Ribosomes are also located in mitochondria and chloroplasts within eukaryotic organisms, these are smaller than those found in the cytoplasm and can be compared to bacterial ribosomes. The size of the ribosome denoted S (S = Svedberg unit) is derived from their rate of sedimentation relating their molecular weight, as well as their three dimensional structure. This is not additive so the total for example in prokaryotic ribosomes is 70S consisting of 30S and 50S constituent parts, the way in which these fit together alter the Svedberg unit for the whole ribosome.
Figure 1, Prokaryotic ribosome Figure 2, Eukaryotic ribosome
Ribosomal components – Prokaryotes
Prokaryotic ribosomes have a sedimentation value of 70S as shown in figure 1. The larger of the two subunits (total 50S) itself is made of two separate rRNA molecules each having a sedimentation value and nucleotide sequence length of 23S of 2904 nucleotides and 5S comprising of 120 nucleotides, as well as 31 proteins. The small subunit, 30S is made of 16S rRNA of 1541 nucleotides and just 21 proteins, illustrated in figure 3.
Eukaryotes
The eukaryotic ribosome of 80 s (figure 2) is made from a large 60 S subunit consisting of 28S rRNA (4718 nucleotides), 5S (120 nucleotides) and 5.8S (160 nucleotides) as well as 49 proteins. The small subunit of 40S comprises of 18S rRNA (1874 nucleotides) and 33 proteins, depicted in figure 3.
Figure 3, an illustrative table showing each component part of both prokaryotic and eukaryotic ribosomes.
RNA molecules are used to provide the catalytic functions associated with translation and the proteins are thought to promote binding of various molecules during translation. The genes responsible for the production of ribosomal RNA have, using hybridization techniques, been shown to be present in multiple copies, creating some redundancy of RNA genes (rDNA). E.coli for example has several copies of a single gene sequence coding for rRNA within its genome; a single transcript is used to firstly create the 30S transcript, which is cleaved enzymatically into the smaller 16S and 5S segments. This ensures that following transcription, equal amounts of all three types of rRNA are present. Eukaryotes have many copies of the sequence coding for the 28S and 18S components, drosophila has ca. 120 copies per haploid genome transcribed into a molecule of 34S from which the 28S, 18S and 5.8S are cleaved, Xenopus has over 500 copies present in a haploid genome. Mammals have clusters of rDNA genes grouped through the genome, in humans they are found localised near the ends of chromosomes 13, 14, 15, 21, and 22, these tandem repeats are separated by a spacer, a non-coding DNA sequence. 5S however, is unique and is not found as part of this rDNA sequence but located separately, in humans on chromosome 1.
Transfer RNA (tRNA) and its role in translation
tRNA in association with ribosomes and mRNA are part of the mechanism to transcribe the correct sequence of RNA to a polypeptide of corresponding amino acids. Small in size between only 75 - 90 nucleotides and stable, they have been researched thoroughly and are the best characterised out of all the RNA molecules. There is very little variation when comparing eukaryotic and prokaryotic tRNA both are transcribed as larger precursors and are cleaved into 4S molecules. All have a sequence of pCpCpA-3’(p?) located at the 3’-end of the acceptor arm, where the amino acid is covalently bonded to the terminal adenine residue, also they all contain 5’-Gp found at the other end of the molecule. The lengths of the arms and loops are similar between tRNA molecules and each tRNA contains an anticodon loop present in the same location. Though while there is 20 differing amino acids there is thought to be 32 different tRNAs. The two-dimensional cloverleaf model of tRNA was proposed by Robert Holley in 1965 after sequencing tRNAala isolated from yeast.
After sequencing it was found that some of the nucleotides were unique only to tRNA, including inosinic acid (I) which contained purines. These modified or unusual bases are made after transcription as part of post transcriptional modifications; a base is added then changed via enzymatic reactions which makes chemical modifications to the base. By looking for the anticodon for alanine, itself coded for by GCU, GCC and GCA, Holley found CGI, the last nitrogenous base I which can form hydrogen bonds with U, C and A, the last base in the codon, thus providing proof of the anticodon loop. Other unusual bases include 1-methyl inosinic acid (Im), 1-methyl guanylic acid (Gm), NN-dimethyl guanylic acid (Gm), pseudouridylic acid (Ψ) and ribothymidylic acid (T).
Prior to translation, tRNA must be bonded to their respective amino acids, this process called charging occurs with a group of enzymes known as aminoacyl tRNA synthetases, there are 20 different synthetases one for each amino acid. During charging amino acids are converted in a reaction with ATP to form aminoacyledenylic acid, its activated form. A convalent bond is formed between the amino acid at the carboxyl end and the 5’-phosphate group of the ATP, this molecule forms a complex with the synthetase enzyme which then reacts with a specific tRNA molecule. Aminoacyl tRNA synthetases are highly specific and only recognise only one amino acid. In reacting with the target tRNA molecule the amino acid is covalently bonded to the tRNA to the adenine residue (common to all tRNA), so charged tRNA may directly participate in protein synthesis.
The complex process of transcription and translation, of DNA being transcribed with an RNA polymerase, the development and maturation of mRNA in its various forms, depending on whether it is found in eukaryotes or prokaryotes. To the correct insertion of an amino acid after mRNA processing between the ribosomal subunits and subsequent association with tRNA, adding specificity to the selection of the correct corresponding amino acid. This process includes the fact that many amino acids are coded for by more than one codon, by using unusual bases in their corresponding anticodons, changed chemically for the purpose. These various steps and components form together sequentially in a way in that is only just beginning to be understood and yet happens each day within life all around us.
ER - providing a membrane network involved in the synthesis, modification, and transport of cellular materials