The mixing and matching of exons from the same gene can lead to proteins with different functions. Therefore, introns are a way to generate different proteins or different amounts of proteins that are unique to a cell type. Introns might also allow for faster evolution.

Introns are noncoding sections of an RNA transcript, or the DNA encoding it, that are spliced out before the RNA molecule is translated into a protein. The sections of DNA (or RNA) that code for proteins are called exons.

On average, there are 8.8 exons and 7.8 introns per gene. About 80% of the exons on each chromosome are < 200 bp in length. < 0.01% of the introns are < 20 bp in length and < 10% of introns are more than 11,000 bp in length.

An intron is a long stretch of noncoding DNA found between exons (or coding regions) in a gene. Genes that contain introns are known as discontinuous or split genes as the coding regions are not continuous. Introns are found only in eukaryotic organisms.

Although the evidence is circumstantial, it is widely thought that spliceosomal introns originated from group-II introns—self-splicing introns that are widely found in fungi, plants, protists, and bacteria—which invaded the uninterrupted nuclear genes of an early eukaryote and subsequently lost the ability to self- …

Introns are removed from primary transcripts by cleavage at conserved sequences called splice sites. These sites are found at the 5′ and 3′ ends of introns. Most commonly, the RNA sequence that is removed begins with the dinucleotide GU at its 5′ end, and ends with AG at its 3′ end.

All introns in a pre-mRNA must be completely and precisely removed before protein synthesis. If the process errs by even a single nucleotide, the reading frame of the rejoined exons would shift, and the resulting protein would be dysfunctional. The process of removing introns and reconnecting exons is called splicing.

Abstract. After transcription of a eukaryotic pre-mRNA, its introns are removed by the spliceosome, joining exons for translation. Other intron products have long half-lives and can be exported to the cytoplasm, suggesting that they have roles in translation.

Introns are removed from the pre-mRNA by the activity of a complex called the spliceosome. The spliceosome is made up of proteins and small RNAs that are associated to form protein-RNA enzymes called small nuclear ribonucleoproteins or snRNPs (pronounced SNURPS).

nucleus

Ribosomes are the sites in a cell in which protein synthesis takes place. Within the ribosome, the rRNA molecules direct the catalytic steps of protein synthesis — the stitching together of amino acids to make a protein molecule. In fact, rRNA is sometimes called a ribozyme or catalytic RNA to reflect this function.

Ribosomes are minute particles consisting of RNA and associated proteins that function to synthesize proteins. Proteins are needed for many cellular functions such as repairing damage or directing chemical processes. Ribosomes can be found floating within the cytoplasm or attached to the endoplasmic reticulum.

The Golgi apparatus (GA), also called Golgi body or Golgi complex and found universally in both plant and animal cells, is typically comprised of a series of five to eight cup-shaped, membrane-covered sacs called cisternae that look something like a stack of deflated balloons.

Ribosomes are found ‘free’ in the cytoplasm or bound to the endoplasmic reticulum (ER) to form rough ER. In a mammalian cell there can be as many as 10 million ribosomes. Several ribosomes can be attached to the same mRNA strand, this structure is called a polysome.