Cellular conveyor belt during protein synthesis. Structures formed by the plasmalemma
- 1. OBJECTIVE OF THE LESSON: to study the structure of the interphase nucleus in fixed preparations. Consider the structural features of cell nuclei with different functional activities. The main components of the nucleus are: nuclear envelope (karyolemma), chromatin, nucleolus, nuclear juice. Under light microscopy, the nuclear envelope presents a clear line outlined from the side of the nucleus and cytoplasm. When considering the diagram of the ultramicroscopic structure of the nucleus, one should pay attention to the structural features of the karyolemma and the connection of its membranes with the endoplasmic reticulum of the cytoplasm. Understand the morphological characteristics of chromatin and its chemical composition. Chromatin in the nucleus can be in the form of clumps (condensed chromatin) or dispersed (dispersed chromatin). The different state of chromatin is an indicator of the biosynthetic activity of the cell. Cells that actively synthesize protein have a nucleus with dispersed chromatin and a well-developed nucleolus. In the nuclei of cells that do not synthesize protein, the chromatin is condensed, and the nucleoli are poorly visible.
- 2. Test questions: 1. Core. The concept of the interphase nucleus. Structural components of the nucleus according to light and electron microscopy: nuclear envelope, chromatin, nucleolus, nuclear juice. The importance and functions of the nucleus in the life of the cell. 2. Nuclear-cytoplasmic ratios in cells with different levels of metabolism. 3. Structure of the nuclear envelope in SM and EM. Molecular organization and functional value nuclear lamina. 4. Nuclear pore and nuclear pore complex. Participation in nuclear import and export of substances. 5. Chromatin of the interphase nucleus. Euchromatin and heterochromatin. Chromatin as an indicator of cell biosynthetic activity. 6. Molecular organization of DNA in chromosomes. Levels of chromatin folding. The role of histone proteins in ensuring the structure of chromatin and the implementation of genetic information. 7. Nucleolus. Structure of the nucleolus in SM and EM. Main components of the nucleolus. The role of the nucleolus in rRNA synthesis and ribosome formation. 8. Synthesis and transport of biopolymers in the cell. Cellular conveyor belt during protein synthesis. Morphological characteristics of a cell that synthesizes proteins. 9. Cellular conveyor during the synthesis of carbohydrates and lipids. Morphological characteristics of a cell that synthesizes carbohydrates and lipids.
- 3. Drug 1. Kernel structures. Ovary. Hematoxylin-eosin staining. Make it under low magnification general overview microslide, find a growing follicle with an egg. Under high magnification, find a large round cell—an egg—and examine the structure of the nucleus. Pay attention to the nuclear envelope, nucleolus, and chromatin state. Draw an egg cell and label the structures of the interphase nucleus. Study the electron diffraction pattern of the nucleus. Draw the structure of the karyolemma and nuclear pore complex.
- 4. Preparation 1. Kernel structures. Ovary. Egg. Hematoxylin-eosin staining
- 5. Specimen 2. Pancreas. Hematoxylin-eosin staining. A cell that synthesizes protein. Under low magnification, make a general overview of the microscopic specimen and locate the exocrine part of the pancreas. Under high magnification, examine one cell, paying attention to the presence of a nucleolus and euchromatin in the nucleus, note the basophilia of the cytoplasm in the basal part of the cell and oxyphilia in the apical part.
- 6. Specimen 2. Pancreas. Hematoxylin-eosin staining. Cells that synthesize proteins
- 7. Preparation 3. Liver. Glycogen in liver cells. CHIC reaction. A cell that synthesizes carbohydrates. Under low magnification, make a general overview of the microslide and find a group of hepatocytes. Under high magnification, examine red-violet glycogen clumps in the cytoplasm of the hepatocyte.
- 8. Preparation 3. Liver. Glycogen in liver cells. CHIC reaction. A cell that synthesizes carbohydrates.
- 9. Specimen 4. Lipid inclusions in liver cells. Staining with osmic acid. Cell that synthesizes lipids. Under low magnification, make a general overview of the microslide and find a group of hepatocytes. Under high magnification, examine the cytoplasm of the hepatocyte, paying attention to lipid droplets colored black.
- 10. Specimen 4. Lipid inclusions in liver cells. Staining with osmic acid. Cells that synthesize lipids.
Protein synthesis
The most important functions of the body: metabolism, development, growth, movement - are carried out by biochemical reactions involving proteins.
Therefore, proteins are continuously synthesized in cells: enzyme proteins, hormone proteins, contractile proteins, protective proteins.
The primary structure of a protein (the order of amino acids in a protein) is encoded in DNA molecules. Each triplet (a group of three adjacent nucleotides) encodes one specific amino acid out of twenty on a DNA strand.
The sequence of triplets on a strand of DNA represents the genetic code.
Knowing the sequence of triplets on a DNA strand, that is, the genetic code, it is possible to determine the sequence of amino acids in a protein.
To date, triplets for all twenty amino acids have been deciphered.
For example
The amino acid lysine is encoded on the DNA strand by the triplet TTT.
The amino acid tryptophan is encoded by the ACC triplet, etc.
Several different proteins can be encoded in one DNA molecule. The section of DNA on which a protein is encoded is called a gene.
DNA sections are separated from each other by special triplets, which are punctuation marks. They mark the beginning and end of protein synthesis.
Since DNA, which stores genetic information about the protein, does not directly participate in protein synthesis, is contained in the nucleus, and protein synthesis occurs in the cytoplasm on ribosomes, there is an intermediary - mRNA. mRNA reads the genetic information about a protein from a section of DNA and transfers this information from the DNA strand to the ribosome. mRNA is synthesized on a piece of DNA according to the principle of complementarity.
Opposite the nitrogenous base adenine (A) on the DNA strand is uracil.
(U) on the mRNA strand, opposite the nitrogenous base thymine (T) on the DNA strand is adenine (A) on mRNA, opposite the nitrogenous base guanine (G) on the DNA strand is cytazine (C).
The process of reading genetic information about a protein from a section of DNA by mRNA is called transcription. This process proceeds as template synthesis, since one of the DNA strands is a template.
Protein synthesis occurs on ribosomes. There is usually a group of ribosomes located on the mRNA strand. This group of ribosomes is called a polysome.
Ribosomes move along the mRNA strand from triplet to triplet.
Each triplet on the mRNA strand codes for one specific amino acid out of twenty amino acids.
Transfer RNAs attach specific amino acids (each tRNA attaches one specific amino acid) and bring them to the ribosomes.
In this case, the anticodon of each tRNA must be complementary to one of the triplets (codons) on the mRNA.
For example
The AGC anticodon on the tRNA must be complementary to the UGC codon on the mRNA strand. rRNA, together with enzyme proteins, is involved in connecting amino acids with each other, as a result of which a certain protein is synthesized on ribosomes.
This process is called translation.
Having reached the final site on the mRNA strand, the ribosomes separate from the RNA strand. The synthesized protein molecule has a primary structure. Then it acquires secondary, tertiary and quaternary structures.
A large number of enzymes take part in protein synthesis. Protein synthesis uses ATP energy.
The protein then enters the channels of the endoplasmic reticulum, in which it is transported to certain parts of the cell.
Ribosome - a minifactory for the production of proteins
One of the most complex processes carried out by living beings is, perhaps, the synthesis of proteins - the most important structural and functional “building blocks” of any organism. A true understanding of the molecular processes underlying it could shed light on incredibly long-standing events associated with the mystery of the origin of Life itself...
In all living organisms, from the simplest bacteria to humans, proteins are synthesized by special cellular devices called ribosomes. In these unique factories, a protein chain is formed from individual amino acids.
In cells that conduct intensive protein synthesis, there are a lot of ribosomes: for example, one bacterial cell contains about 10 thousand of these minifactories, constituting up to 30% of the total dry mass of the cell! The cells of higher organisms contain fewer ribosomes - their number depends on the type of tissue and the level of cell metabolism.
The ribosome synthesizes protein at an average rate of 10-20 amino acids per second. The accuracy of translation is extremely high - the erroneous inclusion of an “incorrect” amino acid residue in a protein chain averages one amino acid per 3 thousand units (with medium length human protein chain of 500 amino acid residues), i.e. only one error per six proteins.
About the genetic code
The program that specifies the sequence of amino acid residues in a protein is written in the genome of the cell: about half a century ago it was established that the amino acid sequences of all proteins are directly encoded in DNA using the so-called genetic code. According to this code, universal for all living organisms, each of the twenty existing amino acids has its own codon- a triple of nucleotides, which are the elementary units of the DNA chain. Any protein is encoded in DNA by a specific sequence of codons. This sequence is called genome.
How does this genetic information get to the ribosome? On a separate gene, as on a matrix, a chain of another information molecule is synthesized - ribonucleic acid acids (RNA). This gene copying process, called transcription, is carried out by special enzymes - RNA polymerases.
But the RNA obtained in this way is not yet a matrix for protein synthesis: certain “non-coding” pieces of the nucleotide sequence are cut out of it (the process splicing).
The accuracy of protein synthesis by the ribosome is extremely high - in humans the error is one in three thousand “wrong” amino acid residuesThe result is messenger RNA (mRNA), which is used by ribosomes as a program for protein synthesis. The synthesis itself, i.e. the translation of genetic information from the language of the nucleotide sequence of mRNA into the language of the amino acid sequence of a protein is called translation.
Decoding and synthesis
In eukaryotic cells, one mRNA is usually translated by many ribosomes at once, forming so-called polysomes, which can be clearly seen using electron microscopy, which allows magnification of tens of thousands of times.
How do amino acids, which are the building blocks for protein synthesis, enter the ribosome? Back in the 50s of the last century, special “carriers” that delivered amino acids to the ribosome were discovered - short ones (less than 80 nucleotides long) transport RNA (tRNA). A special enzyme attaches an amino acid to one end of the tRNA, and each amino acid corresponds to a strictly defined tRNA. Protein synthesis on the ribosome includes three main stages: beginning, elongation of the polypeptide chain, and termination.
The ribosome itself - one of the most complexly organized molecular machines of the cell - consists of two unequal parts, the so-called subparticles (small and large). It can be easily divided into parts by centrifugation at ultra-high speeds in special tubes with a sucrose solution, the concentration of which increases from top to bottom. Since the small subparticle is half the weight of the large one, they move from the top of the test tube to the bottom at different speeds.
The small subparticle is responsible for decoding genetic information. It consists of high molecular weight ribosomal RNA (rRNA) and several dozen proteins (about 20 in prokaryotes and more than 30 in eukaryotes).
IN cancer cells the level of some ribosomal proteins increases sharply. Possible reason- failures in the mechanisms of autoregulation of their productionThe large subunit responsible for the formation of peptide bonds between amino acid residues consists of several rRNAs: one high molecular weight and one (or two in the case of eukaryotes) low molecular weight, as well as several dozen proteins (more than 30 in prokaryotes and up to 50 in eukaryotes). The scale of ribosome activity can be judged at least by the fact that ribosomal RNA makes up about 80% of the total RNA of the cell, tRNA, which transports amino acids, is about 15%, while messenger RNA, which carries information about the protein sequence, is only 5%!
It should be noted that ribosomal proteins are endowed with many other, additional functions that can manifest themselves at different stages of cell life. For example, human ribosomal protein S3 - one of the key proteins of the mRNA binding center on the ribosome - is also involved in the “repair” of DNA damage (Kim et al., 1995), participates in apoptosis(programmed cell death) (Jung et al., 2004), and also protects heat shock protein from destruction (Kim et al., 2006).
In addition, excessively intense synthesis of certain ribosomal proteins may indicate the development of malignant transformation of the cell. For example, significant increases in the levels of five ribosomal proteins were found in colon tumor cells (Zhang et al., 1999). Recently, employees of the laboratory of structure and function of ribosomes of the Institute of Biology and Biochemistry of the Siberian Branch of the Russian Academy of Sciences discovered a new mechanism of autoregulation of the biosynthesis of ribosomal proteins in humans, based on the feedback principle. Uncontrolled synthesis of ribosomal proteins, characteristic of tumor cells, is probably caused by failures in this mechanism. Further research in this area is of particular interest not only for scientists, but also for doctors.
Works as a "ribozyme"
Surprisingly, despite the billions of years of evolution separating bacteria and humans, the secondary structure of their ribosomal RNAs differs little.
Until recently, little was known about how rRNA is folded into subparticles and how it interacts with ribosomal proteins. A revolutionary shift in understanding the structure of the ribosome at the molecular level occurred at the turn of the new millennium, when, using X-ray diffraction analysis, it was possible to decipher the structure of ribosomes in the simplest organisms and their model complexes with mRNA and tRNA at the level of individual atoms. This made it possible to understand the molecular mechanisms of decoding genetic information and the formation of bonds in a protein molecule.
It turned out that both the most important functional centers of the ribosome - both the decoding center on the small subparticle and the one responsible for the synthesis of the protein chain on the large subparticle - are formed not by proteins, but by ribosomal RNA. That is, the ribosome works like ribozymes - unusual enzymes consisting not of proteins, but of RNA.
Ribosomal proteins, however, also play an important role in the functioning of the ribosome. In the absence of these proteins, ribosomal RNAs are completely unable to decode genetic information or catalyze the formation of peptide bonds. Proteins provide the complex “laying” of rRNA in functional centers necessary for the work of the ribosome, serve as “transmitters” of changes in the spatial structure of the ribosome necessary during the work process, and also bind various molecules that affect the speed and accuracy of the protein synthesis process.
The working scheme of the protein cycle itself is, in principle, the same for the ribosomes of all living beings. However, it is still unknown to what extent the molecular mechanisms of ribosomes are similar in different organisms. There is a particularly lack of information about the structure of the functional centers of ribosomes in higher organisms, which have been studied much less well than the ribosomes of protozoa.
This is due to the fact that many of the methods successfully used to study ribosomes in prokaryotes turned out to be inapplicable to eukaryotes. Thus, from the ribosomes of higher organisms it is not possible to obtain crystals suitable for X-ray structural analysis, and their subparticles cannot be “assembled” in a test tube from a mixture of ribosomal proteins and rRNA, as is done in protozoa.
From the lowest to the highest
And yet, methods for obtaining information about the structure of the functional centers of ribosomes in higher organisms exist. One such method is the method chemical affinity crosslinking, developed 35 years ago in the Department of Biochemistry of the Scientific Research Institute of Organic Chemistry of the Siberian Branch of the USSR Academy of Sciences (now IKhBFM SB RAS) under the leadership of Academician D. G. Knorre.
The method is based on the use of short synthetic mRNAs that carry chemically active (“cross-linking”) groups in a selected position, which can be activated at the right time (for example, by irradiation with soft ultraviolet light).
This method is still the main one today for studying the structural and functional organization of ribosomes in higher organisms.
The advantage of this method is that a cross-linking group can be attached to almost any nucleotide residue of mRNA and, as a result, detailed information about its environment on the ribosome can be obtained. Using a set of short mRNAs with different locations of the cross-linking group, we were able to identify ribosomal proteins and rRNA nucleotides of the human ribosome, which form a channel for reading genetic information during translation.
For the first time, it was experimentally possible to show that all rRNA nucleotides of the human small ribosomal particle adjacent to the mRNA codons are located in conserved regions of the secondary structure of the rRNA molecule. Moreover, their location coincides with the position of the corresponding nucleotides in the secondary structure of rRNA ribosomes of lower organisms. This led to the conclusion that this part of the small subunit of ribosomal RNA constitutes an evolutionarily conserved “core” of the ribosome, structurally identical in all organisms.
On the other hand, a number of fundamental differences have been discovered in the structure of the ribosomal mRNA-binding channel in humans and lower organisms. It turned out that in higher organisms, ribosomal proteins play a much larger role in the formation of this channel than in prokaryotes; in addition, proteins that do not have “doubles” (homologs) in lower organisms also participate in this.
Why, despite the fact that the function of the ribosome has remained virtually unchanged during the process of evolution, has specific features appeared in the organization of the decoding center of ribosomes in higher organisms? This is probably due to the more complex and multi-stage regulation of protein synthesis in eukaryotes compared to prokaryotes, during which ribosomal proteins of the mRNA-binding channel can interact not only with mRNA, but also with various factors affecting the efficiency and accuracy of translation. Whether this is true, further research will show.
4. Membrane proteins, associated with carbohydrates.
Peripheral proteins – protein-protein interactions.
An example of these proteins:
1. Spectrin
2. Fibronectin,
Proteins –
integral proteins perform the following functions:
a) ion channel proteins
b) receptor proteins
Ion channels
aquaporins(erythrocytes, kidney, eye).
Supramembrane component
Function of the glycocalyx: 1. Play a role receptors.
2. Intercellular recognition.
(adhesive interactions).
4. R histocompatibility receptors.
5. Enzyme adsorption zone(parietal digestion).
6. Hormone receptors.
Submembrane component
Structures formed by the plasmalemma
The contours of the cell, even at the light-optical level, do not appear even and smooth, and electron microscopy has made it possible to detect and describe various structures in the cell that reflect the nature of its functional specialization. The following structures are distinguished:
1. Microvilli – protrusion of cytoplasm covered with plasmalemma. The microvillus cytoskeleton is formed by a bundle of actin microfilaments, which are woven into the terminal network of the apical part of the cells (Fig. 5). Single microvilli are not visible at the light optical level. If there are a significant number of them (up to 2000-3000) in the apical part of the cell, even with light microscopy a “brush border” is distinguished.
2. Eyelashes – are located in the apical zone of the cell and have two parts (Fig. 6): a) outer - axoneme
b) internal – becal body
Axoneme consists of a complex of microtubules (9 + 1 pairs) and associated proteins. Microtubules are formed by the protein tubulin, and the handles are formed by the protein dynein - these proteins together form the tubulin-dynein chemomechanical transducer.
Basal body consists of 9 triplets of microtubules located at the base of the cilium and serves as a matrix for organizing the axoneme.
3. Basal labyrinth- These are deep invaginations of the basal plasmalemma with mitochondria lying between them. This is a mechanism for active absorption of water, as well as ions against a concentration gradient.
1. Transport low molecular weight compounds carried out in three ways:
1. Simple diffusion
2. Facilitated diffusion
3. Active transport
Simple diffusion– low molecular weight hydrophobic organic compounds (fatty acids, urea) and neutral molecules (HO, CO, O). As the difference in concentration between the compartments separated by the membrane increases, the rate of diffusion also increases.
Facilitated diffusion– the substance passes through the membrane also in the direction of the concentration gradient, but with the help of a transport protein – translocases. These are integral proteins that have specificity for transported substances. These are, for example, anion channels (erythrocyte), K channels (plasmolemma of excited cells) and Ca channels (sarcoplasmic reticulum). Translocase for H O it is aquaporin.
Mechanism of action of translocase:
1. The presence of an open hydrophilic channel for substances of a certain size and charge.
2. The channel opens only when a specific ligand binds.
3. There is no channel as such, and the translocase molecule itself, having bound the ligand, rotates 180 in the plane of the membrane.
Active transport– this is transport using the same transport protein (translocases), but against a concentration gradient. This movement requires energy.
Transport of high molecular weight compounds across membranes
The transition of particles through the plasmalemma always occurs in the composition membrane vesicle: 1. Endocytosis: A. pinocytosis, b. phagocytosis, c. receptor-mediated endocytosis.
2. Exocytosis: A. secretion, b. excretion, c. recreation is a transfer solids through the cell, phagocytosis and excretion are combined here.
Receptor-mediated endocytosis
1. Accumulation of ligand-binding receptors in a specific area of the plasmalemma – bordered pits(one ligand, one receptor).
2. The surface of the pit on the cytosolic side is covered with an amorphous dense substance - clathrin(LDL transport proteins and iron transport proteins, transferrin, enter this way.
3. Formation of a bordered bubble.
4. Fusion of the bordered vesicle with the acidic endosome.
rice. H endosome
5. The fate of the receptor and ligand is determined by the type of endocytosis.
A). The receptor returns, the ligand is destroyed.
rice. lysosome
b) The receptor returns, the ligand returns.
rice. lysosome
c) The receptor is destroyed, the ligand is destroyed.
rice. lysosome
d) The receptor is transported, the ligand is transported.
rice. lysosome
Pathology - Hypercholesterolemia
1. Increased LDL levels.
2. LDL is not taken up by cells.
3. Plasma LDL level.
4. Atherosclerotic plaques of the coronary vessels are formed.
LECTURE
TOPIC “ORGANELLS OF GENERAL VALUE”
Organelles- This functional systems(devices) cells. The following systems are distinguished: 1 Synthetic apparatus
2. Energy apparatus
3. Intracellular digestion apparatus (endosomal - lysosomal)
4. Cytoskeleton
Hyaloplasma- this is a colloidal system, which makes up 55% of the total volume of the cell, organelles and inclusions are suspended in it, it contains proteins, polysaccharides, nucleic acids, and ions. This is where the interstitial exchange takes place.
There are several types of endoplasmic reticulum: 1. Rough (granular endoplasmic reticulum) - GES
2. Smooth (agranular endoplasmic reticulum) - AES
3. Intermediate (transport system)
Granular endoplasmic reticulum is a system of flattened cisterns, vacuoles and channels bounded by membranes on the surface of which ribosomes are located.
Ribosomes consist of RNA and histones (1: 1), associated with membranes by the protein ribophorin. Meaning: 1. Unite protein components in space
2. Provide mutual recognition of the complex - ribosomal RNA - tRNA
3. Provide enzymes that catalyze the formation of peptide bonds
Endoplasmic reticulum – synthesis of proteins, lipids and carbohydrates – post-translational changes.
Functions of hydroelectric power station: 1. Synthesis of membrane proteins
2. Synthesis of proteins for export
3. Initial stages of glycosylation
4. Post-translational changes
In the process of protein synthesis, changes occur, designated by the following terms: 1. Initiation– this is the binding of m-RNA to ribosomes
2. Elongation– lengthening of the peptide chain
3. Folding– folding of the peptide chain into the correct three-dimensional structure.
The light-optical analogue of GES is the phenomenon of cytoplasmic basophilia, which can manifest itself in two forms: a) diffuse staining of the cytoplasm,
b) the presence of basophilic-stained lumps and granules in the cell.
At the same time basophilia- this is the result of the presence of ribosomes on the membranes of the hydroelectric power station, which include phosphoric acid residues (a triplet component), which initiates a negative charge that binds the main dye ( phenomenon of basophilia).
Protein synthesis: 1. Begins with synthesis on polysomes.
2. As a result of the interaction of mRNA and ribosomes, a signal peptide (20-25 amino acids) is formed.
3. Binding of the signal peptide with the ribonucleoprotein complex (SRP - signal recognition particle).
4. This binding stops protein synthesis.
5. Binding of HSR to a specific receptor on the EPS membrane (this is the so-called mooring protein).
6. After binding to the membrane receptor, the HSR is separated from the polysome.
7. The synthesis of the protein molecule is unblocked.
8. Integral receptor proteins - ribophorins - ensure the attachment of the large subunit of ribosomes.
9. In the lumen of the GEPS, the signal peptide is cleaved off by an enzyme signal peptidase.
10. Inside the tank, the peptide undergoes post-translational modification:
hydroxylation, phosphorylation, sulfation, etc.
Functions of the Golgi complex
1. Synthesis of polysaccharides and glycoproteins (glycocalyx, mucus).
2. Processing of molecules:
a) terminal glycosylation
b) phosphorylation
c) sulfation
d) proteolytic cleavage (parts of protein molecules)
3. Condensation of the secretory product.
4. Packaging of the secretory product
5. Sorting of proteins in the trans-Golgi network zone (due to specific receptor membrane proteins that recognize signal sites on macromolecules and direct them to the corresponding vesicles). Transport from the Golgi complex occurs in the form of 3 streams:
1. Hydrolase vesicles (or primary lysosomes)
2. Into the plasmalemma (as part of bordered bubbles)
3. In secretory granules
Endosomes - membrane vesicles with acidifying contents and ensuring the transfer of molecules into the cell. The type of substance transfer by the endosome system is different:
1. With digestion of macromolecules (complete)
2. With their partial splitting
3. No change during transport
The process of transport and subsequent breakdown of substances in the cell using endosomes consists of the following sequential components:
1. Early(peripheral) endosome
2. Late(perinuclear) endosome prelysosomal stage of digestion
3. Lysosome
Early endosome– a vesicle lacking clathrin at the cell periphery. The pH of the environment is 6.0, a limited and regulated cleavage process occurs here (the ligand is separated from the receptor) --- return receptors into the cell membrane. The early endosome is also known as Curl.
Late (perinuclear) endosome: a) more acidic content pH 5.5
b) larger diameter up to 800 nm
c) deeper level of digestion
This is the digestion of the ligand (peripheral endosome + perinuclear endosome) --- multivesicular body.
Lysosomes
1. Phagolysosome– it is formed by the fusion of a late endosome or lysosome with a phagosome. The process of destruction of this material is called heterophagy.
2.Autophagolysosome– it is formed by the fusion of a late endosome or lysosome with an autophagosome.
3. Multivesicular body– a large vacuole (800 nm), consisting of small 40-80 nm vesicles surrounded by a moderately dense matrix. It is formed as a result of the fusion of early and late endosomes.
4. Residual bodies- This is undigested material. The most famous component of this type are lipofuscin granules - vesicles with a diameter of 0.3 – 3 µm, containing lipofuscin pigment.
Cytoskeleton– is a system of microtubules, microfilaments (intermediate, microtrabeculae). They all form a three-dimensional network, interacting with networks of other components.
1. Microtubules– hollow cylinders dia. 24-25 nm, wall thickness 5 nm, dia. lumen – 14-15 nm. The wall consists of helically arranged filaments (they are called protofilaments) 5 nm thick. These threads are formed by dimers of tubulin. This labile system, in which one end (denoted “__”) is fixed, and the other (“+”) is free and participates in the depolymerization process.
Microtubules are associated with a number of proteins, which have the general name MAP - they connect microtubules with other cytoskeletal elements and organelles. Kinesin – (the step of its movement along the surface of the microtubule is 8 nm).
Organelle
rice. Microtubule
Microfilaments– these are two intertwined filaments of F-actin, composed of g-actin. Their diameter is 6 nm. Microfilaments are polar; g-actin attaches to the (“+”) end. They form clusters
along the cell periphery and are connected to the plasmalemma through intermediate proteins (actin, vinculin, talin).
Function: 1. Change in cytosol (transition from sol to gel and back).
2. Endocytosis and exocytosis.
3. Motility of non-muscle cells.
4. Stabilization of local protrusions of the plasma membrane.
Intermediate threads have d 8-11 nm, consist of proteins characteristic of certain cell types. They form an intracellular framework that ensures cell elasticity and the ordered arrangement of cytoplasmic components. Intermediate filaments are formed by thread-like protein molecules woven together like a rope.
Functions: 1. Structural
2. Participation in the formation of the horny substance
3. Maintaining the shape and processes of nerve cells
4. Attachment of myofibrils to the plasmalemma.
Microtrabeculae- an openwork network of thin filaments that exists in combination with microtubules and can participate in the transport of organelles and influence the viscosity of the cytosol.
LECTURE
TOPIC: “CORE. STRUCTURE OF THE INTERPHASE NUCLEUS. BASICS OF BIOSYNTHETIC ACTIVITY OF CELLS”
Core is the main part of the cell that encodes information about the structure and function of the organ. This information is contained in the genetic material, DNA, which is a complex of DNP with the main proteins (histones). With some exceptions (mitochondria), DNA is localized exclusively in the nucleus. DNA is capable of replicating itself, thereby ensuring the transmission of the genetic code to daughter cells under conditions of cell division.
The nucleus plays a central role in the synthesis of proteins and polypeptides, being the carrier of genetic information. All cell nuclei in the body contain the same genes; some cells differ in their structure, function and the nature of the substances produced by the cell. Nuclear control is carried out by
repression or depression (expression) of the activity of various genes. Translation about the nature of protein synthesis is associated with the formation of m-RNA. Many RNAs are a complex of protein and RNA, i.e. RNP. The interphase nucleus in most cells is a round or oval formation several mm in diameter. In leukocytes and cells connective tissue the nucleus is lobulated and is designated by the term polymorphic.
Interphase nucleus has several different structures: nuclear envelope, chromatin, karyolymph and nucleolus.
Nuclear envelope
1. Outer nuclear membrane– ribosomes are located on the surface, where proteins are synthesized and enter the perinuclear cisterns. On the cytoplasmic side, it is surrounded by a loose network of intermediate (vimentin) filaments.
2. Perinuclear cisterns– part of the perinuclear cisterns is associated with the granular endoplasmic reticulum (20-50 nm).
3. Inner nuclear membrane – separated from the contents of the nucleus by the nuclear lamina.
4. Nuclear lamina 80-300 nm thick, participates in the organization of the nuclear membrane and perinuclear chromatin, contains intermediate filament proteins - lamins A, B and C.
5. Nuclear time– from 3-4 thousand specialized communications, carry out transport between the nucleus and the cytoplasm. Nuclear pore d 80 nm, has: a) pore channel – 9 nm
b) nuclear pore complex, the latter contains a receptor protein that responds to nuclear import signals ( entrance ticket into the nucleus). The diameter of the nuclear pore can increase the diameter of the pore channel and ensure the transfer of large macromolecules (DNA-RNA polymerase) into the nucleus.
Nuclear time consists of 2 parallel rings, one on each surface of the karyolemma. A ring with a diameter of 80 nm, they are formed by 8 protein granules, from each granule a thread (5 nm) stretches towards the center, which forms a partition (diaphragm). In the center there is a central granule. The set of these structures is called nuclear pore complex. A channel with a diameter of 9 nm is formed here; such a channel is called a water channel, since small water-soluble molecules and ions move through it.
Functions of the nuclear pore: 1. Selective transport;
2. Active transfer into the nucleus of proteins with a sequence characteristic of proteins of nuclear localization;
3. Transfer of ribosomal subunits into the cytoplasm with a change in the conformation of the pore complex.
Inner nuclear membrane- smooth and connected with the help of integral proteins to the nuclear lamina, which is a layer 80-300 nm thick. This record or lamina– consists of intertwined intermediate filaments (10 nm) that form the karyoskeleton. Its functions:
1. Saving structural organization pore complexes;
2. Maintaining the shape of the core;
3. Ordered chromatin packing.
It is formed as a result of spontaneous association of 3 main polypeptides. This is the structural framework of the nuclear envelope with sites for specific chromatin binding.
Chromatin
Under a light microscope, it consists of an irregularly packed mass of low density, differing in the degree of density, quantity and size in cells various types. Chromatin clumps are called karyosomes, i.e. they have an affinity for basic dyes. Chromatin of the interphase nucleus is the DNP of chromosomes. The chromosomes in the interphase nucleus are very thin, long, resembling threads in a ball.
There was a time when it was believed that this mass consisted of one individual chromosome, which was called spirella.
Dense chromatin is referred to as heterochromatin, as opposed to loose euchromatin. At the light-optical level, chromosome elements are visible only if they form aggregates 0.2 microns in size (heterochromatin). The mass of heterochromatin is an indicator of cellular activity; cells containing large blocks of heterochromatin are characterized by an inactive phase in protein synthesis and, therefore, in the production of m-RNA.
Nucleolus
This is a dense granule with a diameter of 1-3 microns, intensely stained with basic dyes. The main component of the nucleolus is a specialized region of chromosomes (loops), or the organizer of the nucleolus. Such regions are found in five chromosomes: 13th, 14th, 15th, 21st and 22nd; This is where numerous copies of genes encoding ribosomal RNAs are located.
In EM, 3 components are described in the nucleolus:
1. Fibrillar component- many thin (5-8 nm) filaments, with a predominant localization in the inner part of the nucleolus. These are primary rRNA transcripts.
2. Granular component- this is a cluster of dense particles with a diameter of 10-20 nm; they correspond to the most mature precursors of ribosomal subunits.
3. Amorphous component– This is the zone where nucleolar organizers are located, a very pale colored zone. There are large DNA loops involved in the transcription of ribosomal RNA, as well as proteins that specifically bind to RNA. Granules and fibrils form nucleolar filament (nucleolonema), thickness 60-80 nm. Since the nucleolus is surrounded by chromatin, it is called perinuclear chromatin, and its part penetrating into the nucleolus is intranucleolar chromatin.
Cell Conveyor is the assembly of a secretory product on a living conveyor belt with the participation of various cellular organelles. In this case, the assembly process consists of a number of stages that occur in a certain sequence in areas of the cell that are quite far removed from the place of direct action of the nucleic acids that exercise genetic control.
The cellular conveyor belt for protein synthesis involves the usual sequence of processes outlined in the section describing the granular endoplasmic reticulum. Here it is appropriate to present the mechanism of synthesis of non-protein substances.
Membrane proteins associated with lipids.
4. Membrane proteins, associated with carbohydrates.
Peripheral proteins – are not immersed in the lipid bilayer and are not covalently linked to it. They are held together by ionic interactions. Peripheral proteins are associated with integral proteins in the membrane due to interaction - protein-protein interactions.
An example of these proteins:
1. Spectrin, which is located on the inner surface of the cell
2. Fibronectin, localized on the outer surface of the membrane
Proteins – usually constitute up to 50% of the membrane mass. At the same time
integral proteins perform the following functions:
a) ion channel proteins
b) receptor proteins
2. Peripheral membrane proteins(fibrillar, globular) perform the following functions:
a) external (receptor and adhesion proteins)
b) internal – cytoskeleton proteins (spectrin, ankyrin), proteins of the second messenger system.
Ion channels– these are channels formed by integral proteins; they form a small pore through which ions pass along an electrochemical gradient. The most well-known channels are the channels for Na, K, Ca 2, Cl.
There are also water channels - these are aquaporins(erythrocytes, kidney, eye).
Supramembrane component– glycocalyx, thickness 50 nm. These are carbohydrate regions of glycoproteins and glycolipids that provide a negative charge. Under EM is a loose layer of moderate density covering outer surface plasma membranes. In addition to carbohydrate components, the glycocalyx contains peripheral membrane proteins (semi-integral). Their functional areas are located in the supra-membrane zone - these are immunoglobulins (Fig. 4).
Function of the glycocalyx: 1. Play a role receptors.
2. Intercellular recognition.
3. Intercellular interactions(adhesive interactions).
4. R histocompatibility receptors.
5. Enzyme adsorption zone(parietal digestion).
6. Hormone receptors.
Submembrane component or the outermost zone of the cytoplasm, usually has relative rigidity and this zone is especially rich in filaments (d 5-10 nm). It is assumed that the integral proteins that make up the cell membrane are directly or indirectly associated with actin filaments lying in the submembrane zone. At the same time, it has been experimentally proven that during the aggregation of integral proteins, actin and myosin located in this zone also aggregate, which indicates the participation of actin filaments in the regulation of cell shape.
Eukaryotic cells have a developed system of internal structures surrounded by membranes called organelles
Each organelle has a unique composition of (glyco)proteins and (glyco)lipids and performs a specific set of functions
Each organelle contains one or more membrane-bounded compartments
Organelles perform their functions autonomously or in groups
In endocytosis and exocytosis, transported proteins (cargo proteins) are transported between compartments through transport vesicles, which are formed by budding from the surface of the organelle and then fuse with the target membrane of the acceptor compartment
Transport vesicles can selectively include transported material and exclude those components that should remain in the organelle from which the vesicles were formed
Selective incorporation into vesicles is mediated by signals present in the primary structure of the protein or in the carbohydrate structure
Transport vesicles contain proteins that guide them to their destination and binding sites. Subsequently, the vesicles merge with the acceptor site of the membrane
Membrane-bounded compartments in a typical animal cell.One of characteristic features eukaryotic cell is presence in it developed system internal structures surrounded by membranes called organelles. Eukaryotic cells are characterized by the presence of membranes that divide their internal contents into functionally different compartments, while all cells of living organisms have an outer two-layer membrane.
One of the advantages compartmentalization is that the cell has the ability to create the necessary environment to perform functions that require a certain chemical composition environment.
The structure and diversity are illustrated organelles membrane-bearing cells that are normally present in a eukaryotic cell (in this case, a typical animal cell). Each organelle contains one or more compartments. For example, the endoplasmic reticulum (ER) is one compartment; in contrast, the Golgi apparatus consists of several membrane-enclosed compartments that have specific biochemical functions.
Mitochondria are characterized by two compartment, matrix and intermembrane space containing a set of certain macromolecules.
Cytosol can be considered one compartment, bounded by the plasma membrane and in contact with the outer part of the membrane of all intracellular organelles. Cytoplasm consists of cytosol and organelles. Likewise, the nucleoplasm is bounded by the inner nuclear membrane.
Each organelle contains unique set of proteins(both membrane and soluble), lipids and other molecules necessary to perform its functions. Some lipids and proteins are covalently linked to oligosaccharides. As cells grow and divide, new components must be synthesized that are necessary for growth, division, and the final distribution of intracellular material between the two daughter cells. During cell differentiation and development, as well as in response to external factors such as stress, the synthesis of organelle components occurs.
However components are not always formed in the organelle where they function. Typically, various macromolecules are formed at sites specifically designed for their synthesis. For example, most proteins are formed on ribosomes in the cytosol, which is the optimal environment for ribosome function and protein synthesis.
The next question arises: how do components organelles fall into the places of their functioning? Since the early 1970s. this question was central to cell biology. As the figure below suggests, there are at least eight major types of organelles, each consisting of hundreds or thousands various proteins and lipids.
Exocytosis and endocytosis. Exocytosis involves the endoplasmic reticulum (including the nuclear envelope)
and the Golgi apparatus (one stack of cisternae shown).
Endocytosis occurs with the participation of early and late endosomes and lysosomes.
All these molecules must be transported into the organelles in which they perform their functions. Most are formed in the cytosol, and therefore the question arises: how are they delivered to the corresponding organelles or leave the cell if they belong to secreted proteins? In many cases, the answer to this question is the presence of special signals in the protein molecule, usually called sorting signals or addressing signals. They are short sequences of amino acids present in the primary structure of those proteins that should be localized outside the cytosol. Each destination address of a protein molecule is associated with one or more various types signals.
Sorting signals are recognized special cell systems as the protein moves towards its destination. As shown in the figure below, there are two main transport mechanism: exocytosis (or secretory pathway) and endocytosis, in which material (cargo) is transported out of and into the cell, respectively.
For all newly synthesized proteins, intended for secretion from the cell, or for entry into organelles by exo- or endocytosis, there is a common entry point on the ER membrane. Signal sequences serve as signals for protein translocation across the ER membrane. In this chapter, we look at the sorting signals that guide proteins to their destinations.
While in EPR, the protein cannot be transported through the cytoplasm, and the only way for it to enter other organelles surrounded by membranes is vesicular transport. Transport vesicles are primarily composed of proteins and lipids and are said to “bud off” from the membrane. After the vesicle has budded, it fuses with the next compartment in its path. The compartment from which the vesicle originates is usually called the donor compartment (or source compartment), and the destination (or target) compartment is usually called the acceptor compartment.
Transport vesicles directly or indirectly transfer proteins from the ER to all other compartments along the path of exo- or endocytosis. During endocytosis, vesicles are formed at the plasma membrane. These vesicles transport the material they contain into endosomes, which form other vesicles that transport the material to other compartments. Thus, the composition of transport vesicles varies depending on their origin and destination compartment.
Vesicular transport creates a problem for the organelles with which the vesicles exchange. For normal functioning, a certain internal composition of organelles must be maintained. However, how can this be achieved if the vesicles constantly change this composition? The scale of the problem becomes apparent when calculating transport efficiency. Through the endocytosis pathway, amounts of membrane proteins and lipids equivalent to their total content in the plasma membrane can be transported across organelles in less than an hour. When compared to the time required to synthesize a new organelle (usually one day), this speed is impressive.
The solution to this problems associated with the selectivity of the transport process. During budding, only those proteins that need to be transported pass into the vesicle. Resident proteins of the organelle do not enter the vesicle. The vesicle holds these proteins and transfers them to the next vesicle along the way. To maintain homeostasis between organelles, by its nature, vesicular transport should always be bidirectional, i.e., components of the donor compartment should not be continuously transferred to the acceptor compartment.