Over the last three decades, those long-held beliefs have been shattered. For this, they earned the Nobel Prize in ! What is RNA? Deoxyribonucleic acid DNA is a molecule you may already be familiar with; it contains our genetic code, the blueprint of life. The order of these bases determines the genetic blueprint, similar to the way the order of letters in the alphabet are used to form words.
The RNA-specific pyrimidine uracil forms a complementary base pair with adenine and is used instead of the thymine used in DNA. Even though RNA is single stranded, most types of RNA molecules show extensive intramolecular base pairing between complementary sequences within the RNA strand, creating a predictable three-dimensional structure essential for their function Figure 1 and Figure 2. Figure 1. Figure 2. Cells access the information stored in DNA by creating RNA to direct the synthesis of proteins through the process of translation.
Proteins within a cell have many functions, including building cellular structures and serving as enzyme catalysts for cellular chemical reactions that give cells their specific characteristics. If DNA serves as the complete library of cellular information, mRNA serves as a photocopy of specific information needed at a particular point in time that serves as the instructions to make a protein. The mRNA then interacts with ribosomes and other cellular machinery Figure 3 to direct the synthesis of the protein it encodes during the process of translation see Protein Synthesis.
Figure 3. In eukaryotes, synthesis, cutting, and assembly of rRNA into ribosomes takes place in the nucleolus region of the nucleus, but these activities occur in the cytoplasm of prokaryotes. Neither of these types of RNA carries instructions to direct the synthesis of a polypeptide, but they play other important roles in protein synthesis. Ribosomes are composed of rRNA and protein.
The rRNA ensures the proper alignment of the mRNA, tRNA, and the ribosomes; the rRNA of the ribosome also has an enzymatic activity peptidyl transferase and catalyzes the formation of the peptide bonds between two aligned amino acids during protein synthesis.
Although rRNA had long been thought to serve primarily a structural role, its catalytic role within the ribosome was proven in Because of the importance of this work, Steitz shared the Nobel Prize in Chemistry with other scientists who made significant contributions to the understanding of ribosome structure. It carries the correct amino acid to the site of protein synthesis in the ribosome. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to be inserted in the polypeptide chain being synthesized Figure 4.
Any mutations in the tRNA or rRNA can result in global problems for the cell because both are necessary for proper protein synthesis Table 1. Figure 4. These molecules play a critical role in gene regulation by way of RNA splicing. Small Interfering RNAs. Figure 1. Figure Detail. Small Nucleolar RNAs. Noncoding RNAs in Prokaryotes. Listeria monocytogenes. An increase in temperature melts the secondary structure around the ribosome binding site RBS and start codon, allowing ribosome binding and translation initiation.
DsrA RNA pairs with the translational operator of the rpoS gene using two sequences colored blue and light blue located within helices 1 and 2.
This base pairing exposes translation initiation signals for ribosome binding and increases mRNA stability. Johansson, J. An RNA thermosensor controls expression of virulence genes in Listeria monocytogenes. Cell , — All rights reserved. Altuvia, S. Switching on and off with RNA.
PNAS 97 , In several places, the strands bow outwards to form a hollow loop in the molecule. A red region on one loop is the ribosome binding site RBS ; a red region on a second loop is the start codon. A ribosome, depicted as two elongated, green ovals, is shown hovering outside the mRNA strand.
A second illustration adjacent to the first shows the RNA molecule after the structure around the RBS has melted due to high temperatures.
The RBS and start codon loops have fused to become a single, giant loop. The ribosome is bound to one strand of this large,unified loop, and the riboswitch is in an activated state. The bottom, sense strand contains the ribosome binding site and the start codon, both shown in red. A short RNA, or sRNA, is shown with three short sections of paired bases with a hairpin loop on the end of each section.
The loop on section 1 is dark blue, and the bases between section 1 and section 2 is light blue. These colored sections form complementary base pairs with the antisense strand of the rpoS mRNA. This base pairing releases the ribosome binding site and start codon, turning on translation. Catalytic RNA. Significance of Noncoding RNAs. References and Recommended Reading.
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