What is a computer network called? Computer networks

Ribosomes were first discovered in an animal cell in 1955. In the same year, evidence was obtained that they perform important functions in metabolism - they are centers of protein biosynthesis. Ribosomes contribute to the implementation of the cell’s hereditary information and ensure the uniqueness of each type of organism due to the formation of proteins specific to it.

Localization

Ribosomes are present in all types of cells. They are formed in the nucleus, then come out of it and located in:

cytoplasm;
mitochondria;
plastids;
on the membranes of the endoplasmic reticulum (ER).

Rice. 1. Ribosomes on the membranes of rough ER.

Structure

The ribosome measures about 25 - 30 nm and consists of two unequal particles called the large and small subunits.

Rice. 2. Structure of the ribosome.

Each subunit performs its own function in the process of protein synthesis. In terms of their chemical composition, ribosomes are a complex of proteins and RNA, and it is RNA that determines their properties.

Protein synthesis

The process of protein biosynthesis is extremely complex and energy-intensive.
It involves:

regulatory proteins;
catalyst proteins;
ATP and GTP as energy sources;
transport and messenger RNA molecules;
magnesium ions.

Ribosomes are the centers and organizers of the protein synthesizing system, which can also work outside the cell.

Transcription

Information about the structure of a protein is found in the gene.

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During the process of transcription, a copy of the gene is formed in the cell nucleus in the form of messenger RNA (mRNA). The information in this case is a certain sequence of nucleotides, the constituent parts of mRNA.

The nucleotide sequence of mRNA encodes the nucleotide sequence of DNA (a gene is a section of DNA).

Broadcast

After mRNA exits the nucleus into the cytoplasm, a ribosome attaches to it, thereby initiating the assembly of the protein synthesizing system.

Then the process of translation begins - the synthesis of protein molecules from amino acids, which are delivered to the ribosome by transport RNAs (tRNAs).

Rice. 3. Scheme of protein biosynthesis on the ribosome.

After each new amino acid is added, the ribosome subunits move along the mRNA chain by one codon. A codon is three nucleotides that code for a specific amino acid.

In total, information about the composition of the protein is copied twice, first from DNA to mRNA, then from mRNA to the protein itself. Information in a protein is a sequence of amino acids, and in DNA and mRNA it is a sequence of nucleotides.

During the process of protein biosynthesis, the following functions of ribosomes are performed:

binding and retention of components of the protein synthesizing system;
catalysis of reactions leading to the formation of peptide bonds;
catalysis of GTP hydrolysis;
mechanical movement along the mRNA chain.

Differences in the structure and functions of ribosomal subunits are presented in the table.

The function of moving the ribosome along mRNA is carried out jointly by two subunits.

What have we learned?

We found out what function ribosomes perform in a cell. They are the main part of the protein synthesizing system. The assembly of protein molecules occurs on ribosomes. Proteins that make up the ribosomes themselves regulate and catalyze the processes of protein synthesis.

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The structure of the ribosome. Ribosomes are found in the cells of all organisms. These are microscopic round bodies with a diameter of 15-20 nm. Each ribosome consists of two particles of unequal size, small and large.
One cell contains many thousands of ribosomes; they are located either on the membranes of the granular endoplasmic reticulum or lie freely in the cytoplasm. Ribosomes contain proteins and RNA. The function of ribosomes is protein synthesis. Protein synthesis is a complex process that is carried out not by one ribosome, but by a whole group, including up to several dozen united ribosomes. This group of ribosomes is called a polysome. Synthesized proteins first accumulate in the channels and cavities of the endoplasmic reticulum and are then transported to organelles and cell sites where they are consumed. The endoplasmic reticulum and ribosomes located on its membranes represent a single apparatus for the biosynthesis and transport of proteins.

Chemical composition of ribosomes The eukaryotic type ribosomes contain 4 rRNA molecules and about 100 protein molecules, the prokaryotic type - 3 rRNA molecules and about 55 protein molecules. During protein biosynthesis, ribosomes can “work” individually or combine into complexes - polyribosomes (polysomes). In such complexes they are linked to each other by one mRNA molecule. Prokaryotic cells have only 70S-type ribosomes. Eukaryotic cells have both 80S-type ribosomes (rough EPS membranes, cytoplasm) and 70S-type (mitochondria, chloroplasts). Eukaryotic ribosome subunits are formed in the nucleolus. The combination of subunits into a whole ribosome occurs in the cytoplasm, usually during protein biosynthesis.

Function of ribosomes: assembly of a polypeptide chain (protein synthesis).

Free ribosomes, polyribosomes, their connection with other structural components of the cell.

There are single ribosomes and complex ribosomes (polysomes). Ribosomes can be located freely in the hyaloplasm and be associated with the membranes of the endoplasmic reticulum. Free ribosomes form proteins mainly for the cell’s own needs; bound ribosomes provide the synthesis of proteins “for export.”

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Histology

Histology from the Greek histos tissue logos is the study of the structure, development and vital activity of tissues of living organisms.. The formation of histology is closely related to the development of microscopic technology and.. In the history of the study of tissues and the microscopic structure of organs, two periods are distinguished: pre-croscopic and..

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Which consist of RNA and proteins. They are responsible for the biosynthesis of proteins. Depending on the level of protein in a particular cell, the number of ribosomes can reach millions.

Distinctive characteristics

Ribosomes usually consist of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized into the nucleolus and cross the nuclear membrane through nuclear pores. These two subunits come together when the ribosome attaches to messenger RNA (mRNA) during protein synthesis. Ribosomes, along with another RNA molecule, transfer RNA (tRNA), help convert protein-coding mRNAs into proteins. Ribosomes link amino acids together to form polypeptide chains, which are further modified before becoming functional proteins.

Location in the cage

There are two places where ribosomes typically exist: suspended in the cytosol (free ribosomes) and associated with the endoplasmic reticulum (bound ribosomes). In both cases, ribosomes typically form aggregates called polysomes or polyribosomes during protein synthesis. Polyribosomes are clusters of ribosomes that attach to an mRNA molecule during protein biosynthesis.

This allows you to synthesize several copies of the protein from one mRNA molecule at once. Free ribosomes typically produce proteins that function in the cytosol (the liquid component of the cytoplasm), while bound ribosomes typically synthesize proteins that are exported from the cell or incorporated into the cell.

Interestingly, free ribosomes and bound ribosomes are interchangeable, and the cell can change their number according to metabolic needs.

Organelles, like those in eukaryotic organisms, have their own ribosomes, which are more similar to the ribosomes found in bacteria. The subunits containing ribosomes in mitochondria and chloroplasts are smaller (30S - 50S) than the subunits found throughout the rest of the cell (40S - 60S).

Protein synthesis occurs under the influence of transcription and translation processes. In transcription, the genetic code contained in DNA is transcribed into an RNA version of the code, known as messenger RNA (mRNA). Translation produces a growing amino acid chain, also called a polypeptide chain. Ribosomes help transform mRNA and link amino acids together to produce a polypeptide chain that eventually becomes a fully functioning protein. Proteins are very important biological polymers in our cells as they are involved in almost every function.

All living organisms are characterized bystrictly ordered structure. This orderlinessdetermined by genetic information recordedin each organism in the form of a specific and strictspecific sequence of DNA nucleotides.In prokaryotes, hereditary information isin the nuclear substance (bacterial chromosome), and in eucaryotov - in the core. It is the core, due to the presence in itDNA is the information center of eukaryotestic cell, the place of storage and reproduction of hereditary information, which determines everythingcharacteristics of a given cell and the organism as a whole and serves as a control center for metabolism in the cell.

The nucleus is the most important organelle of the cell. Most cells have one nucleus. Often the cage containstwo or three (for example, in liver cells) or more nuclei.The shape of the nucleus is spherical, lenticular, ver.shadow-shaped or multi-lobed.

The nucleus is separated from the cytoplasm by a nuclear envelope consisting of two membranes. The space between the membranes is called perinuclear. The outer membrane passes directly into the endoplasmic reticulum. Exchange substances between the nucleus and the cytoplasm occurs in two main ways. Firstly, the nuclear envelope is penetrated by numerous pores through which molecules are exchanged between the nucleus and the cytoplasm. Secondly, substances from the nucleus into the cytoplasm and back can enter through the release of protrusions and outgrowths of the nuclear membrane.

The internal contents of the nucleus are divided into karyoplasm (nuclear juice), chromatin and nucleolus.

Karyoplasmis represented by a gel-like matrix (RNA, proteins, free nucleotides and other substances), in which chromatin and one or more nucleoli are located.

Chromatinrepresents DNA molecules associated with proteins. It can be in the form of thin threads that are indistinguishable under a light microscope (euchromatin) and in the form of clumps lying mainly along the periphery of the nucleus (heterochromatin). Different degrees of condensation (spiralization) of chromatin are due todifferent genetic activity of those located in it sections of DNA.

Nucleolus- a dense round body, not limited by a membrane. The number of nucleoli in the nucleus ranges from one to five, seven or more. The nucleolus does not showwith an independent core structure. It is formedaround the region of the chromosome in which it is encodedinformation about rRNA structure. This area is lamesoma is called nucleolar organizer On himrRNA synthesis occurs. In addition to rRNA in the nucleolusribosomal subunits are formed (rRNA connectswith protein molecules).Thus, the nucleolus is an accumulation of rRNA and ribosomal subunits at different stages of formation, which is based on a section of the chromosome - the nucleolar organizer.Main functions the kernels are:

1) storage of genetic information and its transferdaughter cells during the process of division;

2) control of cell metabolism by determining which proteins should be synthesized, at what time and in what quantities. This is done through the synthesis of mRNA and the implementation of genetic information during translation.

All cells that have nuclei are calledeukaryoteslogical,and organisms with such cells -eukaryotes.These include plants, animals, protists and mushrooms.

Ribosomes (Fig. 1) are present in the cells of both eukaryotes and prokaryotes, since they perform an important function in biosynthesis of proteins. Each cell contains tens, hundreds of thousands (up to several millions) of these small round organelles. It is a round ribonucleoprotein particle. Its diameter is 20-30 nm. The ribosome consists of large and small subunits, which are combined in the presence of a strand of m-RNA (messenger, or information, RNA). A complex of a group of ribosomes united by one m-RNA molecule like a string of beads is called polysome. These structures are either freely located in the cytoplasm or attached to the membranes of granular EPS (in both cases, protein synthesis actively occurs on them).

Fig.1. Diagram of the structure of a ribosome sitting on the membrane of the endoplasmic reticulum: 1 - small subunit; 2 mRNA; 3 - aminoacyl-tRNA; 4 - amino acid; 5 - large subunit; 6 - - membrane of the endoplasmic reticulum; 7 - synthesized polypeptide chain

Polysomes of granular EPS form proteins that are excreted from the cell and used for the needs of the whole organism (for example, digestive enzymes, proteins in human breast milk). In addition, ribosomes are present on the inner surface of mitochondrial membranes, where they also take an active part in the synthesis of protein molecules.

Ribosomes, intracellular particles that carry out protein biosynthesis

In the process of functioning (i.e. protein synthesis)
Ribosomes perform several functions:

1) specific binding and retention of components of the protein synthesizing system [information, or template, RNA (mRNA): aminoacyl-tRNA; peptidyl-tRNA; guanosine triphosphate (GTP); protein translation factors EF - T and EF - G]:

2) catalytic functions (formation of peptide bonds, hydrolysis of GTP): 3) functions of mechanical movement of substrates (mRNA, tRNA), or translocation. The functions of binding (retention) of components and catalysis are distributed between two ribosomal subunits. The small ribosomal subunit contains sites for binding mRNA and aminoacyl-tRNA and, apparently, does not have catalytic functions. The large subparticle contains a catalytic site for the synthesis of the peptide bond, as well as a center involved in the hydrolysis of GTP: in addition, during protein biosynthesis, it holds the growing protein chain in the form of peptidyl-tRNA.

Each of the subunits can exhibit the functions associated with it separately, without connection with another subparticle. However, none of the subparticles individually has the function of translocation, which is carried out only by the complete Ribosome

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The ribosome is an elementary cellular machine for the synthesis of any cell proteins. All of them are built in the same way in the cell, have the same molecular composition, perform the same function - protein synthesis - therefore they can also be considered cellular organelles. Unlike other organelles of the cytoplasm (plastids, mitochondria, cell center, membrane vacuolar system, etc.), they are represented in a cell in a huge number: 1 x 107 of them are formed per cell cycle. Therefore, the bulk of cellular RNA is ribosomal RNA. Ribosomal RNA is relatively stable, and ribosomes can exist in tissue culture cells for several cell cycles. In liver cells, the half-life of ribosomes is 50-120 hours.

Ribosomes are complex ribonucleoprotein particles, which include many molecules of individual (non-repeated) proteins and several RNA molecules. Ribosomes of prokaryotes and eukaryotes differ in size and molecular characteristics, although they have common principles of organization and functioning. To date, the structure of ribosomes has been completely deciphered using high-resolution X-ray diffraction analysis.

A complete, working ribosome consists of two unequal subunits, which are easily reversibly dissociated into a large subunit and a small one. The size of a complete prokaryotic ribosome is 20 x 17 x 17 nm, eukaryotic - 25 x 20 x 20. A complete prokaryotic ribosome has a sedimentation coefficient of 70S and dissociates into two subunits: 50S and 30S. The complete eukaryotic ribosome, the 80S ribosome, dissociates into 60S and 40S subunits. The shape and detailed outline of ribosomes from a variety of organisms and cells, including both prokaryotic and eukaryotic ones, are strikingly similar, although they differ in a number of details. The small ribosomal subunit is rod-shaped with several small protrusions (see Fig. 81), its length is about 23 nm and its width is 12 nm. The large subunit looks like a hemisphere with three protruding protrusions. When associated into a complete 70S ribosome, the small subparticle rests with one end on one of the protrusions of the 50S particle and the other in its groove. The small subunits contain one RNA molecule, and the large subunits contain several: in prokaryotes - two, and in eukaryotes - 3 molecules. Characteristics of the molecular composition of ribosomes are given in Table 9.

Table 9. Molecular characteristics of ribosomes

Thus, the eukaryotic ribosome includes four RNA molecules of different lengths: 28S RNA contains 5000 nucleotides, 18SRNA – 2000, 5.8S RNA – 160, 5SRNA – 120. Ribosomal RNAs have a complex secondary and tertiary structure, forming complex loops and hairpins on complementary areas, which leads to self-packing, self-organization of these molecules into a body of complex shape. For example, the 18S RNA molecule itself under physiological ionic conditions forms a rod-shaped particle that determines the shape of the small ribosomal subunit.

Under the influence of low ionic forces, especially when magnesium ions are removed, dense ribosomal subunits can unfold into loose ribonucleoprotein strands, where clusters of individual proteins can be observed, but there are no regular structures, such as nucleosomes, because there are no groups of similar proteins: in the ribosome all 80 proteins are different.

In order for ribosomes to form, the presence of four types of ribosomal RNA in equimolar ratios and the presence of all ribosomal proteins are necessary. Ribosome assembly can occur spontaneously in vitro, if proteins are sequentially added to RNA in a certain sequence.

Therefore, the biosynthesis of ribosomes requires the synthesis of many special ribosomal proteins and 4 types of ribosomal RNA. Where this RNA is synthesized, on how many genes, where these genes are localized, how they are organized within the DNA of chromosomes - all these questions have been successfully resolved in recent decades by studying the structure and function of nucleoli.

Structure of ribosomes

Ribosomes are non-membrane organelles. They are very small (about 20 nm), but numerous (thousands and even millions per cell), consist of twoparts –subunits. The subparticles include ribosomal RNA (rRNA) and ribosomal proteins, i.e. ribosomes in chemical composition are ribonucleoproteinsdami. However, they also contain small amounts of low molecular weight compounds. Due to the large number of ribosomes, rRNA makes up more than half of the cell's total RNA.

One of the subunits is called “small”, the second – “large”.

In a ribosome assembled from subunits, two (according to some sources) or three (according to others) sections are distinguished, which are called sites. One of the sections is designated A (aminoacyl) and is called aminoacyl, the second - P (peptidyl) - peptidyl. These sites are the main catalytic centers of reactions occurring on ribosomes. The third section is designated E (exit), through which transfer RNA (tRNA), released from the synthesized polypeptide, leaves the ribosome.

In addition to the listed sites on ribosomes, there are other sites used for binding various enzymes.

When the subunits are dissociated (separated), site specificity is lost, i.e. they are determined by a combination of the corresponding regions of both subunits.

Differences between ribosomes of prokaryotes and eukaryotes

The ratio of the mass of proteins and RNA in the ribosome is approximately equal. However, prokaryotes have fewer proteins (about 40%).

The sizes of both the ribosomes themselves and the subunits are expressed in the rate of their sedimentation (sedimentation) during centrifugation. In this case, S denotes the Svedberg constant - a unit that characterizes the rate of sedimentation in a centrifuge (the larger S, the faster the particle settles, and therefore heavier). In prokaryotes, ribosomes are 70S in size, while in eukaryotes they are 80S in size (i.e., they are heavier and larger). In this case, the subunits of prokaryotic ribosomes have values of 30S and 50S, and of eukaryotic ones - 40S and 60S. The sizes of ribosomes in mitochondria and chloroplasts of eukaryotes are similar to prokaryotes (although they have a certain variability in size), which may indicate their origin from ancient prokaryotic organisms.

In prokaryotes, the large ribosomal subunit contains two rRNA molecules and more than 30 protein molecules, and the small ribosomal subunit contains one rRNA molecule and about 20 proteins. Eukaryotes have more protein molecules in their subunits, and also have three rRNA molecules in their large subunit. The proteins and rRNA molecules that make up the ribosome have the ability to self-assemble and ultimately form a complex three-dimensional structure. The rRNA structure is supported by magnesium ions.

rRNA synthesis

In eukaryotes, ribosomes contain 4 types of rRNA. In this case, three are formed from one precursor transcript - 45S rRNA. It is synthesized in the nucleolus (on the chromosome loops that form it) using RNA polymerase-1. rRNA genes have many copies (tens or hundreds) and are usually located at the ends of different pairs of chromosomes. After synthesis, 45S rRNA is cut into 18S, 5.8S and 28S rRNA, each of which undergoes various modifications.

The fourth type of rRNA is synthesized outside the nucleolus using the enzyme RNA polymerase-3. This is 5S RNA, which after synthesis does not require processing.

The tertiary structure of rRNA within ribosomes is very complex and compact.

Differences between prokaryotes and eukaryotes

It serves as a scaffold for housing ribosomal proteins, which perform auxiliary functions to maintain structure and functionality.

Ribosome function

Functionally, ribosomes are the binding site for molecules involved in synthesis (mRNA, tRNA, various factors). It is in the ribosome that molecules can occupy a position relative to each other that allows a chemical reaction to occur quickly.

In eukaryotic cells, ribosomes can be free in the cytoplasm or attached with the help of special proteins to the EPS (endoplasmic reticulum, also known as ER - endoplasmic reticulum).

During translation, the ribosome moves along the mRNA. Often several (or many) ribosomes move along one stranded mRNA, forming the so-called polysome(polyribosome).

Functional centers of the ribosome (A-site, P-site, PTF-site, M-site, E-site)

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Types of ribosomes. The structure of the ribosome, small and large subunits. The composition of the subunits is ribosomal RNA, ribosomal proteins.

Complete ribosomal particles and their subunits are designated according to the sedimentation coefficient, expressed in Svedberg units.

All prokaryotes have 70S ribosomes. The ratio of protein to RNA is 2:1. Consists of two subunits: 50S and 30S. Each contains rRNA and a certain number of small proteins. In E. coli, the small subunit consists of 1 rRNA (16S) and 21 ribosomal proteins (S1, S2, S3, etc.). The large subunit contains 2 rRNA (23S, 5S) and 31 proteins (L1, L2, L3, etc.). The complete ribosome has an asymmetric structure. There are 4 sections on the small one: head, neck, body and base/platform. In the large one, the central protrusion/proturbanium is clearly distinguishable, which contains 5S rRNA, the main mass, which contains 5S rRNA. there is protein L7, and the groove is located between them, in the cat. there is a peptidyl transferase site. A cavity is formed between the large and small subunits, in which most of the active sites of the ribosome open.

Eukaryotes have 80S ribosomes. Have more rRNA and proteins. Their ratio is 1:1. They consist of a small (40S) and a large (60S) subunit. The small one contains 18SpRNA and 33 ribosomal proteins. Large - 3 rRNA chains (5S, 5.8S, 28S) and 45-50 proteins.

Ribosomes organelles differ from cytompasmatic ones.

2.2. Ribosomes of prokaryotes and eukaryotes

Functional centers of the ribosome (A-site, P-site, PTF-site, M-site, E-site).

The ribosome is a cooperative structure that depends on the interaction of its active sites. Site A is involved in the binding of the next aminoacyl-tRNA; it contains an mRNA codon, which dictates to the ribosome the type of incoming aminoacyl-tRNA/next amino acid of the growing polypeptide. Site P – peptidyl-tRNA binding site – growing pettide, cat. connected at its C-terminus to tRNA, cat. brought the last amino acid residue to the ribosome. Site E is the site where tRNA exits the ribosome. Deacylated tRNA is retained by site E for a short time. Ueukaryotes do not have this site; they go directly from the P-site into the cytoplasm. The catalytic site of peptidyl transferase is located on the border of the A and P sites and catalyzes the formation of a peptide bond. GTPase center - the site of GTP landing, promotes the initiation of ATP hydrolysis

Ribosome biosynthesis, stages of rRNA processing. Chemical modifications of rRNA. Features of the structure and maturation of the eukaryotic ribosome.

rRNA processing: cutting of the primary transcript, methylation, splicing. In eukaryotes, all rRNAs are synthesized as part of a single transcript. It is cut into mature rRNA by exo and endonucleases. The precursor contains 18, 5.8, 28S rRNA and is called 45S RNA. Processing of rRNA requires the participation of snRNA. In some organisms, the 28S RNA precursor contains inserts/intrans, cat. are removed as a result of processing and RNA fragments are stitched together as a result of splicing.

Uprokaryotic rRNA precursor contains 16, 23, 5SrRNA + several tRNA precursors. The 3 and 5' ends are brought closer together due to complementary adjacent base pairs. This structure is cut by RNaseIII. The remaining ribonucleotides are cut off by exonucleases/trimming.

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Cells of bacteria, blue-green algae and actinomycetes contain ribosomes with a sedimentation coefficient of 70S. This coefficient is a measure of the relative buoyant density of particles when they are centrifuged in a density gradient of cesium chloride or sucrose. The unit of buoyant density S (svedberg) is named after the inventor of the ultracentrifuge, the Swedish scientist T. Svedberg. The sedimentation coefficient depends on both the mass and the shape of the particle. The molecular weight of prokaryotic ribosomes is 2.5 mD, round in shape with an average diameter of 25 nm. The total number of ribosomes in a bacterial cell reaches 30% of its dry weight. The relative amount of protein in them is two times less than RNA.

Ribosomes of the prokaryotic type with a sedimentation coefficient of 70S are also found in the chloroplasts of higher plants. However, mitochondrial ribosomes, although similar to bacterial ones, have a higher species specificity. In particular, yeast mitochondrial ribosomes are somewhat larger than typical prokaryotic ribosomes (75S), while mammalian mitochondrial ribosomes, on the contrary, are much smaller than bacterial ones (55S).

Cells of animals, plants, fungi and protozoa contain ribosomes with a sedimentation coefficient of 80S. Their molecular weight is 4 mD and their average diameter is 30 nm. The relative amount of protein in them is approximately equal to the amount of RNA. The eukaryotic type of ribosomes has no species differences.

Morphology of ribosomes

At low magnification of an electron microscope (up to 20,000x), ribosomes look like electron-dense round particles with a diameter of 25-30 nm. At high magnification (above 100,000x) it is clear that they are divided by a groove into two unequal parts, representing small and large subunits with a mass ratio of 1:2.

Under physiological conditions, ribosomes reversibly dissociate into subunits. In this case, prokaryotic ribosomes dissociate according to the following scheme:

70S<=>30S + 50S,

whereas eukaryotic ribosomes dissociate according to the scheme:

80S<=>40S+60S

The sedimentation coefficient deficiency is due to the fact that the buoyant density of ribosomes depends not only on the mass of the subunits, but also on their shape.

Small subunit The prokaryotic ribosome 30S has an oblong shape, its length is 23 nm and its width is 12 nm. It is divided into lobes called the “head”, “body” and “lateral protrusion”. The most pronounced is the transverse groove, which separates the head and body. The small subunit of the eukaryotic ribosome 40S is similar to the small prokaryotic subunit 30S, but has two additional features - a protrusion of the head on the side opposite the lateral protrusion of the body, as well as a bifurcation of the distal end of the body.

Large subunit The prokaryotic 50S ribosome, with a diameter of 25 nm, is externally identical to the large subunit of the eukaryotic 60S ribosome. The large subunit has three projections: the middle projection or “head,” the lateral lobe or “handle,” and the rod-shaped process or “spout.” In general, the shape of the large subunit resembles a teapot.

The combination of subunits into a complete ribosome occurs in a strictly regular manner. In this case, the heads and lateral protrusions of the small and large subunits are oriented in one direction and overlap each other. The flattened surfaces of the subunits also complement each other in space.

Chemical composition of ribosomes

The ribosome consists of RNA and proteins, and the main structural and functional properties of this organelle are determined by ribosomal RNA.

Prokaryotic ribosomes contain three, and eukaryotic ribosomes contain four molecules of ribosomal RNA.

Ribosomal RNA

Small subunit RNA with sedimentation coefficients 16S and 18S has from 1500 to 1800 nucleotide residues. It has significant internal complementarity, due to which about three dozen short double-helix sections - “hairpins” are formed, which determine the shape of the small subparticle.

A long RNA molecule of the large subunit with a sedimentation coefficient of 18S or 26S contains from 3000 to 4800 nucleotide residues. Due to internal complementarity, more than 100 double helices are formed in it, which determine the shape of the subunit.

In addition to long RNA, the large subunit of prokaryotic and eukaryotic ribosomes also contains short 5S RNA, consisting of 120 nucleotide residues, which, due to internal complementarity, forms a T-shaped structure with 5 helical regions.

The large subunit of eukaryotic ribosomes contains an additional 5.8S RNA.

Ribosomes of prokaryotes and eukaryotes

It consists of 160 nucleotide residues and is complementarily associated with 26S RNA. It should be noted that the 5.8S RNA of the large subunit of eukaryotic ribosomes is homologous to the 5’ end of the bacterial 23S RNA.

Thus, the main function of ribosomal RNAs is to form the molecular skeleton of the small and large subunits of the ribosome.

Ribosomes contain 50-70 different proteins, most of them represented by only one molecule. The molecular weight of ribosomal proteins ranges from 10-30 kDa, although individual polypeptides reach a mass of 70 kDa. Among ribosomal proteins, basic polypeptides predominate, but neutral and acidic proteins are also found. The small subunit of the prokaryotic ribosome contains 20 proteins, and the large subunit contains 30 proteins. Eukaryotic ribosomes have significantly more proteins: the small subunit contains 30 proteins, and the large subunit contains 40.

Ribosomal proteins perform various functions related to the role of the ribosome as the organizer of protein biosynthesis:

form sections of small and large subunits;
form molecular binding centers;
catalyze chemical reactions;
participate in the regulation of protein biosynthesis;

Many ribosomal proteins perform several functions simultaneously.

Protein synthesis system

Hereditary information is encoded in the primary structure of DNA, which in eukaryotic cells is concentrated in the cell nucleus. Regions of DNA encoding the primary structure of a polypeptide—structural genes—are templates for the synthesis of messenger RNA (mRNA). The process of producing functional copies of genes in the form of mRNA is called transcription.

The mRNAs edited during splicing then enter the cytoplasm, where they bind to ribosomes. Using information encoded in mRNA, ribosomes synthesize the polypeptide in a process called broadcast. Synthesis of polypeptide from amino acids is carried out in accordance with genetic code, which represents the rules for matching amino acids to nucleotide triplets in mRNA ( codons).

In addition to mRNA and ribosomes, a number of other molecules are required for translation to occur. Ribosomes, together with molecules involved in translation, form protein synthesizing system, which can function outside the cell. The compositions of minimal and complete cell-free translation systems on prokaryotic ribosomes are presented in the following table.

Ribosomes of eukaryotes and prokaryotes, similarities and differences

Ribosome (from “RNA” and soma - body) is a cellular non-membrane organelle that carries out translation (reading the mRNA code and synthesizing polypeptides). A protein molecule is born in the cytoplasm of a cell on free ribosomes or on tanks of the transport-storage system. Special chaperone proteins arrange the growing chain into an openwork structure. Then, if necessary, the protein is completed. There are 2 main types of ribosomes - prokaryotic and eukaryotic. Mitochondria and chloroplasts also contain ribosomes, which are similar to the ribosomes of prokaryotes. 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.

Ribosomes of prokaryotes

In prokaryotes, the genetic material is not isolated from the translation apparatus, and prokaryotic ribosomes occupy almost the entire cytoplasmic compartment. The relative (compared to other organelles) number of ribosomes in prokaryotes is higher than in eukaryotes, and this ensures a higher activity of their metabolism, as well as a higher rate of their growth and reproduction. Ribosomes are multiple general microcompartments that are located in the cytoplasmic compartment and act as a universal protein-synthesizing organelle. Except in rare cases where polypeptides are synthesized by a non-ribosomal route, amino acids are linked into a linear chain only by the enzymatic activity of ribosomes.

The biosynthetic process of translation is unique because information about the order of nucleotide triplets (mRNA code) is translated into information about the order of amino acids (polypeptide code). The intermediary between these codes, or “adapter” (English adapter - audio system pickup) is tRNA. It delivers the amino acid to the peptidyl transferase center and at the same time recognizes its codon in the mRNA molecule.

Types of ribosomes. A ribosome is a multimolecular complex consisting of rRNA and ribosomal proteins in a mass ratio of 2:1. In working condition, a ribosome, or “monosome,” is a particle with a diameter of 25 nm, which consists of two subunits - a large L-subunit (large) and a small S-subunit (small). They have different compositions, different morphologies and perform different functions.

Based on quantitative characteristics, all ribosomes are divided into two types - prokaryotic and eukaryotic. The prokaryotic ribosome has a sedimentation coefficient of 70S (50S and 30S subunits), and the eukaryotic ribosome has a sedimentation coefficient of 80S (60S and 40S subunits). The prokaryotic ribosome contains three rRNA molecules - 23S (~3000 nucleotides), 16S (~1500 nucleotides) and 5S (~120 nucleotides), as well as 53-65 single-copy proteins. The eukaryotic ribosome is more complex than the prokaryotic one. It contains not three, but four rRNA molecules -28S (4000-6000 nucleotides), 18S (1750-1850 nucleotides), 5S (~120 nucleotides) and 5.8S (~150 nucleotides), as well as a richer set of single-copy proteins ( 70-84).

Translation mechanism.

Although modern ideas about the architecture of ribosomes and the translation process have developed on the basis of data obtained in bacteria, it has been proven that the work of the ribosome is universal in all three global domains. The results of X-ray structural analysis with a resolution level of 5.5A and cryoelectron microscopy gave the E. coli ribosome a picture of a geometric body of a complex configuration, consisting of mutually intertwined rRNA and protein molecules. Inside it, as well as on its surface, there are channels, grooves, depressions, platforms, protrusions and bridges.

Aminoacyl-tRNA serves as substrates for the biosynthesis of the polypeptide chain, and for each amino acid there is its own tRNA and its own aminoacyl-tRNA synthetase. Specific tRNAs (~75 nucleotides) vary in primary structure, but they all have a standard L-shaped tertiary structure. At the distal end of the long “elbow” there is an anticodon, complementary to the mRNA triplet, which encodes a specific amino acid. At the distal end of the short “elbow” of all tRNAs there is a 3′-terminal sequence CCA. The α-carboxyl group of a specific amino acid is attached to adenosine (at its 2′- or 3′-hydroxyl radical). During translation, the anticodon of the long “elbow” recognizes the mRNA codon on the S-subunit, and the short “elbow” with the amino acid interacts on the L-subunit with the peptidyltransferase center, which catalyzes the formation of a peptide bond.

For a long time it was believed that translation is provided by ribosomal proteins, and rRNA serves only as a framework for their assembly. However, it has now been proven that rRNA plays the role of the main translation catalyst, and proteins perform a structural function.

There is a division of labor between the ribosomal subunits. The small subunit contains a decoding center that mediates the interaction between mRNA and tRNA. The large subunit contains a peptidyl transferase center. The organization of the decoding center involves 16S rRNA and ribosomal proteins, while the peptidyl transferase center is formed only by 23S rRNA. The Shine-Dalgarno sequence (J. Shine, L. Dalgarno), preceding the start codon of the mRNA, pairs with a complementary sequence at the 3' end of the 16S rRNA. The anticodon end of tRNA also interacts with 16S rRNA, while the acceptor end of tRNA interacts with 23S rRNA.

The ribosome forms peptide bonds step by step in the direction from the N-terminus to the C-terminus. To initiate the polypeptide chain in bacteria, a special amino acid is used - formylmethionine, which is delivered to the ribosome using a specific tRNA. A spontaneous peptidyl transferase reaction occurs: the nucleophilic α-amino group of aminoacyl-tRNA attacks the electrophilic carbonyl group (*) in the ester bond between the peptide (or formyl-methionine primer) and another tRNA. The 16S rRNA and 23S rRNA molecules form three sites - P, A and E, each of which is represented by subsites in both subunits of the ribosome. The P-site (from the English peptide) binds peptidyl-tRNA, the A-site (from the English amino acid) binds aminoacyl-tRNA, and the E-site (from the English exit) binds deacylated tRNA.

The ribosome's work cycle consists of four stages, or states.

1. In the initial state P/P-A/A, peptidyl-tRNA is in the P-site, aminoacyl-tRNA is in the A-site, and the E-site is free. A specific aminoacyl-tRNA binds to the A site using the elongation factor EF-Tu. For this purpose, the energy of GTP hydrolysis is used. The ternary aminoacyl-tRNA/(EG-Ti) × GTP complex binds tightly to the ribosome only if the anticodon is complementary to the codon in the decoding subsite A.

2. In the “pre-translocation” state of P/P-A/A, a peptidyl transferase reaction occurs. In this case, the amino acid located in the A-site forms a bond with the peptide (or formylmethionine, if the chain is initiated), which is located in the P-site. In both cases, a dipeptide or polypeptide chain lengthened by one amino acid residue is transferred to the A site. In order for the next aminoacyl-tRNA molecule to enter the A-site, the peptidyl-tRNA must release it and move to the P-site. This process is called "translocation". During translocation, tRNA and mRNA interacting with each other move within the ribosome over a distance of up to 50A.

3. In the “hybrid translocation” state E/P-P/A, the peptide-bound end of the tRNA moves on the large subunit from the A-subsite to the P-subsite, and the acceptor CCA end of the deacylated tRNA moves from the P-subsite to the E-subsite. This translocation step resembles a “domino effect” and is dependent on rRNA. Its mechanism is still unknown.

4. In the “homogeneous translocation” state E/E-P/P, the anticodon end of the tRNA associated with the peptide moves on the small subunit from the A-subsite to its P-subsite, and the anticodon end of the deacylated tRNA moves from the P-subsite to E -subsite. As a result, the mRNA shifts one codon in the small subunit. The A site can now accept the next aminoacyl-tRNA molecule, and the deacylated tRNA molecule can leave the ribosome. Although this translocation step is rRNA dependent, it is accelerated by the elongation factor EF-G, which harnesses the energy from GTP hydrolysis.

The action of many antibiotics (kanamycin, neomycin, oleandomycin, streptomycin, tetracycline, chloramphenicol, etc.) is based on their binding to elongation factors and sites that rRNA forms.

Structure of eukaryotic ribosomes

Ribosomes are made up of two different subunits, each made up of ribosomal RNA and many proteins. Ribosomes and their subparticles are usually classified not by mass, but in accordance with sedimentation coefficients. So. The sedimentation coefficient of the complete eukaryotic ribosome is about 80 Svedberg units (80S), and the sedimentation coefficient of its subunits is 40S and 60S.

The smaller 40S subunit consists of one 18S rRNA molecule and 30-40 protein molecules. The large 60S subunit contains three types of rRNA with sedimentation coefficients of 5S, 5.8S and 28S and 40-50 proteins (for example, rat hepatocyte ribosomes include 49 proteins). In the presence of mRNA (mRNA), the subparticles combine to form a complete ribosome, which weighs approximately 650 times the mass of a hemoglobin molecule. Ribosomes have a diameter of 20-200 nm and can be seen with an electron microscope. The structural organization of ribosomes has not been fully elucidated. However, it is known that the mRNA molecule passes through a gap near the characteristic “horn”-shaped structure on the small subparticle, and this gap is oriented precisely into the gap between the two subparticles. tRNAs also bind near this site. For comparison, the diagram shows a tRNA molecule on the same scale.

In eukaryotic cells, ribosomes are formed in the nucleolus, where r-RNA is synthesized on DNA, to which proteins are then attached. Subparticles of the ribosome leave the nucleus into the cytoplasm, and here the formation of full-fledged ribosomes is completed. In the cytoplasm, ribosomes are free in the cytoplasmic matrix (hyaloplasm) or attached to the outer membranes of the nucleus and the endoplasmic reticulum. Free ribosomes synthesize proteins for the internal needs of the cell. Ribosomes on membranes form complexes - polyribosomes, which synthesize proteins that enter the Golgi apparatus through the endoplasmic reticulum and are then secreted by the cell. The number of ribosomes in a cell depends on the intensity of protein biosynthesis - there are more of them in the cells of actively growing tissues (plant meristems, embryos, etc.). Chloroplasts and mitochondria have their own small ribosomes; they provide these organelles with autonomous (nucleus-independent) protein biosynthesis.

Each ribosome consists of two subparticles - large and small. Ribosomes consist of approximately equal (by mass) amounts of RNA and protein (i.e., they are ribonucleoprotein particles). The RNA they contain, called ribosomal RNA (rRNA), is synthesized in the nucleolus.

Together, both form a complex three-dimensional structure that has the ability to self-assemble. During protein synthesis on ribosomes, the amino acids from which the polypeptide chain is built are added one after another to the growing chain. The ribosome serves as a binding site for molecules involved in synthesis, i.e., a place where these molecules can take a very specific position in relation to each other.

The synthesis involves: messenger RNA (mRNA), which carries genetic instructions from the cell nucleus, transport RNA (tRNA), which delivers the required amino acids to the ribosome, a growing polypeptide chain, as well as a number of factors responsible for the initiation, elongation and termination of the chain. In eukaryotic cells, two populations of ribosomes are clearly visible - free ribosomes and ribosomes attached to the endoplasmic reticulum. The structure of both is identical, but some of the ribosomes are connected to the endoplasmic reticulum through the proteins that they synthesize. Such proteins are usually secreted. An example of a protein synthesized by free ribosomes is hemoglobin, which is formed in young red blood cells. During protein synthesis, the ribosome moves along the thread-like mRNA molecule. The process is more efficient when not just one ribosome moves along the mRNA, but many ribosomes at the same time, resembling beads on a string in this case. Such chains of ribosomes are called polyribosomes or polysomes. On the endoplasmic reticulum, polysomes are found in the form of characteristic curls.

Ribosomal protein synthesis is a multistep process. The first stage (initiation) begins with the attachment of messenger RNA (mRNA) to the small ribosomal subunit, which is not associated with the large subunit. It is characteristic that a dissociated ribosome is required to begin the process. To the resulting so-called a large ribosomal subunit is attached to the initiation complex. Specialists participate in the initiation stage. initiation codon (see Genetic code), initiation transfer RNA (tRNA) and specific. proteins (so-called initiation factors). Having passed the initiation stage, the ribosome moves on to the sequence. reading the codons of mRNA in the direction from the 5′ to the 3′ end, which is accompanied by the synthesis of the polypeptide chain of the protein encoded by this mRNA. In this process, the ribosome functions as a cyclically working mol. car.

The working cycle of the ribosome during elongation consists of three cycles: 1) codon-dependent binding of aminoacyl-tRNA (supplies amino acids to the ribosome), 2) transpeptidation - transfer of the C-terminus of the growing peptide to aminoacyl-tRNA, i.e. elongation of the protein chain under construction by one link, 3) translocation-movement of the matrix (mRNA) and peptidyl-tRNA relative to the ribosome and the transition of the ribosome to its original state, when it can perceive the trace. aminoacyl-tRNA. When the ribosome reaches the special stop codon of the mRNA, polypeptide synthesis stops. With the participation of specific proteins (so-called termination factors) synthesized. the polypeptide is released from the ribosome. After termination, the ribosome can repeat the entire cycle with another strand of mRNA or another coding sequence of the same strand.

In cells with intense protein secretion and developed endoplasmic. reticulum means. part of the cytoplasmic ribosome is attached to its membrane on the surface facing the cytoplasm. These ribosomes synthesize polypeptides that are directly transported across the membrane for further secretion. The synthesis of polypeptides for intracellular needs occurs mainly on free (not membrane-bound) ribosomes in the cytoplasm. In this case, translating ribosomes are not evenly dispersed in the cytoplasm, but are collected in groups. Such ribosome aggregates are structures where the mRNA is associated with many ribosomes that are in the process of translation; these structures are called polyribosome or polysome.

With intensive protein synthesis, the distance between ribosomes along the mRNA chain in a polyribosome can be extremely short, i.e. ribosomes are located almost close to each other. The ribosomes included in polyribosomes work independently and each of them synthesizes a complete polypeptide chain.

Differences in the structure of ribosomes between prokaryotes and eukaryotes

A prokaryotic cell contains several thousand ribosomes, while a eukaryotic cell contains tens of times more. Ribosomes of prokaryotes and eukaryotes differ in size (in prokaryotes they are smaller than in eukaryotes), but the principle of their structure is the same. Ribosomes consist of two parts: large and small subunits. In addition to proteins, they contain RNA. These RNAs are called ribosomal RNAs, rRNAs.

The size of ribosomes and their constituent parts is usually indicated in special units - S (Svedberg). S is the sedimentation coefficient, which characterizes the speed of movement of molecules or particles in a centrifugal field during centrifugation. The speed of movement depends on the mass of the particles, their size and shape. The size of ribosomes in prokaryotes and eukaryotes is 70S and 80S, respectively.

Prokaryotic ribosomes contain three different types of rRNA molecules (16S rRNA in the small subunit; 23S rRNA and 5S rRNA in the large subunit) and 55 different proteins (21 in the small and 34 in the large subunit). Eukaryotic ribosomes include four types of rRNA molecules (18S rRNA - small; 28S rRNA, 5.8S rRNA and 5S rRNA - large subunits) and about 80 proteins. Ribosomes are also found in mitochondria and chloroplasts. They are characterized by the same properties and parameters as prokaryotic ribosomes.

rRNA molecules interact with each other and with proteins, forming compact structures - ribosomal subunits. In eukaryotes, the connection of rRNA with ribosomal proteins occurs in the nucleolus. In the center of the nucleolus there is a section of the chromosome in which the ribosomal RNA genes are located. The synthesized rRNAs are combined with ribosomal proteins, which entered through nuclear pores from the cytoplasm, where they were synthesized on pre-existing ribosomes. They combine with rRNA molecules to form ribosomal subunits. The finished subunits exit through the pores into the cytoplasm, where they will participate in protein synthesis.

Thus, the nucleolus is not only the site of ribosomal RNA synthesis, but also the site of assembly of ribosomal subunits. Ribosomes are needed in huge quantities, since protein synthesis processes are constantly going on in the cell. Therefore, on the chromosomes in those places where the rRNA genes are located, there is a huge accumulation of molecules: synthesized rRNA, ribosomal proteins coming from the cytoplasm, assembled and ready-made ribosomal subunits. It is clear why the nucleolus is the densest part of the nucleus and cell. The size of the nucleolus depends on the functional state of the cells. If protein biosynthesis processes are actively occurring in the cell, the nucleolus can occupy up to 25% of the volume of the nucleus.

The nucleolus is formed on those chromosomes where there are rRNA genes. These regions of chromosomes are called nucleolar organizers. For example, in humans, ten chromosomes are capable of forming nucleoli. Each nucleolar organizer is a huge chromatin loop, since it contains several tens and even hundreds of identical sequences - rRNA genes. These sequences are located one after another and rRNA synthesis occurs simultaneously from all copies. This increases the intensity of rRNA synthesis, which accounts for more than 90% of the total RNA in the cell. Nucleoli formed by different chromosomes very often merge with each other. In the nuclei of human cells, one, two or three nucleoli are usually observed.

At the beginning of translation, the small subunit of the ribosome binds to a certain region of mRNA, a tRNA with an amino acid attaches to them, and then the large subunit binds to this complex. After this, the ribosome is ready to perform its function - protein synthesis. Ribosomal proteins are able to perform their functions only as part of a ribosome; only in combination with rRNA and other ribosomal proteins do they acquire the required conformation.

Eukaryotic transcription is separated from translation in space and time. Transcription along with RNA processing occur in the nucleoplasm, and translation, depending on the cell type, occurs primarily in the cytosol or on the rough endoplasmic reticulum (RER). Integral proteins are integrated into the RER membrane cotranslationally, and secreted proteins are released into the cavity of the RER cistern through a toroidal adapter between the ribosome exit portal and the membrane translocon (it is formed by the Sec61 protein).

In prokaryotes, there is no spatiotemporal isolation of the processes of transcription and translation. Cytoplasmic ribosomes attach to the 5′ end of the mRNA before the completion of the short-lived transcript. Cotranslational insertion of integral proteins is known only from the example of “rough thylakoids” of cyanobacteria. Hydrophobic proteins are presented to the translocon, a component of the general secretion system Sec, using SRP particles.

Transfer RNA resembles an unfolded clover leaf. The amino acid is attached to the “cloverleaf petiole,” and at the top of the leaf there is a triplet that interacts with a codon in the mRNA—an anticodon. The role of the “capital letter” in the translation of the amino acid sequence in prokaryotes is performed by a modified form of the amino acid methionine - formylmethionine. It corresponds to the codon AUG. After completion of the synthesis of the polypeptide chain, formylmethionine is cleaved off and is absent from the finished protein. In the case when the AUG triplet is inside the gene, it encodes the unchanged amino acid methionine.

If the codon and anticodon are complementary to each other, then the ribosome moves relative to the mRNA, and the next codon becomes available for interaction with the next tRNA. The first amino acid is detached from the first tRNA and attached to the amino acid brought by the second tRNA. During the movement of the ribosome relative to the mRNA, the first tRNA free of an amino acid leaves the ribosome. The second tRNA remains connected to a peptide of two amino acid residues, and the third mRNA codon enters the ribosome to interact with the next tRNA, etc.

When one of the three triplets (UAA, UAG, UGA) appears in the ribosome, no tRNA can take the place opposite it, since there are no tRNAs with anticodons complementary to these sequences. The polypeptide chain has nothing to join and leaves the ribosome. Protein synthesis is complete. Thus, the ribosome connects translation participants in one place: mRNA and amino acids in complex with tRNA, while the RNA molecules are oriented relative to each other in such a way that codon-anticodon interaction becomes possible. The formation of a peptide bond is controlled by the correctness of the codon-anticodon interaction. The ribosome forms a peptide bond and moves relative to mRNA.

The messenger RNA molecule interacts not with one ribosome, but with several. Each ribosome goes all the way from the “head” codon to the termination codon, synthesizing one protein molecule. The more ribosomes pass along the mRNA, the more protein molecules will be synthesized. A messenger RNA molecule with several ribosomes looks like a string of beads and is called a polyribosome, or polysome.