Ribosomal RNA rRNA , which is a major constituent of the cellular particles called ribosomes on which protein synthesis actually takes place. A set of transfer RNA tRNA molecules, each of which incorporates a particular amino acid subunit into the growing protein when it recognizes a specific group of three adjacent bases in the mRNA.
DNA maintains genetic information in the nucleus. RNA takes that information into the cytoplasm, where the cell uses it to construct specific proteins, RNA synthesis is transcription; protein synthesis is translation. RNA differs from DNA in that it is single stranded, contains Uracil instead of Thymine and ribose instead of deoxyribose, and has different functions. The central dogma depicts RNA as a messenger between gene and protein, but does not adequately describe RNA's other function.
Transcription takes place in the nucleus. RNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. Translation reads the genetic code in mRNA and makes a protein. Figure 1 shows how this occurs. Figure 1. Perhaps not surprisingly, abnormal splicing patterns can lead to disease states including cancer. This process, catalyzed by reverse transcriptase enzymes, allows retroviruses, including the human immunodeficiency virus HIV , to use RNA as their genetic material.
The mRNA formed in transcription is transported out of the nucleus, into the cytoplasm, to the ribosome the cell's protein synthesis factory. Here, it directs protein synthesis.
The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA triplet is known as a codon , and one codon contains the information for a specific amino acid. This tRNA molecule carries an amino acid at its 3'-terminus, which is incorporated into the growing protein chain. The tRNA is then expelled from the ribosome. Figure 7 shows the steps involved in protein synthesis.
Transfer RNA adopts a well defined tertiary structure which is normally represented in two dimensions as a cloverleaf shape, as in Figure 7. The structure of tRNA is shown in more detail in Figure 8. The reaction of esters with amines is generally favourable but the rate of reaction is increased greatly in the ribosome. Each transfer RNA molecule has a well defined tertiary structure that is recognized by the enzyme aminoacyl tRNA synthetase, which adds the correct amino acid to the 3'-end of the uncharged tRNA.
The presence of modified nucleosides is important in stabilizing the tRNA structure. Some of these modifications are shown in Figure The genetic code is almost universal. It is the basis of the transmission of hereditary information by nucleic acids in all organisms. In theory only 22 codes are required: one for each of the 20 naturally occurring amino acids, with the addition of a start codon and a stop codon to indicate the beginning and end of a protein sequence.
Many amino acids have several codes degeneracy , so that all 64 possible triplet codes are used. Sequence determinants of N-terminal protein processing. European Journal of Biochemistry , — Grunberger, D. Codon recognition by enzymatically mischarged valine transfer ribonucleic acid. Science , — doi Kozak, M. Point mutations close to the AUG initiator codon affect the efficiency of translation of rat preproinsulin in vivo. Nature , — doi Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes.
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Tandem Repeats and Morphological Variation. Chemical Structure of RNA. Eukaryotic Genome Complexity. RNA Functions. Citation: Clancy, S.
Nature Education 1 1 How does the cell convert DNA into working proteins? The process of translation can be seen as the decoding of instructions for making proteins, involving mRNA in transcription as well as tRNA.
Aa Aa Aa. Figure Detail. Where Translation Occurs. Figure 3: A DNA transcription unit. A DNA transcription unit is composed, from its 3' to 5' end, of an RNA-coding region pink rectangle flanked by a promoter region green rectangle and a terminator region black rectangle. Genetics: A Conceptual Approach , 2nd ed. All rights reserved. The Elongation Phase.
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