Ribosomal RNA
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Ribosomal RNA (rRNA), a type of RNA synthesized in the nucleolus by RNA Pol I, is the central component of the ribosome, the protein manufacturing machinery of all living cells. The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.
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[edit] Inside the ribosome
The ribosome is composed of two subunits, named for how far away they segment when subject to centrifugation. tRNA is sandwiched between the small and large subunits and the ribosome catalyzes the formation of a peptide bond between the 2 amino acids that are contained in the tRNA.
The ribosome also has 3 binding sites called A, P, and E.
- The A site in the ribosome binds to an aminoacyl-tRNA (a tRNA bound to an amino acid).
- The NH2 group of the aminoacyl-tRNA which contains the new amino acid, attacks the carboxyl group of peptidyl-tRNA (contained within the P site) which contains the last amino acid of the growing chain called peptidyl transferase reaction.
- The tRNA that was holding on the the last amino acid is moved to the E site, and what used to be the aminoacyl-tRNA is now the peptidyl-tRNA.
A single mRNA can be translated simultaneously by multiple ribosomes.
[edit] Prokaryotes vs. Eukaryotes
The prokarytoic 70S ribosome is composed of the 50S (5S, 23S) large subunit and the 30S (16S) small, while the eukaryotic 80S ribosome is composed of the 60S (5S, 5.8S, 28S) large subunit and 40S (18S) small. Note that the S units cannot be added because it is a crude representation of how far away the centrifuge it travels.
[edit] Prokaryotes
In Prokaryotes a small 30S ribosomal subunit contains the 16S rRNA, where the S in 16S represents Svedberg units.
The large 50S ribosomal subunit contains two rRNA species (the 5S and 23S rRNAs).
Bacterial 16S, 23S, and 5S rRNA genes are typically organized as a co-transcribed operon.
There may be one or more copies of the operon dispersed in the genome (for example, Escherichia coli has seven).
Archaea contains either a single rDNA operon or multiple copies of the operon.
[edit] Eukaryotes
In contrast, Eukaryotes generally have many copies of the rRNA genes organized in tandem repeats; in humans approximately 300–400 rDNA repeats are present in five clusters (on chromosomes 13, 14, 15, 21 and 22).
The 18S rRNA in most eukaryotes is in the small ribosomal subunit, and the large subunit contains three rRNA species (the 5S, 5.8S and 28S rRNAs).
[edit] Translation
Translation is the net effect of proteins being synthesized by ribosomes, from a copy (mRNA) of the DNA template in the nucleus. One of the components of the ribosome (16s rRNA) base pairs complementary to a sequence upstream of the start codon in mRNA.
[edit] Importance of rRNA
Ribosomal RNA characteristics are important in medicine and in evolution.
- rRNA is the target of several clinically relevant antibiotics: Chloramphenicol, Erythromycin, Kasugamycin, Micrococcin, Paromomycin, Ricin, Sarcin, Spectinomycin, Streptomycin, and Thiostrepton.
- rRNA is the most conserved (least variable) gene in all cells. For this reason, genes that encode the rRNA (rDNA) are sequenced to identify an organism's taxonomic group, calculate related groups, and estimate rates of species divergence.
[edit] Nucleolar dominance
Nucleolar dominance has also been shown for rRNA genes. In some organisms, particularly plants, when two nuclei are combined into a single cell during hybridization the developing organism can 'choose' one set of rRNA genes for transcription. The rRNA genes of the other parent are suppressed and not generally transcribed, though reactivation of the suppressed or 'inferior' rRNA genes may occasionally occur. This selective preference of transcription of rRNA genes is termed nucleolar dominance.
[edit] External links
- Ribosomal RNA by Denis LJ Lafontaine and David Tollervey
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| Major Families of Biochemicals | ||
| Peptides | Amino acids | Nucleic acids | Carbohydrates | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides | ||
| Analogues of nucleic acids: | Types of Nucleic Acids | Analogues of nucleic acids: |
| Nucleobases: | Adenine | Thymine | Uracil | Guanine | Cytosine | Purine | Pyrimidine | |
|---|---|---|
| Nucleosides: | Adenosine | Uridine | Guanosine | Cytidine | Deoxyadenosine | Thymidine | Deoxyguanosine | Deoxycytidine | |
| Nucleotides: | AMP | UMP | GMP | CMP | ADP | UDP | GDP | CDP | ATP | UTP | GTP | CTP | cAMP | cADPR | cGMP | |
| Deoxynucleotides: | dAMP | TMP | dGMP | dCMP | dADP | TDP | dGDP | dCDP | dATP | TTP | dGTP | dCTP | |
| Ribonucleic acids: | RNA | mRNA | piRNA | tRNA | rRNA | ncRNA | sgRNA | shRNA | siRNA | snRNA | miRNA | snoRNA | LNA | |
| Deoxyribonucleic acids: | DNA | mtDNA | cDNA | plasmid | Cosmid | BAC | YAC | HAC | |
| Analogues of nucleic acids: | GNA | PNA | TNA | morpholino | |
