The expression of the genes contained in a cell's DNA is accomplished by three general kinds of RNA molecules. Ribosomal RNA (rRNA) molecules form the core of a cell's ribosomes, the structures in which protein synthesis occurs; messenger RNA (mRNA) molecules carry the coding sequences for protein synthesis and are called transcripts; and transfer RNA (tRNA) molecules transport amino acids to the ribosomes during protein synthesis. In prokaryotic cells, a single RNA polymerase synthesizes the many classes of RNA, whereas in eukaryotic cells, each class of RNA has its own polymerase. There are other forms of RNA as well, but their functions are not as well understood. Nevertheless, they seem to regulate gene expression and aid in the defense against invasive viruses.
There are literally thousands of distinct mRNA molecules present in a cell at any given time, making mRNA the most changeable class of RNA. As is frequently the case with transcripts that encode structural proteins, certain mRNA molecules are many, numbering in the hundreds or thousands. As is occasionally the case with transcripts that encode signaling proteins, other mRNAs are extremely uncommon and may only exist in a single copy. There is considerable variation in the longevity of mRNAs. Transcripts encoding structural proteins in eukaryotes have the potential to persist for more than ten hours, but those encoding signaling proteins may disintegrate in less than ten minutes.
The task of pairing amino acids with the correct codons in mRNA is carried out by tRNA molecules. Every tRNA molecule possesses two unique ends, one of which attaches to a particular amino acid and the other to the matching mRNA codon. These tRNAs transport amino acids to the ribosome during translation, where they bind with corresponding codons. Subsequently, the ribosome, accompanied by its resident rRNAs, unites the formed amino acids by moving in a ratchet-like manner along the mRNA molecule. A significant amount of chemical energy is needed to synthesize the resultant molecules, which can result in protein chains that are hundreds of amino acids long.
The instructions for constructing the different proteins that a cell needs to survive are found in its cellular DNA. Certain genes included in a cell's DNA must first be translated into mRNA molecules in order for the transcripts to be translated into chains of amino acids, which eventually fold into fully functioning proteins. Despite the fact that every cell in a multicellular creature has the same genetic makeup, each cell's transcriptome is unique based on its structure and role within the organism.
References: https://www.nature.com/scitable/topicpage/ribosomes-transcription-and-translation-14120660/ https://www.news-medical.net/life-sciences/-Types-of-RNA-mRNA-rRNA-and-tRNA.aspx
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