single-stranded RNA molecule that is complementary to the DNA. It provides the template for protein synthesis, and in eukaryotes, is synthesised in the nucleus from DNA through a process called transcription, which is catalysed by RNA polymerase II.

The pre-mRNA, also known as hnRNA or heterogeneous nuclear RNA, undergoes processing to form the mature mRNA. This mature mRNA is then transported out of the nucleus and translation or protein synthesis takes place in the cytoplasm.

The pre-mRNA contains both exons and introns, which are coding and non-coding regions respectively. After undergoing post-transcriptional processing, such as splicing, capping and tailing, the hnRNA is converted into a mature mRNA that is functional.

Splicing - It is the process of cutting out the introns (non-coding regions) from the primary transcript and joining together the exons. This process is catalysed by spliceosomes.

Capping - It is the process of adding a cap or methyl guanosine triphosphate to the 5’ end of the primary transcript.

Tailing - In the process, 200-300 adenylate residues are added to the 3’ end, which is referred to as a poly(A) tail.

The fully functional mRNA, which is the result of processing of hnRNA, is transported out of the nucleus through the nuclear pore complex and acts as a template for protein synthesis.

Now Let’s Learn in Detail About Spliceosomes

Spliceosomes

Spliceosomes catalyse the process of splicing, which is one of the steps in post-transcriptional processing, necessary for the synthesis of functional mRNA in eukaryotes. This process occurs in the nucleus and involves the removal of non-coding intervening sequences (introns) of pre-mRNA and the joining together of coding sequences (exons).

Sometimes the RNA itself catalyses its own splicing, which is called a ribozyme. Spliceosomes are not required for this process of self-splicing.

The Spliceosome is a huge complex comprised of small nuclear ribonucleoproteins (snRNPs, also known as ‘snurps’). The Spliceosome complex is multi-megadalton in size and is composed of roughly 300 different types of proteins. Each cell contains around 100,000 Spliceosomes which are responsible for removing various intron sequences.

The two types of spliceosomes are:

  1. Proteins specific to RNA that drive the process to ensure accuracy.
  2. Small nuclear riboprotein complexes (snRNPs) composed of small nuclear RNAs (snRNAs) that are rich in uridine (U1, U2, U4, U5, U6, etc.).

Major Spliceosomes: They account for the removal of 99.5% of introns and are composed of U1, U2, U4, U5, and U6 snRNPs.

Minor Spliceosomes - They are responsible for the removal of 0.5% of introns and are composed of U11, U12, U4atac, U5, and U6atac snRNPs.

Mechanism of Splicing

Splicing is the process of removal of introns and ligation of exons in the pre-mRNA or hnRNA.

The assembly of spliceosomes occurs at the exon-intron junction of hnRNA. The U1 snRNP recognizes the 5’ end of the intron, while the U2 snRNP recognizes a specific site near the 3’ end. The other components of the spliceosome then assemble and rearrangement takes place.

The splicing process occurs in two steps:

  1. The intron is cleaved at the 5’ splice site as the first step.

  2. The first step is the linking of exons and the cleavage at the 3’ splice site of the intron.

  3. The second step is also the linking of exons and the cleavage at the 3’ splice site of the intron. Both of these reactions occur together.

Spliceosomes then disassemble from the removed intron, and the intron is then degraded.

Most genes in humans contain introns, and thus spliceosomes have an effect on gene expression. Additionally, some genes demonstrate alternative splicing, wherein a single gene can generate multiple RNAs that code for different proteins.

Incorrect splicing of a gene or misregulation of a spliceosome can result in mutation and genetic diseases. Furthermore, a single point mutation can cause a significant alteration in protein structure and its abundance.

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