Many one amino acid. In eukaryotic cells once

Many RNAs do not code for protein however (about 97% of the transcriptional output is non-protein-coding in eukaryotes. These so-called non-coding RNAs (“ncRNA”) can be en­coded by their own genes (RNA genes) but can also derive from mRNA introns. The most prominent examples of non-coding RNAs are transfer RNA (tRNA).and ribosomal RNA (rRNA) both of which are involved in the process of translation. There are also non-coding RNAs involved in gene regulation, RNA processing and other roles. Certain RNAs are able to catalyze chemical reac­tions such as cutting and ligating other RNA molecules and the catalysis of peptide bond formation in the ribosome; these are known as ribozymes. Messenger RNA (mRNA) carries information about a protein sequence to the ribosomes: the protein synthesis factories in the cell. It is coded so that every three nucleotides (a codon) corre­spond to one amino acid.

In eukaryotic cells once precursor mRNA (pre-mRNA) has been transcribed from DNA, it is pro­cessed to mature mRNA. This removes its introns non-coding sec­tions of the pre-mRNA. The mRNA is then exported from the nucleus to the cytoplasm where it is bound to ribosomes and trans­lated into its corresponding protein form with the help of tRNA. In prokaryotic cells which do not have nucleus and cytoplasm com­partments, mRNA can bind to ribosomes while it is being tran­scribed from DNA. After a certain amount of time the message degrades into its component nucleotides with the assistance of ribonucleases. Transfer RNA (tRNA) is a small RNA chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation.

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It has sites for amino acid attachment and an anticodon region for codon recognition that binds to a specific sequence on the messenger RNA chain through hydrogen bonding. 2. Ribosomal RNA (rRNA): Is the catalytic component of the ribosomes. Eukaryotic ribosomes contain four different rRNA molecules: 18S, 5.

8S, 28S and 5S rRNA. Three of the rRNA mol­ecules are synthesized in the nucleolus and one is synthesized elsewhere. In the cytoplasm, ribosomal RNA and protein combine to form a nucleoprotein called a ribosome. The ribosome binds mRNA and carries out protein synthesis. Several ribosomes may be attached to a single mRNA at any time. rRNA is extremely abundant and makes up 80% of the 10 mg/ml RNA found in a typical eukaryotic cytoplasm.

3. Transfer RNA (tRNA): Is a small RNA molecule (usually about 74-95 nucleotides) that transfers a specific active aminoacid to a growing polypeptide chain at the ribosomal site of pro­tein synthesis during translation. It has a 3? terminal site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase. It also contains a three base region called the anticodon that can base pair to the corresponding three base codon region on mRNA. Each type of tRNA molecule can be attached to only one type of amino acid but because the genetic code contains multiple codons that specify the same amino acid, tRNA molecules bearing different anticodons may also carry the same amino acid.

4. Transfer-messenger RNA (tmRNA): A prominent feature of every tmRNA is the conserved tRNA-like domain (TLD) com­posed of helices 1, 12, and 2a (analogs of the tRNA acceptor stem, T-stem and variable stem, respectively) and containing the 5? monophosphate and alanylatable 3? CCA ends. The mRNA-like region (MLR) is in standard tmRNA a large loop containing pseudoknots and a coding sequence (CDS) for the tag peptide marked by the resume codon and the stop codon. The encoded tag peptide (ANDENYALAA in E. coli) varies among bacteria per­haps depending on the set of proteases and adaptors available.


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