Protein synthesis in muscle cells. Protein biosynthesis: brief and clear
1. What functions do proteins perform in a cell?
Answer. Proteins play an extremely important role in the life processes of cells and organisms; they are characterized by the following functions.
1. Structural. They are part of intracellular structures, tissues and organs. For example, collagen and elastin serve as components of connective tissue: bones, tendons, cartilage; fibroin is part of silk, spider webs; keratin is part of the epidermis and its derivatives (hair, horns, feathers). They form shells (capsids) of viruses.
2. Enzymatic. All chemical reactions in the cell occur with the participation of biological catalysts - enzymes (oxidoreductases, hydrolases, ligases, transferases, isomerases, and lyases).
3. Regulatory. For example, the hormones insulin and glucagon regulate glucose metabolism. Histone proteins are involved in the spatial organization of chromatin, and thereby influence gene expression.
4. Transport. Hemoglobin carries oxygen in the blood of vertebrates, hemocyanin in the hemolymph of some invertebrates, and myoglobin in muscles. Serum albumin serves for the transport of fatty acids, lipids, etc. Membrane transport proteins ensure the active transport of substances across cell membranes. Cytochromes transport electrons along the electron transport chains of mitochondria and chloroplasts.
5. Protective. For example, antibodies (immunoglobulins) form complexes with bacterial antigens and with foreign proteins. Interferons block viral protein synthesis in an infected cell. Fibrinogen and thrombin are involved in blood clotting processes.
6. Contractile (motor). The proteins actin and myosin provide the processes of muscle contraction and contraction of cytoskeletal elements.
7. Signal (receptor). Cell membrane proteins are part of receptors and surface antigens.
Storage proteins. Milk casein, chicken egg albumin, ferritin (stores iron in the spleen).
8. Toxin proteins. Diphtheria toxin.
9. Energy function. When 1 g of protein breaks down to the final metabolic products (CO2, H2O, NH3, H2S, SO2), 17.6 kJ or 4.2 kcal of energy is released.
2. What do proteins consist of?
Answer. Proteins are high-molecular organic substances consisting of amino acids connected in a chain by a peptide bond. In living organisms, the amino acid composition of proteins is determined by the genetic code; in most cases, 20 standard amino acids are used during synthesis. Many of their combinations create protein molecules with a wide variety of properties.
Questions after §26
1. What is a gene?
Answer. A gene is a material carrier of hereditary information, the totality of which parents transmit to their descendants during reproduction. Currently, in molecular biology it has been established that genes are sections of DNA that carry some kind of integral information - about the structure of one protein molecule or one RNA molecule. These and other functional molecules determine the growth and functioning of the body.
2. What process is called transcription?
Answer. The carrier of genetic information is DNA, located in the cell nucleus. Protein synthesis itself occurs in the cytoplasm on ribosomes. From the nucleus to the cytoplasm, information about the structure of the protein comes in the form of messenger RNA (mRNA). In order to synthesize mRNA, a section of double-stranded DNA is unwinded, and then an mRNA molecule is synthesized on one of the DNA strands according to the principle of complementarity. This happens as follows: against, for example, G of a DNA molecule becomes C of an RNA molecule, against A of a DNA molecule - Y of an RNA molecule (remember that instead of thymine, RNA carries uracil, or Y), against T of a DNA molecule - A of an RNA molecule and against C DNA molecules - RNA molecules. Thus, an mRNA chain is formed, which is an exact copy of the second (non-template) DNA chain (only uracil is included instead of thymine). This is how information about the sequence of amino acids in a protein is translated from the “language of DNA” to the “language of RNA.” This process is called transcription.
3. Where and how does protein biosynthesis occur?
Answer. The process of protein synthesis occurs in the cytoplasm, which is also called translation. Translation is the translation of the nucleotide sequence of an mRNA molecule into the amino acid sequence of a protein molecule. The ribosome interacts with the end of the mRNA from which protein synthesis should begin. In this case, the beginning of the future protein is indicated by the triplet AUG, which is a sign of the beginning of translation. Since this codon codes for the amino acid methionine, all proteins (except in special cases) begin with methionine. After binding, the ribosome begins to move along the mRNA, stopping at each section of it, which includes two codons (i.e. 3 + 3 = 6 nucleotides). The delay time is only 0.2 s. During this time, the tRNA molecule, whose anticodon is complementary to the codon located in the ribosome, manages to recognize it. The amino acid that was associated with this tRNA is separated from the “petiole” and joins the growing protein chain to form a peptide bond. At the same moment, the next tRNA approaches the ribosome, the anticodon of which is complementary to the next triplet in the mRNA, and the next amino acid brought by this tRNA is included in the growing chain. After this, the ribosome moves along the mRNA, stops at the next nucleotides, and everything repeats all over again.
4. What is a stop codon?
Answer. Stop codons (UAA, UAG, or UGA) do not code for amino acids; they merely indicate that protein synthesis must be completed. The protein chain is detached from the ribosome, enters the cytoplasm and forms the secondary, tertiary and quaternary structures inherent in this protein
5. How many types of tRNA are involved in the synthesis of proteins in the cell?
Answer. Not less than 20 (number of amino acids), not more than 61 (number of sense codons). Typically there are about 43 tRNAs in prokaryotes. In humans, about 50 different tRNAs ensure the incorporation of amino acids into protein.
6. What does a polysome consist of?
Answer. A cell needs not one, but many molecules of each protein. Therefore, as soon as the ribosome, which was the first to begin protein synthesis on an mRNA molecule, moves forward, a second ribosome is immediately strung on this mRNA, which begins to synthesize the same protein. The same mRNA can be strung with a third and a fourth ribosome, etc. All ribosomes that synthesize protein on one mRNA molecule are called a polysome.
7. Do protein synthesis processes require energy? Or, on the contrary, does energy release occur in the processes of protein synthesis?
Answer. Like any synthetic process, protein synthesis is an endothermic reaction and, therefore, requires energy. Protein biosynthesis represents a chain of synthetic reactions: 1) synthesis of mRNA; 2) connection of amino acids with t-RNA; 3) “protein assembly”. All these reactions require high energy costs - up to 24.2 kcal/mol. The energy for protein synthesis is provided by the cleavage reaction of ATP.
The role of proteins in the cell and the body
The role of protein in the life of a cell and the main stages of its synthesis. Structure and functions of ribosomes. The role of ribosomes in the process of protein synthesis.
Proteins play an extremely important role in the life processes of cells and organisms; they are characterized by the following functions.
Structural. They are part of intracellular structures, tissues and organs. For example, collagen and elastin serve as components of connective tissue: bones, tendons, cartilage; fibroin is part of silk, spider webs; keratin is part of the epidermis and its derivatives (hair, horns, feathers). They form shells (capsids) of viruses.
Enzymatic. All chemical reactions in the cell occur with the participation of biological catalysts - enzymes (oxidoreductases, hydrolases, ligases, transferases, isomerases, and lyases).
Regulatory. For example, the hormones insulin and glucagon regulate glucose metabolism. Histone proteins are involved in the spatial organization of chromatin, and thereby influence gene expression.
Transport. Hemoglobin carries oxygen in the blood of vertebrates, hemocyanin in the hemolymph of some invertebrates, and myoglobin in muscles. Serum albumin serves for the transport of fatty acids, lipids, etc. Membrane transport proteins provide active transport of substances across cell membranes (Na+, K+-ATPase). Cytochromes transport electrons along the electron transport chains of mitochondria and chloroplasts.
Protective. For example, antibodies (immunoglobulins) form complexes with bacterial antigens and with foreign proteins. Interferons block viral protein synthesis in an infected cell. Fibrinogen and thrombin are involved in blood clotting processes.
Contractile (motor). The proteins actin and myosin provide the processes of muscle contraction and contraction of cytoskeletal elements.
Signal (receptor). Cell membrane proteins are part of receptors and surface antigens.
Storage proteins. Milk casein, chicken egg albumin, ferritin (stores iron in the spleen).
Toxin proteins. Diphtheria toxin.
Energy function. When 1 g of protein breaks down to the final metabolic products (CO2, H2O, NH3, H2S, SO2), 17.6 kJ or 4.2 kcal of energy is released.
Protein biosynthesis occurs in every living cell. It is most active in young growing cells, where proteins are synthesized to build their organelles, as well as in secretory cells, where enzyme proteins and hormone proteins are synthesized.
Main role in determining the structure of proteins belongs to DNA. A piece of DNA containing information about the structure of one protein is called a gene. A DNA molecule contains several hundred genes. The DNA molecule contains a code for the sequence of amino acids in a protein in the form of specifically matching nucleotides.
Protein synthesis - a complex multi-stage process representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.
In protein biosynthesis, the following stages are determined, occurring in different parts of the cell:
First stage - mRNA synthesis occurs in the nucleus, during which the information contained in the DNA gene is transcribed into mRNA. This process is called transcription (from the Latin “transcript” - rewriting).
At the second stage amino acids are combined with tRNA molecules, which sequentially consist of three nucleotides - anticodons, with the help of which their triplet codon is determined.
Third stage - This is the process of direct synthesis of polypeptide bonds, called translation. It occurs in ribosomes.
At the fourth stage the formation of the secondary and tertiary structure of the protein occurs, that is, the formation of the final structure of the protein.
Thus, in the process of protein biosynthesis, new protein molecules are formed in accordance with the exact information contained in the DNA. This process ensures the renewal of proteins, metabolic processes, cell growth and development, that is, all the life processes of the cell.
Lesson outline : “Protein synthesis in the cell”
(For specialized 10th grade, lesson time - 2 hours)
Teacher: Mastyukhina Anna Aleksandrovna
Municipal educational institution "Secondary school named after General Zakharkin I.G."
Lesson objective:
Educational: studyfeatures of protein biosynthesis in the cell, learn concepts:gene, genetic code, triplet, codon, anticodon, transcription, translation, polysome; ncontinue to develop knowledge about the mechanisms of protein biosynthesis using the example of translation; find out the role of transfer RNAs in the process of protein biosynthesis; reveal the mechanisms of template synthesis of the polypeptide chain on ribosomes.
Developmental: in order to develop the cognitive interest of studentsprepare messages in advance(“Interesting facts about the gene”, “Genetic code”, “Transcription and translation”). To develop practical skillswill make a syncwine. In order to develop logical thinkinglearn to solve problems.
Educational: In order to form a scientific worldview, prove the importance and significance of protein synthesis in cells, as well as their vital necessity.
F.O.U.R .: lesson.
Lesson type : combined
Lesson type : with the presentation “Protein synthesis in the cell” and demonstration of magnetic models.
Equipment:
presentation “Protein synthesis in the cell”; table "Genetic code"; Scheme “Formation of mRNA from a DNA template (transcription)”; Scheme “Structure of t-RNA”; Scheme “Protein synthesis in ribosomes (translation)”; Scheme “Protein synthesis on a polysome”; Task cards and crossword puzzle; magnetic models.
Methods and methodological techniques:
I .Class organization.
In previous lessons we studied substances called nucleic acids. As a consequence
then we looked at their two types: DNA and RNA, and got acquainted with their structure and functions. It was found that each of the nucleic acids contains four different nitrogenous bases, which are connected to each other according to the principle of complementarity. We will need all this knowledge when studying today's new topic. So write down its name in your workbooks “Protein synthesis in the cell.”
II .Learning new material:
1) Updating knowledge:
Before starting to study a new topic, let’s remember: what is metabolism (metabolism):
METABOLISM is the totality of all enzymatic reactions of a cell connected with each other and with the external environment, consisting of plastic
and energy exchanges.
Let's make a syncwine, the first word of which is metabolism. (1-metabolism
2-plastic, energy
3-flows, absorbs, releases
4-set of enzymatic reactions of the cell
5-metabolism)
Protein biosynthesisrefers to plastic exchange reactions.
Protein biosynthesis – the most important process in living nature. This is the creation of protein molecules based on information about the sequence of amino acids in its primary structure contained in the structure of DNAAssignment: complete the sentences by filling in the missing terms.
1. Photosynthesis is...(synthesis of organic substances in the light).
2. The process of photosynthesis is carried out in cell organelles - ...(chloroplasts).
3. Free oxygen is released during photosynthesis during the breakdown of...(water).
4. At what stage of photosynthesis is free oxygen formed? On...(light).
5. During the light stage... ATP.(Synthesized.)
6. In the dark stage,... is formed in the chloroplast...(primary carbohydrate is glucose).
7. When the sun hits chlorophyll,...(excitation of electrons).
8. Photosynthesis occurs in cells...(green plants).
9. The light phase of photosynthesis occurs in...(thylakoids).
10. The dark phase occurs in...(any) Times of Day.
The most important process of assimilation in the cell is its inherent proteins.
Each cell contains thousands of proteins, including those unique to this type of cell. Since all proteins are destroyed sooner or later in the process of life, the cell must continuously synthesize proteins to restore its , organelles, etc. In addition, many cells “manufacture” proteins for the needs of the whole organism, for example, cells of the endocrine glands, which secrete protein hormones into the blood. In such cells, protein synthesis is especially intense.
2)Learning new material:
Protein synthesis requires a lot of energy.
The source of this energy, as for all cellular processes, is . The variety of functions of proteins is determined by their primary structure, i.e. sequence of amino acids in their molecule. In turn, hereditary The primary structure of a protein is contained in the sequence of nucleotides in a DNA molecule. A section of DNA that contains information about the primary structure of one protein is called a gene. One chromosome contains information about the structure of many hundreds of proteins.
Genetic code.
Each amino acid in the protein corresponds to a sequence of three nucleotides located one after another - a triplet. To date, a map of the genetic code has been compiled, that is, it is known which triplet combinations of DNA nucleotides correspond to one or another of the 20 amino acids that make up proteins (Fig. 33). As you know, DNA can contain four nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). The number of combinations of 4 by 3 is: 43 = 64, i.e. 64 different amino acids can be encoded, while only 20 amino acids are encoded. It turned out that many amino acids correspond to not one, but several different triplets - codons.
It is assumed that this property of the genetic code increases the reliability of the storage and transmission of genetic information during cell division. For example, the amino acid alanine corresponds to 4 codons: CGA, CGG, CTG, CGC, and it turns out that a random error in the third nucleotide cannot affect the structure of the protein - it will still be an alanine codon.
Since a DNA molecule contains hundreds of genes, it necessarily includes triplets, which are “punctuation marks” and indicate the beginning and end of a particular gene.
A very important property of the genetic code is specificity, i.e. one triplet always denotes only one single amino acid. The genetic code is universal for all living organisms from bacteria to humans.
Transcription. The carrier of all genetic information is DNA, located in
cells. Protein synthesis itself occurs in the cytoplasm of the cell, on ribosomes. From the nucleus to the cytoplasm, information about the structure of the protein comes in the form of messenger RNA (i-RNA). In order to synthesize mRNA, a section of DNA “unwinds”, despirals, and then, according to the principle of complementarity, RNA molecules are synthesized on one of the DNA chains with the help of enzymes (Fig. 34). This happens as follows: against, for example, guanine of a DNA molecule becomes cytosine of an RNA molecule, against adenine of a DNA molecule - uracil RNA (remember that RNA contains uracil in nucleotides instead of thymine), opposite thymine of DNA - adenine RNA and opposite cytosine of DNA - guanine RNA. Thus, an mRNA chain is formed, which is an exact copy of the second DNA strand (only thymine is replaced by uracil). Thus, information about the nucleotide sequence of a DNA gene is “rewritten” into the nucleotide sequence of mRNA. This process is called transcription. In prokaryotes, synthesized mRNA molecules can immediately interact with ribosomes, and protein synthesis begins. In eukaryotes, mRNA interacts with special proteins in the nucleus and is transported through the nuclear envelope into the cytoplasm.
The cytoplasm must contain a set of amino acids necessary for protein synthesis. These amino acids are formed as a result of the breakdown of food proteins. In addition, this or that amino acid can get to the site of direct protein synthesis, i.e., into the ribosome, only by attaching to a special transfer RNA (t-RNA).
Transfer RNAs.
To transfer each type of amino acid into ribosomes, a separate type of tRNA is needed. Since proteins contain about 20 amino acids, there are the same number of types of tRNA. The structure of all tRNAs is similar (Fig. 35). Their molecules form peculiar structures that resemble a clover leaf in shape. Types of tRNA necessarily differ in the triplet of nucleotides located “at the top”. This triplet, called an anticodon, corresponds in its genetic code to the amino acid that this T-RNA will carry. A special enzyme necessarily attaches to the “leaf petiole” the amino acid that is encoded by the triplet complementary to the anticodon.
Broadcast.
The last stage of protein synthesis—translation—occurs in the cytoplasm. A ribosome is threaded onto the end of the mRNA from which protein synthesis must begin (Fig. 36). The ribosome moves along the mRNA molecule intermittently, in “jumps,” staying on each triplet for approximately 0.2 s. During this instant, one tRNA out of many is able to “identify” with its anticodon the triplet on which the ribosome is located. And if the anticodon is complementary to this mRNA triplet, the amino acid is detached from the “leaf petiole” and attached by a peptide bond to the growing protein chain (Fig. 37). At this moment, the ribosome moves along the mRNA to the next triplet, encoding the next amino acid of the protein being synthesized, and the next t-RNA “brings” the necessary amino acid, which increases the growing protein chain. This operation is repeated as many times as the number of amino acids the protein being built must contain. When there is one set of triplets in the ribosome, which is a “stop signal” between genes, then not a single t-RNA can join such a triplet, since t-RNA does not have anticodons for them. At this point, protein synthesis ends. All the described reactions occur in very short periods of time. It is estimated that the synthesis of a fairly large protein molecule takes only about two minutes.
A cell needs not one, but many molecules of each protein. Therefore, as soon as the ribosome, which was the first to begin protein synthesis on mRNA, moves forward, a second ribosome synthesizing the same protein is behind it on the same mRNA. Then the third, fourth ribosomes, etc. are sequentially strung onto the mRNA. All ribosomes that synthesize the same protein encoded in a given mRNA are called a polysome.
When protein synthesis is completed, the ribosome can find another mRNA and begin to synthesize the protein whose structure is encoded in the new mRNA.
Thus, translation is the translation of the nucleotide sequence of an mRNA molecule into the amino acid sequence of the synthesized protein.
It is estimated that all the proteins in a mammal's body can be encoded by just two percent of the DNA contained in its cells. What is the other 98% of DNA needed for? It turns out that each gene is much more complex than previously thought, and contains not only the section in which the structure of a protein is encoded, but also special sections that can “turn on” or “turn off” the operation of each gene. That is why all cells, for example the human body, which have the same set of chromosomes, are capable of synthesizing different proteins: in some cells, protein synthesis occurs with the help of certain genes, while in others completely different genes are involved. So, in each cell only part of the genetic information contained in its genes is realized.
Protein synthesis requires the participation of a large number of enzymes. And each individual protein synthesis reaction requires specialized enzymes.
IV .Secure the material:
Fill out the table:
B-1
Protein biosynthesis consists of two successive stages: transcription and translation.
Solve problem 1:
The tRNA anticodons are given: GAA, GCA, AAA, ACG. Using the genetic code table, determine the amino acid sequence in the protein molecule, mRNA codons and triplets in the gene fragment encoding this protein.
Solution:
mRNA codons: TSUU – TsGU – UUU – UGC.
Amino acid sequence: leu – arg – phen – cis.
DNA triplets: GAA – GCA – AAA – ACG.
Task 2
TGT-ATSA-TTA-AAA-CCT. Determine the nucleotide sequence of mRNA and the sequence of amino acids in the protein that is synthesized under the control of this gene.
Answer: DNA: TGT-ATSA-TTA-AAA-CCT
mRNA: ACA-UGU-AAU-UUU-GGA
Protein: tre---cis---asp---fen---gli.
V-2
Solve problem 1:
Given is a fragment of a double-stranded DNA molecule. Using the genetic code table, determine the structure of the fragment of the protein molecule encoded by this section of DNA:
AAA – TTT – YYY – CCC
TTT – AAA – TCC – YYY.
Solution:
Since mRNA is always synthesized on only one DNA strand, which is usually depicted in writing as the top strand, then
mRNA: UUU – AAA – CCC – YGG;
protein fragment encoded by the upper chain: fen - lys - pro - gly.
Task 2 : a section of DNA has the following nucleotide sequence:
TGT-ATSA-TTA-AAA-CCT. Determine the nucleotide sequence of mRNA and the amino acid sequence in the protein that is synthesized under the control of this gene.
Answer: DNA: AGG-CCT-TAT-YYY-CGA
mRNA: UCC-GGA-AUA-CCC-GCU
Protein: ser---gli---iso---pro---ala
Now let’s listen to the interesting messages that you have prepared.
"Interesting facts about the gene"
"Genetic code"
"Transcription and Broadcasting"
VI .Summing up the lesson.
1) Conclusion from the lesson: One of the most important processes occurring in a cell is protein synthesis. Each cell contains thousands of proteins, including those unique to this type of cell. Since in the process of life, all proteins sooner or laterare destroyed, the cell must continuously synthesize proteins to restore its membranes, organelles, etc. In addition, many cells produce proteins for the needs of the whole organism, for example, cells of the endocrine glands, which secrete protein hormones into the blood. In such cells, protein synthesis is especially intense. Protein synthesis requires a lot of energy. The source of this energy, as for all cellular processes, is ATP.
2) Evaluate students’ independent work and their work at the board. Also evaluate the activity of conversation participants and speakers.
V II . Homework:
Repeat § 2.13.
Solve the crossword:
1. A specific sequence of nucleotides located at the beginning of each gene.
2. Transition of the nucleotide sequence of an mRNA molecule into the AK sequence of a protein molecule.
3. Broadcast start sign.
4. A carrier of genetic information located in the cell nucleus.
5. A property of the genetic code that increases the reliability of storage and transmission of genetic information during cell division.
6. A section of DNA containing information about the primary structure of one protein.
7. A sequence of three DNA nucleotides located one after another.
8. All ribosomes that synthesize protein on one mRNA molecule.
9. The process of translating information about the AK sequence in a protein from the “DNA language” to the “RNA language”.
10. A codon that does not code for AK, but only indicates that protein synthesis must be completed.
11. Structure, where the sequence of AK in a protein molecule is determined.
12. An important property of the genetic code is that one triplet always encodes only one AK.
13. A “punctuation mark” in a DNA molecule indicating that mRNA synthesis should be stopped.
14. Genetic code... for all living organisms from bacteria to humans.
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The set of reactions of biological synthesis is called plastic exchange, or assimilation. The name of this type of exchange reflects its essence: from simple substances entering the cell from the outside, substances similar to the substances of the cell are formed.
Let's consider one of the most important forms of plastic metabolism - protein biosynthesis. The entire variety of properties of proteins is ultimately determined by the primary structure, i.e., the sequence of amino acids. A huge number of unique combinations of amino acids selected by evolution are reproduced by the synthesis of nucleic acids with a sequence of nitrogenous bases that corresponds to the sequence of amino acids in proteins. Each amino acid in the polypeptide chain corresponds to a combination of three nucleotides - a triplet.
The process of implementing hereditary information in biosynthesis is carried out with the participation of three types of ribonucleic acids: informational (template) - mRNA (mRNA), ribosomal - rRNA and transport - tRNA. All ribonucleic acids are synthesized in the corresponding sections of the DNA molecule. They are much smaller in size than DNA and represent a single chain of nucleotides. Nucleotides contain a phosphoric acid residue (phosphate), a pentose sugar (ribose) and one of four nitrogenous bases - adenine, cytosine, guanine and uracil. The nitrogenous base, uracil, is complementary to adenine.
The biosynthesis process is complex and includes a number of stages - transcription, splicing and translation.
The first stage (transcription) occurs in the cell nucleus: mRNA is synthesized in a section of a specific gene on a DNA molecule. This synthesis is carried out with the participation of a complex of enzymes, the main of which is DNA-dependent RNA polymerase, which attaches to the initial (initial) point of the DNA molecule, unwinds the double helix and, moving along one of the strands, synthesizes a complementary strand of mRNA next to it. As a result of transcription, mRNA contains genetic information in the form of a sequential alternation of nucleotides, the order of which is exactly copied from the corresponding section (gene) of the DNA molecule.
Further studies showed that during the transcription process, the so-called pro-mRNA is synthesized - the precursor of mature mRNA involved in translation. Pro-mRNA is significantly larger and contains fragments that do not code for the synthesis of the corresponding polypeptide chain. In DNA, along with regions encoding rRNA, tRNA and polypeptides, there are fragments that do not contain genetic information. They are called introns in contrast to the coding fragments, which are called exons. Introns are found in many parts of DNA molecules. For example, one gene, the DNA section encoding chicken ovalbumin, contains 7 introns, and the rat serum albumin gene contains 13 introns. The length of the intron varies - from two hundred to a thousand pairs of DNA nucleotides. Introns are read (transcribed) at the same time as exons, so pro-mRNA is much longer than mature mRNA. In the nucleus, introns are cut out in pro-mRNA by special enzymes, and exon fragments are “spliced” together in a strict order. This process is called splicing. During the splicing process, mature mRNA is formed, which contains only the information that is necessary for the synthesis of the corresponding polypeptide, that is, the informative part of the structural gene.
The meaning and functions of introns are still not entirely clear, but it has been established that if only exon sections are read in DNA, mature mRNA is not formed. The splicing process was studied using the example of the ovalbumin gene. It contains one exon and 7 introns. First, pro-mRNA containing 7700 nucleotides is synthesized on DNA. Then in pro-mRNA the number of nucleotides decreases to 6800, then to 5600, 4850, 3800, 3400, etc. until 1372 nucleotides corresponding to the exon. Containing 1372 nucleotides, mRNA leaves the nucleus into the cytoplasm, enters the ribosome and synthesizes the corresponding polypeptide.
The next stage of biosynthesis - translation - occurs in the cytoplasm on ribosomes with the participation of tRNA.
Transfer RNAs are synthesized in the nucleus, but function in a free state in the cytoplasm of the cell. One tRNA molecule contains 76-85 nucleotides and has a rather complex structure, reminiscent of a clover leaf. Three sections of tRNA are of particular importance: 1) an anticodon, consisting of three nucleotides, which determines the site of attachment of the tRNA to the corresponding complementary codon (mRNA) on the ribosome; 2) a region that determines the specificity of tRNA, the ability of a given molecule to attach only to a specific amino acid; 3) acceptor site to which the amino acid is attached. It is the same for all tRNAs and consists of three nucleotides - C-C-A. The addition of an amino acid to tRNA is preceded by its activation by the enzyme aminoacyl-tRNA synthetase. This enzyme is specific for each amino acid. The activated amino acid is attached to the corresponding tRNA and delivered to the ribosome.
The central place in translation belongs to ribosomes - ribonucleoprotein organelles of the cytoplasm, which are present in large numbers in it. The sizes of ribosomes in prokaryotes are on average 30x30x20 nm, in eukaryotes - 40x40x20 nm. Typically, their sizes are determined in sedimentation units (S) - the rate of sedimentation during centrifugation in an appropriate medium. In the bacterium Escherichia coli, the ribosome has a size of 70S and consists of two subunits, one of which has a constant of 30S, the second 50S, and contains 64% ribosomal RNA and 36% protein.
The mRNA molecule leaves the nucleus into the cytoplasm and attaches to the small ribosomal subunit. Translation begins with the so-called start codon (initiator of synthesis) - A-U-G-. When tRNA delivers an activated amino acid to the ribosome, its anticodon is hydrogen bonded to the nucleotides of the complementary codon of the mRNA. The acceptor end of the tRNA with the corresponding amino acid is attached to the surface of the large ribosomal subunit. After the first amino acid, another tRNA delivers the next amino acid, and thus the polypeptide chain is synthesized on the ribosome. An mRNA molecule usually works on several (5-20) ribosomes at once, connected into polysomes. The beginning of the synthesis of a polypeptide chain is called initiation, its growth is called elongation. The sequence of amino acids in a polypeptide chain is determined by the sequence of codons in the mRNA. Synthesis of the polypeptide chain stops when one of the terminator codons appears on the mRNA - UAA, UAG or UGA. The end of the synthesis of a given polypeptide chain is called termination.
It has been established that in animal cells the polypeptide chain lengthens by 7 amino acids in one second, and the mRNA advances on the ribosome by 21 nucleotides. In bacteria, this process occurs two to three times faster.
Consequently, the synthesis of the primary structure of the protein molecule - the polypeptide chain - occurs on the ribosome in accordance with the order of alternation of nucleotides in the matrix ribonucleic acid - mRNA. It does not depend on the structure of the ribosome.
The process of protein biosynthesis is extremely important for the cell. Since proteins are complex substances that play a major role in tissues, they are essential. For this reason, a whole chain of protein biosynthesis processes is implemented in the cell, which occurs in several organelles. This guarantees the cell reproduction and the possibility of existence.
The essence of the protein biosynthesis process
The only place for protein synthesis is the rough one. Here the bulk of the ribosomes are located, which are responsible for the formation of the polypeptide chain. However, before the translation stage (the process of protein synthesis) begins, activation of the gene is required, which stores information about the protein structure. After this, copying of this section of DNA (or RNA, if bacterial biosynthesis is considered) is required.
After DNA is copied, the process of creating messenger RNA is required. On its basis, the synthesis of the protein chain will be performed. Moreover, all stages that occur with the involvement of nucleic acids must occur in However, this is not the place where protein synthesis occurs. where preparation for biosynthesis takes place.
Ribosomal protein biosynthesis
The main place where protein synthesis occurs is a cellular organelle, consisting of two subunits. There are a huge number of such structures in the cell, and they are mainly located on the membranes of the rough endoplasmic reticulum. The biosynthesis itself occurs as follows: messenger RNA formed in the cell nucleus exits through nuclear pores into the cytoplasm and meets the ribosome. The mRNA is then pushed into the gap between the ribosomal subunits, after which the first amino acid is fixed.
Amino acids are supplied to the place where protein synthesis occurs with the help of One such molecule can bring one amino acid at a time. They are attached in turn depending on the codon sequence of the messenger RNA. Also, synthesis may stop for some time.
When moving along mRNA, the ribosome can enter regions (introns) that do not code for amino acids. In these places, the ribosome simply moves along the mRNA, but no amino acids are added to the chain. Once the ribosome reaches the exon, that is, the region that codes for the acid, then it reattaches to the polypeptide.
Postsynthetic modification of proteins
After the ribosome reaches the stop codon of the messenger RNA, the process of direct synthesis is completed. However, the resulting molecule has a primary structure and cannot yet perform the functions reserved for it. In order to fully function, the molecule must be organized into a certain structure: secondary, tertiary or even more complex - quaternary.
Structural organization of protein
Secondary structure is the first stage of structural organization. To achieve this, the primary polypeptide chain must coil (form alpha helices) or fold (create beta sheets). Then, in order to take up even less space along the length, the molecule is further contracted and wound into a ball due to hydrogen, covalent and ionic bonds, as well as interatomic interactions. Thus, we get a globular
Quaternary protein structure
The quaternary structure is the most complex of all. It consists of several sections with a globular structure, connected by fibrillar strands of a polypeptide. In addition, the tertiary and quaternary structure may contain a carbohydrate or lipid residue, which expands the range of functions of the protein. In particular, glycoproteins, proteins and carbohydrates, are immunoglobulins and perform a protective function. Glycoproteins are also located on cell membranes and work as receptors. However, the molecule is modified not where protein synthesis occurs, but in the smooth endoplasmic reticulum. Here there is the possibility of attaching lipids, metals and carbohydrates to protein domains.