Ribosomes were discovered in the mid-1950 by cell biologist George Palade using the electron microscope observations. The term ''ribosomes'' was proposed by scientist Richard Roberts in 1958. In 1974, together with Albert Claude and Christian de Duve, George Palade was awarded the Nobel Prize in Physiology and Medicine. In addition, G.  Palade discovered the cell's protein-making factory and helped explain the way proteins are transported out of the cell (1, 2).   
Prokaryotic and eukaryotic ribosomes have same basic form, but among them are the differences of size and proportions of proteins and ribosomal RNA (rRNA).   
In cytoplasm, ribosomes are suspended (free ribosomes) and bound to endoplasmic reticulum (rough ER). In generally, the free ribosomes synthesize the proteins for cytoplasm and organelles activities and the bound ribosomes for cell membrane and for cell ''export'' (via exocytosis).        
Ribosomes are ribonucleoprotein particles consisting of two subunits that have specific roles in protein synthesis. Bacteria and organelles (mitochondria and chloroplast) ribosomes (70S) have a mass of 2.5 MDa and two subunits. The large subunit (50S) contains two types of the rRNA (23S and 5S) and 31 ribosomal proteins. The small subunit (30S) is composed of the rRNA (16S) and 21 ribosomal proteins. The eukaryotic ribosomes (80S) are larger than those of prokaryotes and organelles. Their mass is 4.2 MDa and the two subunits have the following characteristics: the large subunit (60S) contains three types of ribosomal RNA (28S, 5.8S and 5S) and 49 ribosomal proteins. The small subunit (40S) is composed of the rRNA (18S) and 33 ribosomal proteins (3, 4).          
The protein synthesis is a multi-stage process in  which are involved: ribosomes, messenger RNA (mRNA), transfer RNA (tRNA), the cytoplasm factors (initiation, elongation etc), amino acids and guanosine triphosphate (GTP). In the following section, we'll show some features of the ribosomes and the substances involved in protein synthesis. Then, we'll present the translation that is the second stage of protein synthesis.
Ribosome has three tRNA binding sites: ''site A'' that allows recognition of the aminoacyl-tRNA, ''site P'' that hosting the nascent peptide (peptidyl-tRNA) and ''site E'' for deacylated tRNA. mRNA  is a chain of polynucleotide (AUCG) which contains the genetic information to synthesis of polypeptide chain. It is formed by coping the genetic information of a polynucleotid chain of DNA (ATGC). tRNA has shaped leaf clover and it has a length of 73-93 nucleotides. The molecules of tRNA transfer the amino acids from cytoplasm to ribosomes during protein translation. Protein translation is done with the participation of cytoplasm factors either prokaryote and eukaryote cells. Cytoplasm factors are classified according to their role in: initiation factors, elongation factors and protein release factors. Prokaryotic initiation factors (IF-1, IF-2 and IF-3) are needed for the formation the initiation complex (ribosomal subunit 30S and mRNA) and for tRNA to enter into the complex. Eukaryotic cells have 12 initiation factors: eIF2, eIF2B, eIF3, eIF4A, eIF4B, eIF4E, eIF4E-BP 1, 2, 3, eIF4F, eIF4G, eIF5B. Elongation factor is  elongation factor EF-Tu (bacteria and EF-G) and protein release factors are class I release factors (RF). Amino acids are macromolecules containing an (the) amino group(s) (-NH2), a (the) carboxylic acid group(s) (-COOH) and a side-chain (-R) that is specific for each amino acid. Protein translation is achieved with the participation of 20 amino acids: alanine (Ala - A), arginine (Arg - R), asparagine (Asn - N), aspartic acid (Asp - D), cysteine (Cys - C), glutamic acid (Glu - E), glutamine (Gln - Q), glycine (Gly - G), histidine (His - H), isoleucine (Ile - I), leucine (Leu - L), lysine (Lys - K), methionine (Met - M), phenylalanine (Phe - F), proline (Pro - P), serine (Ser - S), threonine (Thr - T), tryptophan (Trp - N), tyrosine (Tyr - Y) and valine (Val - V).  GTP  is an energy-rich compound involved in activation of amino acids during the protein translation.
Protein translation is a process divided in three stages: initiation, elongation and termination.

For bibliography and more information click on the blue numbers and words.

1 - Transcription and Translation

2 - Protein Synthesis Animation Video

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