Cells have a second type of nucleic acid - RNA (Ribonucleic Acid) which can also carry genetic information. Unlike DNA, which is located primarily in the nucleus, RNA can also be found in the cellular liquid outside the nucleus, which is called the cytoplasm. Like DNA, RNA is also built from purine and pyrimidine nucleotides (Uracil (U) taking the place of Thymine (T)), but forms a single strand (unlike the DNA's double strand).
The messenger RNA (mRNA) is an RNA molecule whose role is to carry genetic information from the DNA in the nucleus to the ribosomes - the intra-cellular constructs that translate RNA into a protein. mRNA is synthesized in the nucleus based on a single DNA strand, using the RNA polymerase enzyme. mRNA is transcribed from a DNA strand only in places where genes are present. When such transcription occurs, the two DNA strands are split apart, and one of them serves as a mold for the generated mRNA molecule, which is reverse complementary to this strand, and therefore, a replica of the other one. Near most of the genes there is a special pattern in the DNA called Promotor which ``tells'' the RNA polymerase where to begin the transcription. At the end of the gene the transcription stops, and a few dozens of Adenine (A) nucleotides are added to the end of the RNA molecule to mark its end. In eukaryotes, the mRNA is formed of fragments, which are alternatingly called exons and introns. Introns are excised from the mRNA in a procedure called Splicing that takes place in the nucleus. After the introns are cut, the exons are glued together to create the final mature mRNA molecule. Typical eukaryotic exons are of average length of 200bp, while the average length of introns is around 10000bp (these lengths can vary greatly between different introns and exons). In many cases, the pattern of the splicing can vary depending on the tissue in which the transcription occurs. For example, an intron that is cut from mRNAs of a certain gene transcribed in the liver, may not be cut from the same mRNA when transcribed in the brain. This variation is called Alternative splicing, and it contributes to the overall protein diversity in the organism. After being synthesized from a DNA strand and edited, the mRNA molecule exits the nucleus, heading for a ribosome, where it is being translated into a protein, as explained below.
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