Eukaryotic cells have different regions or segments within their DNA and RNA. For example, the human genome has groupings called introns and exons in DNA and RNA coding sequences.
Introns are segments that do not code for specific proteins, while exons code for proteins. Some people refer to introns as "junk DNA", but the name is no longer valid in molecular biology because these introns can, and often do, serve a purpose.
What Are Introns and Exons?
You can divide the different regions of eukaryotic DNA and RNA into two main categories: introns and exons.
Exons are the coding regions of DNA sequences that correspond to proteins. On the other hand, introns are the DNA/RNA found in the spaces between exons. They are non-coding, meaning they don't lead to protein synthesis, but they are important for gene expression.
The genetic code consists of the nucleotide sequences that carry the genetic information for an organism. In this triplet code, called a codon, three nucleotides or bases code for one amino acid. The cells can build proteins from the amino acids. Although there are only four base types, the cells can make 20 different amino acids from the protein-coding genes.
When you look at the genetic code, exons make up the coding regions and introns exist between the exons. Introns are "spliced" or "cut" out of the mRNA sequence and are thus not translated into amino acids during the translation process.
Why Are Introns Important?
Introns create extra work for the cell because they replicate with each division, and cells must remove introns to make the final messenger RNA (mRNA) product. Organisms have to devote energy to get rid of them.
So why are they there?
Introns are important for gene expression and regulation. The cell transcribes introns to help form pre-mRNA. Introns can also help control where certain genes are translated.
In human genes, about 97 percent of the sequences are non-coding (the exact percent varies depending on which reference you use), and introns play a vital role in gene expression. The number of introns in your body is greater than exons. When researchers artificially remove intronic sequences, the expression of a single gene or many genes can go down. Introns can have regulatory sequences that control gene expression.
In some cases, introns can make small RNA molecules from the pieces that are cut out. Also, depending on the gene, different areas of the DNA/RNA can change from introns to exons. This is called alternative splicing and it allows for the same sequence of DNA to code for multiple different proteins.
Related article: Nucleic Acids: Structure, Function, Types & Examples
Introns can form micro RNA (miRNA), which helps up- or down-regulate gene expression. Micro RNAs are single strands of RNA molecules that usually have about 22 nucleotides. They are involved in gene expression after transcription and RNA silencing that inhibits gene expression, so the cells stop making particular proteins. One way to think of miRNAs is to imagine they provide minor interference that interrupts mRNA.
How Are Introns Processed?
During transcription, the cell copies the gene to make pre-mRNA and includes both introns and exons. The cell has to remove the non-coding regions from mRNA before translation. RNA splicing allows the cell to remove intron sequences and join the exons to make coding nucleotide sequences. This spliceosomal action creates mature mRNA from the intron loss that can continue on to translation.
Spliceosomes, which are enzyme complexes with a combination of RNAs and protein, carry out RNA splicing in the cells to make mRNA that has only coding sequences. If they do not remove the introns, then the cell can make the wrong proteins or nothing at all. Introns have a marker sequence or splice site that a spliceosome can recognize, so it knows where to cut on each specific intron. Then, the spliceosome can glue or ligate the exon pieces together.
Alternative splicing, as we mentioned earlier, allows cells to form two or more forms of mRNA from the same gene, depending on how it is spliced. The cells in humans and other organisms can make different proteins from mRNA splicing. During alternative splicing, one pre-mRNA is spliced in two or more ways. Splicing creates different mature mRNAs that code for different proteins.