Intron

Inside your cells, genes have special parts.

Some parts are used to make things. These are the good parts. Other parts are not used at all. These are called introns. The cell cuts the introns out. Then, it joins the good parts together. This makes a final piece that the cell can use. Introns are found in most living things. They are in humans and even in some tiny germs. Some introns are very long. Others are very short. It is amazing how cells sort these pieces out.

Inside your genes, there are special parts. Some parts are used to make things. These are called exons. Other parts are not used in the final product. We call these parts introns.

Think of a gene like a long sentence. The exons are the words that make sense. The introns are like extra spaces or marks. To read the sentence, the cell must remove the introns. This set of steps is called splicing. After splicing, the exons join together. This makes a final piece called mature RNA.

Scientists first found introns in 1977. Phillip Allen Sharp and Richard J. Roberts shared a Nobel Prize for this work. Introns are found in most complex living things. They are very common in humans. However, they are rare in some tiny life forms like yeast.

There are different types of introns. Some use a large machine called a spliceosome to cut them out. Others can cut themselves out. This is called self-splicing. Sometimes, the cell makes a mistake during splicing. This can lead to health problems in people.

Inside your genes, there are special parts that work together. Some parts carry the instructions used to make things. We call these parts exons. Other parts are inside the gene but are not used in the final product. These are called introns.

The name intron comes from the term "intragenic region," which means a region inside a gene. Introns are found in most eukaryotes, which are complex living things. They are also found in many eukaryotic viruses. However, they are rare in Bacteria and Archaea.

To make a final messenger, the cell must go through a way it works called splicing. First, the cell makes a long piece of RNA that contains both introns and exons. Then, the cell must remove the introns. The exons are then joined together to form a mature RNA.

There are different ways this happens. Some introns are removed by a large machine called a spliceosome. These are called spliceosomal introns. Other introns are removed by proteins. These are called tRNA introns. Some introns are even able to remove themselves. These are called group I and group II self-splicing introns.

Scientists first discovered that genes were split by introns in 1977. This was found by several different labs at the same time. Phillip Allen Sharp and Richard J. Roberts led these labs. They later shared the Nobel Prize in Physiology or Medicine in 1993. Other scientists like Louise Chow and Thomas Broker also helped with the discovery. Much of the work in the Sharp lab was done by Susan Berget. The term "intron" was introduced by the American biochemist Walter Gilbert in 1978.

Introns can be very different in size and frequency. In humans, protein-coding genes almost always have many introns. One human gene, MST1L, has a very short intron of only 30 base pairs. On the other side, a gene in a fruit fly called Drosophila DhDhc7 has a huge intron. This intron is 3.6 megabase long and takes three days to transcribe.

Introns are common in the cells of humans and mice. They are very rare in the genes of baker's yeast. In vertebrates, the mitochondrial genomes have no introns at all.

Sometimes, the cell makes a mistake during the splicing process. This is called a splicing error. While the process can be very accurate, errors can happen. These mistakes can lead to health problems. For example, a mutation in an intron caused hemophilia in the descendants of Queen Victoria.

This happened because the mutation created a new, incorrect spot for the cell to cut. Most of these incorrect pieces are quickly destroyed by the cell. Scientists study these errors to understand how genes work. They also look at how introns help regulate how genes are used.

An intron is a specific nucleotide sequence found within a gene. These sequences are not expressed or operative in the final RNA product. The term comes from "intragenic region," meaning a region located inside a gene.

While introns are part of the initial genetic code, they are removed during a process called splicing. The remaining sequences that are joined together to form mature RNA are called exons. Introns are common in most eukaryotes, which are complex organisms. They are also found in many eukaryotic viruses. However, they are quite rare in prokaryotes like Bacteria and Archaea.

To understand how this works, we must look at the process of RNA splicing. First, the cell transcribes a long RNA molecule known as a pre-mRNA. This precursor contains both the functional exons and the non-functional introns. During splicing, the cell must identify and remove the introns. Once the introns are gone, the exons are linked together to create a mature messenger RNA. This mature RNA is then ready for translation into proteins.

This mechanism ensures that only the correct instructions reach the protein-making machinery of the cell.

Scientists have identified at least four main types of introns based on how they are removed. Spliceosomal introns are found in nuclear protein-coding genes. These are removed by a large complex called a spliceosome, which uses specific RNA molecules to recognize boundaries. tRNA introns are found in transfer RNA genes. These require specific proteins, called endonucleases and ligases, to perform the removal. Group I and group II introns are unique because they are self-splicing. These introns fold into complex three-dimensional architectures that allow them to remove themselves through RNA catalysis.

Spliceosomal introns are particularly complex. They are characterized by specific sequences at the boundaries between introns and exons. These sequences allow the spliceosome to initiate the reaction. These introns also contain a branch point, which is a nucleotide sequence near the 3' end. During splicing, this sequence becomes covalently linked to the 5' end of the intron, creating a branched structure. While these specific elements are conserved, the rest of the nuclear pre-mRNA intron sequence is highly variable. These introns are often much longer than the exons that surround them.

The discovery of introns changed our understanding of genetics. In 1977, several laboratories independently discovered that genes were split by these intervening sequences. Phillip Allen Sharp and Richard J. Roberts led prominent labs that made these findings. Their work earned them the Nobel Prize in Physiology or Medicine in 1993. Other researchers, such as Louise Chow and Thomas Broker, also contributed to this discovery. Much of the research in the Sharp lab was conducted by postdoctoral fellow Susan Berget. The American biochemist Walter Gilbert introduced the official term "intron" in 1978.

Introns vary greatly in size and frequency across the biological spectrum. In the nuclear genome of jawed vertebrates, like humans and mice, introns are extremely common. For example, the human MST1L gene contains a very short intron of only 30 base pairs. In contrast, the Drosophila DhDhc7 gene in fruit flies contains an enormous intron. This single intron is 3.6 megabase long and takes roughly three days to transcribe.

Distribution also varies by cell type. While introns are common in vertebrate nuclei, they are absent in vertebrate mitochondrial genomes. In baker's yeast, introns are also quite rare.

Splicing is a highly precise but imperfect process. The spliceosome is a massive structure containing up to one hundred proteins and five different RNAs. Under ideal conditions, splicing is 99.999% accurate. However, real-world error rates can be much higher. Some studies suggest errors may occur at a rate of 0.1% per intron, or even 2% to 3% per gene. These errors often happen because of "cryptic" splice sites, which are accidental cutting points. Most incorrect transcripts are quickly destroyed by a process called nonsense-mediated decay.

When splicing errors occur due to genetic mutations, they can cause serious disease. A mutation in an intron can create a new, incorrect splice site. This happened in the descendants of Queen Victoria, leading to hemophilia. In that case, a mutation in a blood clotting factor gene created a cryptic 3' splice site. This resulted in aberrant splicing and a non-functional protein. Beyond disease, introns play a vital role in biology by helping to regulate gene expression. Some introns even encode their own functional RNAs through further processing.