Although they can be localized in an unattached form in the cytoplasm. Often several ribosomes are associated with one mRNA molecule; this structure is called polyribosome(polysome). The synthesis of ribosomes in eukaryotes occurs in a special intranuclear structure - the nucleolus.
Ribosomes are a nucleoprotein, in which the RNA/protein ratio is 1:1 in higher animals and 60-65:35-40 in bacteria. Ribosomal RNA makes up about 70% of the total RNA in a cell. Eukaryotic ribosomes contain four rRNA molecules, of which 18S, 5.8S and 28S rRNA are synthesized in the nucleolus by RNA polymerase I as a single precursor (45S), which is then modified and cut. 5S rRNAs are synthesized by RNA polymerase III in another part of the genome and do not require additional modifications. Almost all rRNA is in the form of a magnesium salt, which is necessary for maintaining structure; When magnesium ions are removed, the ribosome undergoes dissociation into subunits.
Large subunit RNA
The high molecular weight RNA that forms the structural basis of the large subunit of the ribosome is designated 23S rRNA (in the case of bacterial ribosomes) or 23S-like rRNA (in other cases). Bacterial 23S rRNA, like 16S rRNA, is one covalently continuous polyribonucleotide chain. At the same time, 23S-like rRNA of cytoplasmic ribosomes of eukaryotes consists of two tightly associated polyribonucleotide chains - 28S and 5.8S rRNA (5.8S rRNA is the structural equivalent of the 5′-terminal ~160-nucleotide segment of 23S rRNA, which turned out to be “split off” in the form of a covalently isolated fragment). The 23S-like rRNA of plant plastid ribosomes also consists of two tightly associated polyribonucleotide chains and contains 4.5S rRNA, a structural equivalent of the 3′-terminal segment of 23S rRNA. There are known cases of even more profound RNA fragmentation, an example of which is the 23S-like rRNA of the cytoplasmic ribosomes of some protists. Yes, y Crithidia fasciculata it consists of 7 separate fragments, and Euglena gracilis- out of 14.
In addition to the above-mentioned 23S(-like) rRNA, the large subunit usually also contains relatively low molecular weight RNA - the so-called 5S rRNA. Unlike the above-mentioned 5.8S and 4.5S rRNA, 5S rRNA is less tightly associated with 23S(-like) rRNA, is transcribed from a separate gene and, thus, cannot be considered as a cleaved fragment of high-polymeric rRNA. 5S rRNA is part of the large subunit of cytoplasmic ribosomes of all prokaryotes and eukaryotes, but, apparently, is not an essential component of any functional ribosome, since 5S rRNA is absent in mammalian mitochondrial ribosomes (the so-called “miniribosomes”).
The number of nucleotide units, as well as sedimentation constants, for samples of 23S and 23S-like rRNA from different sources can vary significantly. For example, 23S rRNA Escherichia coli consists of 2904 nucleotide residues, cytoplasmic 26S rRNA Saccharomyces cerevisiae- from 3392, mitochondrial 26S rRNA Saccharomyces cerevisiae- from 3273, cytoplasmic 28S rRNA Homo sapiens- out of 5025. Large subunits of mammalian mitochondrial ribosomes contain relatively short 23S-like rRNAs - only 1560-1590 nucleotide residues. The 5.8S rRNA molecule of the 28S 5.8S rRNA complex, characteristic of cytoplasmic eukaryotic ribosomes, is about 160 nucleotide residues long. The length of 5S rRNA is quite conservative and amounts to 115-125 nucleotide residues.
Ribosomal proteins
In addition to rRNA, the ribosome also contains about 50 (prokaryotic ribosomes) or 80 (cytoplasmic ribosomes of eukaryotes) different proteins. Almost each of these proteins is represented by only one copy per ribosome. Moderately basic proteins predominate. Most ribosomal proteins are evolutionarily conserved, and many ribosomal proteins from various sources can be classified as homologues, which is taken into account in the modern universal nomenclature of ribosomal proteins. The ribosome is 30-50% protein.
Low molecular weight components
In addition to biopolymers (RNA and proteins), ribosomes also contain some low-molecular components. These are water molecules, metal ions (mainly Mg 2+ - up to 2% of the dry weight of the ribosome), di- and polyamines (such as putrescine, cadaverine, spermidine, spermine - can account for up to 2.5% of the dry weight of the ribosome).
Translation mechanism
Translation is the synthesis of a protein by a ribosome based on information recorded in messenger RNA (mRNA). In prokaryotes, mRNA binds to the small subunit of the ribosome as a result of the interaction of the 3′-end of the 16S rRNA with the complementary Shine-Dalgarno sequence of the 5′-end of the mRNA (for binding of the small subunit of the eukaryotic ribosome, in addition to a specific motif in the nucleotide sequence of the mRNA, the presence of a cap structure is also required at its 5′ end). Next, the start codon (usually AUG) of the mRNA is positioned on the small subunit. Further association of the small and large subunits occurs with the binding of the initiator tRNA (in prokaryotes, this is formylmethionyl-tRNA, designated fMet-tRNA f Met) and with the participation of initiation factors (IF1, IF2 and IF3 in prokaryotes; in the case of eukaryotic ribosomes, they are involved in translation initiation analogues of prokaryotic factors, as well as additional factors). Thus, anticodon recognition (in tRNA) occurs on the small subunit.
After association, fMet-tRNA f Met is located in the P- (peptidyl-) site of the catalytic (peptidyl transferase) center of the ribosome. The next tRNA, carrying an amino acid at the 3′ end and complementary to the second codon on the mRNA, being in a complex with the charged (GTP) elongation factor EF-Tu, enters the A-(aminoacyl-) site of the ribosome. Then, a peptide bond is formed between formylmethionine (bound to the f Met tRNA located in the P site) and the amino acid brought by the tRNA located in the A site. The mechanism of catalysis of the transpeptidation reaction (formation of a peptide bond in the peptidyl transferase center) has not yet been fully elucidated. There are several hypotheses explaining the details of this process:
It is likely that high catalysis efficiency is achieved by a combination of these factors.
After the formation of a peptide bond, the polypeptide becomes associated with the tRNA located in the A-site. At the next stage, the deacylated tRNA f Met is shifted from the P site to the E site (exit-), the peptidyl tRNA is shifted from the A site to the P site, and the mRNA moves one triplet of nucleotides (codon). This process is called translocation and occurs with energy expenditure (GTP) with the participation of the EF-G factor.
Next, tRNA, complementary to the next codon of the mRNA, binds to the vacated A-site of the ribosome, which leads to the repetition of the described steps, and the resulting polypeptide is lengthened by one amino acid residue with each cycle. Stop codons (UGA, UAG and UAA) signal the end of translation. The process of ending translation and releasing the finished polypeptide, ribosome and mRNA is called termination. In prokaryotes, it occurs with the participation of termination factors RF1, RF2, RF3 and RRF.
History of ribosome research
Ribosomes were first described as compacted particles, or granules, by Romanian-American cell biologist George Palade in the mid-1950s. In 1974, Palade, Claude and Christian De Duve received the Nobel Prize in Physiology or Medicine “for their discoveries concerning the structural and functional organization of the cell.”
The term "ribosome" was proposed by Richard Roberts in 1958 instead of "ribonucleoprotein particle of the microsomal fraction" at the first symposium on these particles and their role in protein biosynthesis. Biochemical and mutational studies of the ribosome since the 1960s have described many of the functional and structural features of the ribosome.
In the early 2000s, models were built with atomic resolution (up to 2.4 Å) of the structures of individual subunits, as well as the complete prokaryotic ribosome associated with various substrates, which made it possible to understand the mechanism of decoding (recognition of a tRNA anticodon complementary to an mRNA codon) and the details of interactions between the ribosome, tRNA, mRNA, translation factors, as well as various antibiotics. This major achievement in molecular biology was awarded the 2009 Nobel Prize in Chemistry (“for studies of the structure and function of the ribosome”). The awards were given to American Thomas Steitz, a British Indian.
Ribosome (from “RNA” and soma - body) is a cellular non-membrane organelle that carries out translation (reading the mRNA code and synthesizing polypeptides).
Eukaryotic ribosomes are located on the membranes of the endoplasmic reticulum (granular ER) and in the cytoplasm. Ribosomes attached to membranes synthesize protein “for export,” and free ribosomes synthesize protein for the needs of the cell itself. There are 2 main types of ribosomes - prokaryotic and eukaryotic. Mitochondria and chloroplasts also contain ribosomes, which are similar to the ribosomes of prokaryotes.
The ribosome consists of two subunits - large and small. In prokaryotic cells they are designated 50S and 30S subunits, in eukaryotic cells - 60S and 40S. (S is a coefficient that characterizes the sedimentation rate of the subunit during ultracentrifugation). Subunits of eukaryotic ribosomes are formed by self-assembly in the nucleolus and enter the cytoplasm through the pores of the nucleus.
Ribosomes in eukaryotic cells consist of four strands of RNA (three rRNA molecules in the large subunit and one rRNA molecule in the small one) and approximately 80 different proteins, i.e. they represent a complex complex of molecules held together by weak, non-covalent bonds. (Ribosomes in prokaryotic cells consist of three strands of RNA; two strands of rRNA are in the large subunit and one rRNA is in the small subunit). The process of translation (protein biosynthesis) begins with the assembly of an active ribosome. This process is called translation initiation. Assembly occurs in a strictly ordered manner, which is ensured by the functional centers of ribosomes. All centers are located on the contacting surfaces of both ribosomal subunits. Each ribosome works like a large biochemical machine, or more precisely, like a superenzyme, which, firstly, correctly orients the participants (mRNA and tRNA) of the process relative to each other, and secondly, catalyzes reactions between amino acids.
Active sites of ribosomes:
1) mRNA binding center (M-center);
2) peptidyl center (P-center). The initiating tRNA binds to this center at the beginning of the translation process; at subsequent stages of translation, tRNA moves from the A-center to the P-center, holding the synthesized part of the peptide chain;
3) amino acid center (A-center) – the site of binding of the mRNA codon with the anticodon of the tRNA carrying the next amino acid.
4) peptidyl transferase center (PTP center): it catalyzes the binding reaction of amino acids. In this case, another peptide bond is formed, and the growing peptide is lengthened by one amino acid.
Scheme of protein synthesis on ribosomes of the granular endoplasmic reticulum.
(Figure from the book Cell Biology, Vol.II)
Schematic representation of a polyribosome. Protein synthesis begins with the binding of a small subunit at the location AUG-codon in a messenger RNA molecule (figure from the book Cell Biology, Vol.II).
Endoplasmic reticulum
Endoplasmic reticulum (syn. endoplasmic reticulum) – organelle of a eukaryotic cell. In cells of different types and under different functional states, this component of the cell may look different, but in all cases it is a labyrinthine extended closed membrane structure, built from communicating tube-like cavities and sacs called cisterns. Outside the membranes of the endoplasmic reticulum there is cytosol (hyaloplasm, the main substance of the cytoplasm), and the lumen of the endoplasmic reticulum is a closed space (compartment) communicating through vesicles (transport vesicles) with the Golgi complex and the environment external to the cell. The endoplasmic reticulum is divided into two functionally distinct structures: the granular (rough) endoplasmic reticulum and the smooth (agranular) endoplasmic reticulum.
The granular endoplasmic reticulum, in protein-secreting cells, is represented by a system of numerous flat membrane cisterns with ribosomes on the outer surface. The membrane complex of the granular endoplasmic reticulum is associated with the outer membrane of the nuclear envelope and the perinuclear (perinuclear) cistern.
In the granular endoplasmic reticulum, the synthesis of proteins and lipids for all cell membranes occurs, lysosome enzymes are synthesized, and the synthesis of secreted proteins is also carried out, i.e. intended for exocytosis. (The remaining proteins are synthesized in the cytoplasm on ribosomes not associated with ES membranes.) In the lumen of the granular ES, the protein is surrounded by a membrane, and the resulting vesicles are separated (budding off) from the ribosome-free regions of the ES, which deliver the contents to another organelle - the Golgi complex - by fusion with its membrane.
That part of the ES, on the membranes of which there are no ribosomes, is called the smooth endoplasmic reticulum. The smooth endoplasmic reticulum does not contain flattened cisterns, but is a system of anastomosing membrane channels
ovs, bubbles and tubes. The smooth network is a continuation of the granular network, but does not contain ribophorins - glycoprotein receptors with which the large subunit of ribosomes connects and is therefore not associated with ribosomes.
The functions of the smooth endoplasmic reticulum are diverse and depend on the cell type. The smooth endoplasmic reticulum is involved in the metabolism of steroids, such as sex hormones. Controlled calcium channels and energy-dependent calcium pumps are localized in its membranes. The cisterns of the smooth endoplasmic reticulum are specialized for the accumulation of Ca 2+ in them by constant pumping of Ca 2+ from the cytosol. Similar Ca 2+ depots exist in skeletal and cardiac muscles, neurons, eggs, endocrine cells, etc. Various signals (for example, hormones, neurotransmitters, growth factors) influence cell activity by changing the concentration of the intracellular messenger - Ca 2+. In the smooth endoplasmic reticulum of liver cells, the neutralization of harmful substances (for example, acetaldehyde formed from alcohol), the metabolic transformation of drugs, the formation of most of the cell's lipids and their accumulation occur, for example, in fatty degeneration. The ES cavity contains many different component molecules. Among them, chaperone proteins are of great importance.
Chaperones(English letters - an elderly lady accompanying a young girl at balls) - a family of specialized intracellular proteins that ensure rapid and correct folding of newly synthesized protein molecules. Binding with chaperones prevents aggregation with other proteins and thereby creates conditions for the formation of the secondary and tertiary structure of the growing peptide. Chaperones belong to three protein families, the so-called heat shock proteins ( 60, Chaperones belong to three protein families, the so-called heat shock proteins ( 70, Chaperones belong to three protein families, the so-called heat shock proteins (hsp90). The synthesis of these proteins is activated under many stresses, in particular during heat shock (hence the name h
eart shocked protein is a heat shock protein, and the number indicates its molecular weight in kilodaltons). These chaperones prevent denaturation of proteins at high temperatures and other extreme factors. By binding to abnormal proteins, they restore their normal conformation and thereby increase the survival of the organism during a sharp deterioration in the physicochemical parameters of the environment.
Ribosomes are submicroscopic non-membrane organelles necessary for protein synthesis. They combine amino acids into a peptide chain to form new protein molecules. Biosynthesis is carried out using messenger RNA by translation.
Ribosomes are located on the granular endoplasmic reticulum or float freely in the cytoplasm. They are attached to the endoplasmic reticulum with their large subunit and synthesize a protein that is transported outside the cell and used by the entire body. Cytoplasmic ribosomes mainly provide the internal needs of the cell.
The shape is spherical or oval, with a diameter of about 20 nm.
During the translation stage, several ribosomes can attach to the mRNA, forming a new structure - a polysome. They themselves are formed in the nucleolus, inside the nucleus.
There are 2 types of ribosomes:
- Small ones are found in prokaryotic cells, as well as in chloroplasts and the mitochondrial matrix. They are not associated with the membrane and are smaller in size (up to 15 nm in diameter).
- Large ones are found in eukaryotic cells, can reach a diameter of up to 23 nm, bind to the endoplasmic reticulum or are attached to the nuclear membrane.
Structure diagram The structure of both types is identical. The ribosome consists of two subunits - large and small, which when combined resemble a mushroom. They are combined with the help of magnesium ions, maintaining a small gap between the contacting surfaces. With magnesium deficiency, the subunits move away, disaggregation occurs, and ribosomes can no longer perform their functions.
Chemical composition
Ribosomes consist of highly polymeric ribosomal RNA and protein in a 1:1 ratio. They contain approximately 90% of all cellular RNA. The small and large subunits contain about four rRNA molecules, which look like threads gathered into a ball. The molecules are surrounded by proteins and together form a ribonucleoprotein.
Polyribosomes are a combination of messenger RNA and ribosomes that are strung on an mRNA strand. During the absence of synthesizing processes, ribosomes separate and exchange subunits. When mRNA arrives, they reassemble into polyribosomes.
The number of ribosomes can vary depending on the functional load on the cell. Tens of thousands are found in cells with high mitotic activity (plant meristem, stem cells).
Education in a cell
Ribosomal subunits are formed in the nucleolus. The template for the synthesis of ribosomal RNA is DNA. To fully mature, they go through several stages:
- Eosome is the first phase, in which only rRNA is synthesized on DNA in the nucleolus;
- neosome - a structure that includes not only rRNA, but also proteins, after a series of modifications it enters the cytoplasm;
- ribisome is a mature organelle consisting of two subunits.
Biosynthesis of proteins on ribosomes
Translation or synthesis of proteins on ribosomes from an mRNA matrix is the final stage in the transformation of genetic information in cells. During translation, information encoded in nucleic acids is transferred into protein molecules with a strict sequence of amino acids.
Translation is a very difficult stage (compared to replication and transcription). To carry out translation, all types of RNA, amino acids, and many enzymes are included in the process, which can correct each other’s errors. The most important participants in translation are ribosomes.
After transcription, the newly formed mRNA molecule leaves the nucleus into the cytoplasm. Here, after several transformations, it connects with the ribosome. In this case, amino acids are activated after interaction with the energy substrate - the ATP molecule.
Amino acids and mRNA have different chemical compositions and cannot interact with each other without outside participation. To overcome this incompatibility, transfer RNA exists. Under the action of enzymes, amino acids are combined with tRNA. In this form, they are transferred to the ribosome and tRNA, with a certain amino acid, is attached to the mRNA in the intended place. Next, ribosomal enzymes form a peptide bond between the attached amino acid and the polypeptide being built. The ribosome then moves along the messenger RNA chain, leaving a site for the attachment of the next amino acid.
The polypeptide grows until the ribosome encounters a “stop codon”, which signals the end of synthesis. To release the newly synthesized peptide from the ribosome, termination factors are activated, finally completing the biosynthesis. A water molecule is attached to the last amino acid, and the ribosome splits into two subunits.
As the ribosome moves further along the mRNA, it releases the initial section of the chain. A ribosome can again join it, which will begin a new synthesis. Thus, using one template for biosynthesis, ribosomes simultaneously create many copies of the protein.
The role of ribosomes in the body
- Ribosomes synthesize proteins for the cell's own needs and beyond. Thus, plasma blood clotting factors are formed in the liver, plasma cells produce gamma globulins.
- Reading encoded information from RNA, combining amino acids in a programmed order to form new protein molecules.
- Catalytic function – formation of peptide bonds, hydrolysis of GTP.
- Ribosomes perform their functions in the cell more actively in the form of polyribosomes. These complexes are capable of simultaneously synthesizing several protein molecules.
Ribosomes- intracellular organelles with a diameter of 20-22 nm that carry out protein biosynthesis. They are found in the cells of all living organisms. The shape of ribosomes is close to spherical. Prokaryotic cells (bacteria, blue-green algae), as well as chloroplasts and mitochondria of eukaryotes, are characterized by 70 S ribosomes; 80 S ribosomes are found in the cytoplasm of all eukaryotes. S is an indicator of the rate of deposition (sedimentation), the higher the number S, the higher the rate of deposition. The location of ribosomes in the cytoplasm can be free, but most often they are associated with the EPS, forming polysomes (associations of ribosomes).
Bosomes in the cytoplasm can be free, but most often they are associated with the EPS, forming polysomes (units of ribosomes using messenger RNA).
Composition and structure of ribosomes. Ribosomes consist of two subunits: large and small. The large subunit of each ribosome is attached to the membrane of the roughest ER, and the small subunit protrudes into the cytoplasmic matrix. The small one combines 1 rRNA molecule and 33 molecules of various proteins, the large one - three rRNA molecules and about 40 proteins. rRNA (ribosomal) functions as a framework for proteins (they play a structural and enzymatic role), and also ensures the binding of ribosomes to a specific nucleotide sequence of mRNA (information RNA K). Education
Ribosomes in cells proceed by self-assembly from pre-synthesized RNA and proteins. Ribosomal RNA precursors are synthesized in the nucleolus on nucleolar DNA.
Functions of ribosomes:
. specific binding and retention of components of the protein synthesizing system (messenger RNA; transport RNA, (GTP) and protein translation factors);
. catalytic functions (formation of peptide bonds, hydrolysis of guanosine triphosphate);
. functions of mechanical movement of substrates (messenger and transport RNA), or translocation.
Broadcast- the process of formation of a polypeptide chain on a matrix and RNA. The synthesis of protein molecules occurs on ribosomes located either freely in the cytoplasm or on the rough ER.
Translation stages (Fig. 13):
Rice. 13. Broadcast scheme
Consecutive stages of polypeptide synthesis:
. the small ribosomal subunit binds to met-tRNA, then to mRNA;
. the ribosome is mixed along the RNA, which is accompanied by multiple repetitions of the cycle of adding the next amino acid to the growing polypeptide chain;
. The ribosome reaches one of the stop codons of the mRNA, and the polypeptide chain is released and separated from the ribosome.
Activation of amino acids. Each of the protein's 20 amino acids is linked by covalent bonds to a specific tRNA using the energy of ATP. The reaction is catalyzed by a specialized enzyme that requires the presence of magnesium ions - aminoacyl-tRNA synthetase.
Initiation of a protein chain. In the small subunit of the ribosome there is a functional center with two sections - peptidyl (P-section) and aminoacyl (A-section). In the first position there is a tRNA carrying a specific amino acid, in the second there is a tRNA that is loaded with a chain of amino acids. The 5" end of the mRNA, which contains information about this protein, binds to the P-site with a small particle of the ribosome and with the initiating amino acid (formylmethionine in prokaryotes; methionine in eukaryotes) attached to the corresponding tRNA. The tRNA is complementary to the triplet contained in the mRNA, signaling the beginning of a protein chain.
Elongation is a cyclically repeating event in which the lengthening of a peptide occurs. The polypeptide chain is lengthened by the sequential addition of amino acids, each of which is delivered to the ribosome and inserted into a specific position using the corresponding tRNA. A peptide bond is formed between an amino acid from a peptide chain and an amino acid connected to a tRNA. The ribosome moves along the mRNA and the tRNA with a chain of amino acids enters the A-site. This sequence of events is repeated until the ribosomes arrive at a terminator codon for which there is no corresponding tRNA.
Termination. After completion of the synthesis of the chain, which is signaled by the so-called. stop codon of mRNA (UAA, UAG, UGA). In this case, water is added to the last amino acid in the peptide chain and its carboxyl end is separated from the tRNA, and the ribosome breaks up into two subparticles.
Peptide synthesis occurs not by one ribosome, but by several thousand, which form a complex - a polysome.
Folding and processing. To take its normal shape, the protein must fold into a specific spatial configuration. Before or after folding, the polypeptide can undergo processing, carried out by enzymes and consisting in the removal of excess amino acids, the addition of phosphate, methyl and other groups, etc.
Ribosomes are the most important components of cells of both prokaryotes and eukaryotes. The structure and functions of ribosomes are associated with protein synthesis in the cell, i.e. the translation process.
According to the chemical composition, ribosomes are ribonucleoproteins, i.e. they consist of RNA and proteins. Ribosomes contain only one type of RNA - rRNA (ribosomal RNA). However, there are 4 types of its molecules.
According to their structure, ribosomes are small, round-shaped, non-membrane cell organelles. Their number in different cells varies from thousands to several millions. The ribosome is not a monolithic structure, it consists of two particles called large and small subunits.
In eukaryotic cells, most ribosomes are attached to the ER, as a result of which the latter becomes rough.
Most of the rRNA that makes up ribosomes is synthesized in the nucleolus. The nucleolus is formed by certain sections of different chromosomes, containing many copies of genes on which the precursor of rRNA molecules is synthesized. After the synthesis of the precursor, it is modified and breaks down into three parts - different rRNA molecules.
One of the four types of rRNA molecules is synthesized not in the nucleolus, but in the nucleus in other parts of the chromosomes.
In the nucleus, individual ribosomal subunits are assembled, which then enter the cytoplasm, where they are combined during protein synthesis.
In structure, both ribosomal subunits are rRNA molecules that take on certain tertiary structures (fold) and are encrusted with dozens of different proteins. Moreover, the large ribosomal subunit contains three rRNA molecules (in prokaryotes, two), while the small ribosomal subunit contains only one.
The only function of ribosomes is to enable chemical reactions to occur during protein biosynthesis in the cell. Messenger RNA, transfer RNA, and many protein factors occupy certain positions in the ribosome, which makes it possible for chemical reactions to occur efficiently.
When subunits combine in the ribosome, “places” are formed - sites. The ribosome moves along the mRNA and “reads” codon by codon. A tRNA with an amino acid attached to it arrives at one site, and at the other site there is a previously arrived tRNA, to which a previously synthesized polypeptide chain is attached. In the ribosome, a peptide bond is formed between an amino acid and a polypeptide. As a result, the polypeptide ends up on the “new” tRNA, and the “old” one leaves the ribosome. The remaining tRNA along with its “tail” (polypeptide) is displaced in its place. The ribosome moves forward along the mRNA by one triplet, and a complementary tRNA is added to it, etc.
Several ribosomes can move along one strand of mRNA one after another, forming polysome.
