Gene expression occurs via two main steps, known as transcription and translation. First, the nucleotide sequence of the gene is transcribed into a pre-mRNA molecule. Since gene sequence contains introns and exons, introns should be spliced off from the pre-mRNA molecule to produce a mature mRNA molecule containing only exons. Spliceosomes are ribonucleoprotein complexes that carry out splicing of introns from pre-mRNA molecules. Afterwards, the mature mRNA molecule leaves the nucleus and reaches the ribosomes in the cytoplasm for translation. Ribosomes are the cellular organelles that carry out protein synthesis from mRNA molecules. Both spliceosomes and ribosomes are multimolecular complexes that contain both RNA and proteins.
1. Overview and Key Difference
2. What are Spliceosomes
3. What are Ribosomes
4. Similarities Between Spliceosomes and Ribosomes
5. Side by Side Comparison – Spliceosomes vs Ribosomes in Tabular Form
What are Spliceosomes?
The spliceosome is the large molecular machine operating inside the nucleus to remove introns from the transcribed pre-mRNA molecule. It is a complex of ribonucleoproteins, consisting of five uridine-rich small nuclear RNAs known as U1, U2, U4, U5 and U6 and numerous protein components. Spliceosome contains approximately 100 proteins. The conformation and composition of spliceosomes are highly dynamic. Dynamic nature of spliceosome is very important to carry out splicing machinery highly accurate and flexible.
The human cell contains about 100,000 spliceosomes. There are two types of spliceosomes in human cells. They are major spliceosomes and minor spliceosomes. Major spliceosomes are responsible for removing 99.5% of introns while the remaining 0.5% is removed by minor spliceosomes.
What are Ribosomes?
The ribosome is a small round organelle present in the cytoplasm of cells. It is the protein factory of the cell. In simple words, it is the organelle that carries out proteins synthesis in living cells. Both prokaryotes and eukaryotes have ribosomes. Prokaryotic ribosomes are 70S while eukaryotic ribosomes are 80S in size.
During gene expression, the transcribed mRNA molecule is translated into a protein at the ribosome. Therefore, translation of mRNA occurs in ribosomes. Ribosomes are composed of ribosomal RNA molecules and proteins. Structurally, the ribosome has two subunits as large subunit and small subunit. Four rRNA molecules hold the structure of ribosome together.
Nucleolus produces ribosomes, and then ribosomes travel to the cytoplasm of the cell. In the cytoplasm, there are two types of ribosomes. They are free form or bound (attached) form ribosomes. Free ribosomes do not stay bound to any organelle. They are freely floating in the cytoplasm and move all around the cell. Bound ribosomes associate with the endoplasmic reticulum. They are present on the surface of the ER. Once these ribosomes are attached, they cannot move around the cell. However, both free and bound ribosomes participate in protein synthesis.
What are the Similarities Between Spliceosomes and Ribosomes?
- Ribosomes and spliceosomes are ribonucleoprotein nanomachines.
- They contain both RNA and protein constituents.
- Moreover, they participate in important steps of gene expression and protein synthesis.
- Both work as ribozymes.
What is the Difference Between Spliceosomes and Ribosomes?
Spliceosomes are the multi-megadalton ribonucleoprotein complexes that catalyze the pre-mRNA splicing while ribosomes are the ribonucleoprotein complexes that catalyze protein synthesis from mRNA. So, this is the key difference between spliceosomes and ribosomes. Moreover, the spliceosomes are found only in the eukaryotic nuclei, while the ribosomes are found in both prokaryotic and eukaryotic cells. Therefore, this is another important difference between spliceosomes and ribosomes. Besides, spliceosomes can be seen in eukaryotic nuclei, while ribosomes can be seen in the cytoplasm of cells.
Below infographic summarizes the difference between spliceosomes and ribosomes.
Summary – Spliceosomes vs Ribosomes
Spliceosomes are ribonucleoprotein complexes found in eukaryotic nuclei. They remove noncoding regions or introns from the pre-mRNA molecule and ligate exons together. On the other hand, ribosomes are small organelles found in both prokaryotic and eukaryotic cells. They carry out protein synthesis or translation process. So, this is the summary of the difference between spliceosomes and ribosomes.
1. Will, Cindy L, and Reinhard Lührmann. “Spliceosome Structure and Function.” Cold Spring Harbor Perspectives in Biology, Cold Spring Harbor Laboratory Press, 1 July 2011, Available here.
2. “RNA Splicing: Introns, Exons and Spliceosome.” Scitable. Nature Publishing Group, Available here.