ribosomal RNA
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What is ribosomal RNA (rRNA)?
Ribosomal RNA (rRNA) is a molecule that is part of a ribosome, the protein-synthesizing organelle. After being exported to the cytoplasm, rRNA helps translate the information in messenger RNA (mRNA) into protein.
In the cell nucleus, rRNA molecules are created in the nucleolus. Ribosomes contain large and small rRNAs that combine with ribosomal proteins to make large and small ribosome subunits. The rRNAs play a role in recognizing portions of mRNAs and transfer RNAs.
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What role does rRNA play in making proteins?
Ribosomal RNA (rRNA) is a molecule that is a structural component of ribosomes, the protein-synthesizing organelles within cells. rRNA is transcribed from DNA and combines with ribosomal proteins to form the large and small subunits of the ribosome.
rRNA plays an active role in recognizing messenger RNAs (mRNAs) and transfer RNAs (tRNAs). It also catalyzes the formation of peptide bonds between amino acids, which links them together to form proteins. The order of tRNA molecules determines the amino acid sequence of a protein.
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Where is rRNA found inside a cell?
Ribosomal RNA (rRNA) is found in the nucleolus of eukaryotic cells, a specialized region within the nucleus, where rRNA synthesis and ribosome formation occur. rRNA is also found in the cytoplasm, where it forms part of the protein-synthesizing organelle known as a ribosome.
In the nucleolus, rRNA genes are transcribed, and early ribosome assembly takes place. Ribosomal proteins, synthesized in the cytoplasm, are transported to the nucleolus to combine with rRNA molecules, forming large and small ribosomal subunits. These subunits then return to the cytoplasm for final assembly into functional ribosomes.
Both eukaryotic and prokaryotic cells contain rRNA, but eukaryotes have more rRNA genes and ribosomes.
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How is rRNA different from other types of RNA?
Ribosomal RNA (rRNA) differs from messenger RNA (mRNA) and transfer RNA (tRNA) in several ways. rRNA, along with ribosomal proteins, forms ribosomes, the sites of protein synthesis. Unlike mRNA, which carries genetic code from DNA to ribosomes, rRNA forms the protein subunits that read the code in mRNA. While mRNA molecules determine the order of tRNA molecules, rRNA molecules catalyze the formation of peptide bonds between amino acids.
rRNA is synthesized in the nucleolus, a specialized region of the cell nucleus. Eukaryotic cells can contain anywhere from 50 to 5,000 sets of rRNA genes and up to 10 million ribosomes. The rRNA of Bacteria and Archaea differ from each other and from eukaryotic rRNA, which is important in understanding the evolutionary origins of these organisms.
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How is the genetic code for rRNA stored and used in cells?
Ribosomal RNA (rRNA) genes are stored in the nucleolus region of the cell nucleus in eukaryotes. These genes, of which there can be anywhere from 50 to 5,000 sets, are looped out of the main chromosomal fibers. In prokaryotes such as E. coli, there are fewer sets of rRNA genes.
During the synthesis of RNA, the enzyme RNA polymerase copies the base sequence of a section of double-stranded DNA, called a gene, into a single-stranded nucleic acid. This process is called transcription. The resulting rRNA molecules then form extensive secondary structures and play a role in recognizing mRNA and tRNA, as well as assisting with the catalysis of protein synthesis.
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Why is rRNA important for the structure and function of ribosomes?
Ribosomal RNA (rRNA) is essential to the structure and function of ribosomes. Ribosomes, which are organelles that synthesize proteins, contain both large and small rRNAs. rRNA forms extensive secondary structures and actively recognizes conserved portions of messenger RNAs (mRNAs) and transfer RNAs (tRNAs). rRNA also catalyzes the peptidyl transferase reaction during protein synthesis. The three-dimensional structure of RNA is critical to its stability and function.
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ribosomal RNA (rRNA), molecule in cells that forms part of the protein-synthesizing organelle known as a ribosome and that is exported to the cytoplasm to help translate the information in messenger RNA (mRNA) into protein. The three major types of RNA that occur in cells are rRNA, mRNA, and transfer RNA (tRNA).
Molecules of rRNA are synthesized in a specialized region of the cell nucleus called the nucleolus, which appears as a dense area within the nucleus and contains the genes that encode rRNA. The encoded rRNAs differ in size, being distinguished as either large or small. Each ribosome contains at least one large rRNA and at least one small rRNA. In the nucleolus, the large and small rRNAs combine with ribosomal proteins to form the large and small subunits of the ribosome (e.g., 50S and 30S, respectively, in bacteria). (These subunits generally are named according to their rate of sedimentation, measured in Svedberg units [S], in a centrifugal field.) Ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus for subassembly in the nucleolus. The subunits are then returned to the cytoplasm for final assembly.
The rRNAs form extensive secondary structures and play an active role in recognizing conserved portions of mRNAs and tRNAs. In eukaryotes (organisms that possess a clearly defined nucleus), anywhere from 50 to 5,000 sets of rRNA genes and as many as 10 million ribosomes may be present in a single cell. In contrast, prokaryotes (organisms that lack a nucleus) generally have fewer sets of rRNA genes and ribosomes per cell. For example, in the bacterium Escherichia coli, seven copies of the rRNA genes synthesize about 15,000 ribosomes per cell.
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There are radical differences between prokaryotes in the domains Archaea and Bacteria. These differences, in addition to being evident in the composition of lipids, cell walls, and utilization of different metabolic pathways, are also reflected in rRNA sequences. The rRNAs of Bacteria and Archaea are as different from each other as they are from eukaryotic rRNA. This information is important in understanding the evolutionary origins of these organisms, because it suggests that the bacterial and archaeal lines diverged from a common precursor somewhat before eukaryotic cells developed.
In bacteria the gene that has proved to be the most informative for investigating evolutionary relatedness is 16S rRNA, a sequence of DNA that encodes the RNA component of the smaller subunit of the bacterial ribosome. The 16S rRNA gene is present in all bacteria, and a related form occurs in all cells, including those of eukaryotes. Analysis of the 16S rRNA sequences from many organisms has revealed that some portions of the molecule undergo rapid genetic changes, thereby distinguishing between different species within the same genus. Other positions change very slowly, allowing much broader taxonomic levels to be distinguished.
Other evolutionary implications of rRNA stem from its ability to catalyze the peptidyl transferase reaction during protein synthesis. Catalysts are self-promoting—they facilitate reactions without being consumed themselves. Thus, rRNA, in serving both as a repository of nucleic acids and as a catalyst, is suspected of having played a key role in the early evolution of life on Earth.