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Genes are sections of DNA that code for proteins, known as the genetic code. Introns are lengths of DNA that do not code for proteins, while exons do. Transcription creates RNA from DNA, followed by translation to produce proteins. Three types of RNA are produced during transcription, each with a specific role in translation. Intron removal is important to ensure the correct sequence is included in the gene. Exons are expressed to make proteins, while introns contain sections of RNA that are removed. A single base error during removal can prevent exons from being expressed.
In the nucleus, genes are sections of DNA that code for specific proteins in the cell. This is referred to as the genetic code. All DNA is not included as part of the genetic code. There are lengths of DNA, called introns, that do not code for proteins. The parts of DNA that make up the genetic code are called exons.
To create proteins from DNA, two separate processes take place inside the cell. First, transcription creates RNA from DNA. This is then followed by translation, which produces proteins based on the previously created RNA. Transcription occurs within the nucleus and translation occurs within the cytoplasm of the cell.
During transcription, three types of RNA are produced, based on the DNA in the nucleus. Everyone has a specific role in translation. Messenger RNA, or mRNA, provides the code for building proteins. Ribosomal RNA, also called rRNA, is used to make ribosomes, where translation takes place. Transfer RNA, or tRNA, carries amino acids, the building blocks of proteins, to the ribosome to make protein strands.
The presence of exons and introns was first discovered in 1977 by Phillip A. Sharp, a scientist at MIT. He found that the functional mRNA produced was only about a third of the length expected based on DNA. He found that large regions were removed from mRNA after transcription to make it functional. This occurs within the nucleus, before mRNA is released into the cell to carry out translation.
Intron removal involves at least four different types of small nuclear ribonucleoprotein particles, or snRNPs. snRNPs recognize a specific sequence of RNA at the boundary between the intron and the exon. When the snRNPs have bound to the RNA, they break the bond between the exon and the intron and the intron moves apart. Several snRNPs then splice the remaining two exons together. This occurs along the length of the RNA strand to produce functional mRNA.
Exons are so named because they survive this removal process and are then expressed or used to make proteins within the cell. Introns contain the sections of RNA that are removed. Each of the exons is not necessarily the same as a single gene, and introns can occur within genes and even between genes. This means that intron removal is very important to ensure that the correct sequence is included in the gene. A single base error during removal can mean that the resulting exons cannot be expressed or used to make the proteins required by the cell.
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