Brief description of how protein synthesis occurs in a cell. Protein biosynthesis: concise and understandable
The role of proteins in the cell and the body
The role of protein in cell life and the main stages of its synthesis. The structure and functions of ribosomes. The role of ribosomes in protein synthesis.
Proteins play an extremely important role in the life processes of the cell and the body, 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 a part of silk‚ cobwebs; 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 proceed with the participation of biological catalysts - enzymes (oxidoreductase, hydrolase, ligase, transferase, isomerase, and lyase).
Regulatory. For example, the hormones insulin and glucagon regulate glucose metabolism. Histone proteins are involved in the spatial organization of chromatin, and thus affect gene expression.
Transport. Hemoglobin carries oxygen in the blood of vertebrates, hemocyanin in the hemolymph of some invertebrates, myoglobin in the muscles. Serum albumin serves to transport fatty acids, lipids, etc. Membrane transport proteins provide active transport of substances through cell membranes (Na +, K + -ATPase). Cytochromes carry out the transfer of 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 the synthesis of viral protein in an infected cell. Fibrinogen and thrombin are involved in blood coagulation processes.
Contractile (motor). 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, albumin chicken egg, ferritin (stores iron in the spleen).
Protein toxins. diphtheria toxin.
Energy function. With the breakdown of 1 g of protein to the final metabolic products (CO2, H2O, NH3, H2S, SO2), 17.6 kJ or 4.2 kcal of energy is released.
Protein biosynthesis takes place in every living cell. It is most active in young growing cells, where proteins are synthesized for the construction of 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 a single protein is called a gene. A DNA molecule contains several hundred genes. A DNA molecule contains a code for the sequence of amino acids in a protein in the form of definitely combined nucleotides.
Protein synthesis - a complex multistage process representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.
In protein biosynthesis, the following steps are determined, which go to different parts cells:
First stage - i-RNA synthesis occurs in the nucleus, during which the information contained in the DNA gene is rewritten into i-RNA. This process is called transcription (from the Latin "transcript" - rewriting).
At the second stage there is a connection of amino acids with t-RNA molecules, which sequentially consist of three nucleotides - anticodons, with the help of which its 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, 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 embedded in DNA. This process ensures the renewal of proteins, metabolic processes, growth and development of cells, that is, all the processes of cell vital activity.
To study the processes occurring in the body, you need to know what is happening at the cellular level. Where proteins play an important role. It is necessary to study not only their functions, but also the process of creation. Therefore, it is important to explain briefly and clearly. Grade 9 is the best fit for this. It is at this stage that students have enough knowledge to understand this topic.
Proteins - what is it and what are they for
These macromolecular compounds play a huge role in the life of any organism. Proteins are polymers, that is, they consist of many similar “pieces”. Their number can vary from a few hundred to thousands.
Proteins perform many functions in the cell. Their role is also great at higher levels of organization: tissues and organs largely depend on the correct functioning of various proteins.
For example, all hormones are of protein origin. But it is these substances that control all processes in the body.
Hemoglobin is also a protein, it consists of four chains, which are connected in the center by an iron atom. This structure provides the ability to carry oxygen by erythrocytes.
Recall that all membranes contain proteins. They are necessary for the transport of substances through the cell membrane.
There are many more functions of protein molecules that they perform clearly and unquestioningly. These amazing compounds are very diverse not only in their roles in the cell, but also in structure.
Where does the synthesis take place
The ribosome is the organelle in which the main part of the process called "protein biosynthesis" takes place. Grade 9 in different schools differs in the curriculum for studying biology, but many teachers give material on organelles in advance, before studying translation.
Therefore, it will not be difficult for students to remember the material covered and consolidate it. You should be aware that only one polypeptide chain can be created on one organelle at a time. This is not enough to satisfy all the needs of the cell. Therefore, there are a lot of ribosomes, and most often they are combined with the endoplasmic reticulum.
Such EPS is called rough. The benefit of such “collaboration” is obvious: immediately after synthesis, the protein enters the transport channel and can be sent to its destination without delay.
But if we take into account the very beginning, namely the reading of information from DNA, then we can say that protein biosynthesis in a living cell begins in the nucleus. It is there that the genetic code is synthesized.
The necessary materials are amino acids, the place of synthesis is the ribosome
It seems that it is difficult to explain how protein biosynthesis proceeds, briefly and clearly, the process diagram and numerous drawings are simply necessary. They will help convey all the information, as well as students will be able to remember it easier.
First of all, for the synthesis you need " construction material"- amino acids. Some of them are produced by the body. Others can only be obtained from food, they are called indispensable.
The total number of amino acids is twenty, but due to the huge number of options in which they can be arranged in a long chain, protein molecules are very diverse. These acids are similar in structure, but differ in radicals.
It is the properties of these parts of each amino acid that determine which structure the resulting chain will “fold”, whether it will form a quaternary structure with other chains, and what properties the resulting macromolecule will have.
The process of protein biosynthesis cannot proceed simply in the cytoplasm, it needs a ribosome. consists of two subunits - large and small. At rest, they are separated, but as soon as synthesis begins, they immediately connect and begin to work.
Such different and important ribonucleic acids
In order to bring an amino acid to the ribosome, you need a special RNA called transport. It is abbreviated as tRNA. This single-stranded cloverleaf molecule is able to attach a single amino acid to its free end and ferry it to the site of protein synthesis.
Another RNA involved in protein synthesis is called matrix (information). It carries an equally important component of synthesis - a code that clearly states when which amino acid to chain to the resulting protein chain.
This molecule has a single-stranded structure, consists of nucleotides, as well as DNA. There are some differences in the primary structure of these nucleic acids, which you can read about in the comparative article on RNA and DNA.
Information about the composition of the protein mRNA receives from the main custodian of the genetic code - DNA. The process of reading and synthesizing mRNA is called transcription.
It occurs in the nucleus, from where the resulting mRNA is sent to the ribosome. The DNA itself does not leave the nucleus, its task is only to preserve the genetic code and transfer it to the daughter cell during division.
Summary table of the main participants of the broadcast
In order to describe protein biosynthesis concisely and clearly, a table is simply necessary. In it, we will write down all the components and their role in this process, which is called translation.
The very process of creating a protein chain is divided into three stages. Let's look at each of them in more detail. After that, you can easily explain protein biosynthesis to everyone who wants it in a short and understandable way.
Initiation - the beginning of the process
This initial stage translation, in which the small subunit of the ribosome joins with the very first tRNA. This ribonucleic acid carries the amino acid methionine. Translation always begins with this amino acid, since the start codon is AUG, which encodes this first monomer in the protein chain.
In order for the ribosome to recognize the start codon and not start synthesis from the middle of the gene, where the AUG sequence can also appear, a special nucleotide sequence is located around the start codon. It is from them that the ribosome recognizes the place where its small subunit should sit.
After the formation of the complex with mRNA, the initiation step ends. And the main stage of translation begins.
Elongation - the middle of synthesis
At this stage, a gradual build-up of the protein chain occurs. The duration of elongation depends on the number of amino acids in the protein.
First of all, the large subunit of the ribosome is attached to the small subunit. And the initial t-RNA is in it entirely. Outside, only methionine remains. Next, a second t-RNA carrying another amino acid enters the large subunit.
If the second codon on the mRNA matches the anticodon at the top of the cloverleaf, the second amino acid is attached to the first via a peptide bond.
After that, the ribosome moves along the m-RNA for exactly three nucleotides (one codon), the first t-RNA detaches methionine from itself and separates from the complex. In its place is a second t-RNA, at the end of which there are already two amino acids.
Then a third tRNA enters the large subunit and the process repeats. It will continue until the ribosome hits a codon in the mRNA that signals the end of translation.
Termination
This stage is the last one, it may seem very cruel to some. All the molecules and organelles that have worked so harmoniously to create a polypeptide chain stop as soon as the ribosome hits a terminal codon.
It does not code for any amino acid, so whatever tRNA goes into the large subunit will all be rejected due to a mismatch. This is where termination factors come into play, which separate the finished protein from the ribosome.
The organelle itself can either split into two subunits or continue down the mRNA in search of a new start codon. One mRNA can have several ribosomes at once. Each of them is at its own stage of translation. The newly created protein is provided with markers, with the help of which its destination will be clear to everyone. And by EPS it will be sent to where it is needed.
To understand the role of protein biosynthesis, it is necessary to study what functions it can perform. It depends on the sequence of amino acids in the chain. It is their properties that determine the secondary, tertiary, and sometimes quaternary (if it exists) and its role in the cell. You can read more about the functions of protein molecules in an article on this topic.
How to learn more about broadcasting
This article describes protein biosynthesis in a living cell. Of course, if you study the subject more deeply, it will take many pages to explain the process in all details. But the above material should be enough for a general idea. Video materials in which scientists have simulated all stages of translation can be very useful for understanding. Some of them have been translated into Russian and can serve as an excellent guide for students or just an educational video.
In order to understand the topic better, you should read other articles on related topics. For example, about or about the functions of proteins.
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 indispensable. For this reason, a whole chain of protein biosynthesis processes is realized in the cell, which proceeds in several organelles. This guarantees the cell reproduction and the possibility of existence.
The essence of the process of protein biosynthesis
The only place for protein synthesis is rough. Here is the bulk of the ribosomes that are responsible for the formation of the polypeptide chain. However, before the translation stage (the process of protein synthesis) begins, activation of the gene, which stores information about the protein structure, is required. After this, copying of this section of DNA (or RNA, if bacterial biosynthesis is considered) is required.
After copying the DNA, the process of creating messenger RNA is required. Based on it, the synthesis of the protein chain will be performed. Moreover, all the steps 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 the cell organelle, which consists 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: the messenger RNA formed in the nucleus of the cell exits through the nuclear pores into the cytoplasm and meets with the ribosome. Then mRNA is pushed into the gap between the subunits of the ribosome, after which the first amino acid is fixed.
Amino acids are supplied to the site where protein synthesis occurs with the help of one such molecule can bring one amino acid at a time. They join in turn, depending on the codon sequence of messenger RNA. Also, the synthesis may stop for a while.
When moving along the 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. As soon as the ribosome reaches the exon, that is, the site that codes for the acid, then it reattaches to the polypeptide.
Postsynthetic modification of proteins
After the ribosome reaches the stop codon of 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, a molecule must be organized into a certain structure: secondary, tertiary, or even more complex - quaternary.
Structural organization of a protein
Secondary structure is the first stage of structural organization. To achieve it, the primary polypeptide chain must coil (form alpha helices) or fold (create beta layers). Then, in order to take up even less space along the length, the molecule is even more contracted and coiled 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 filaments of the polypeptide. In addition, the tertiary and quaternary structure can contain a carbohydrate or lipid residue, which expands the spectrum of protein functions. In particular, glycoproteins, protein and carbohydrate, are immunoglobulins and perform a protective function. Also, glycoproteins are 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 attachment of lipids, metals and carbohydrates to protein domains.
First, establish the sequence of steps in protein biosynthesis, starting with transcription. The entire sequence of processes occurring during the synthesis of protein molecules can be combined into 2 stages:
Transcription.
Broadcast.
Structural units of hereditary information are genes - sections of the DNA molecule that encode the synthesis of a particular protein. In terms of chemical organization, the material of heredity and variability of pro- and eukaryotes is not fundamentally different. The genetic material in them is presented in the DNA molecule, the principle of recording hereditary information and the genetic code is also common. The same amino acids in pro- and eukaryotes are encrypted by the same codons.
The genome of modern prokaryotic cells is characterized by a relatively small size, the DNA of Escherichia coli has the form of a ring, about 1 mm long. It contains 4 x 10 6 base pairs, forming about 4000 genes. In 1961, F. Jacob and J. Monod discovered the cistronic, or continuous organization of prokaryotic genes, which consist entirely of coding nucleotide sequences, and they are entirely realized during protein synthesis. The hereditary material of the DNA molecule of prokaryotes is located directly in the cytoplasm of the cell, where the tRNA and enzymes necessary for gene expression are also located. Expression is the functional activity of genes, or gene expression. Therefore, mRNA synthesized with DNA is able to immediately act as a template in the process of translation of protein synthesis.
The eukaryotic genome contains much more hereditary material. In humans, the total length of DNA in the diploid set of chromosomes is about 174 cm. It contains 3 x 10 9 base pairs and includes up to 100,000 genes. In 1977, a discontinuity was discovered in the structure of most eukaryotic genes, which was called the "mosaic" gene. It has coding nucleotide sequences exonic And intron plots. Only exon information is used for protein synthesis. The number of introns varies in different genes. It has been established that the chicken ovalbumin gene includes 7 introns, and the mammalian procollagen gene - 50. The functions of silent DNA - introns have not been completely elucidated. It is assumed that they provide: 1) the structural organization of chromatin; 2) some of them are obviously involved in the regulation of gene expression; 3) introns can be considered as a store of information for variability; 4) they can play a protective role, taking on the action of mutagens.
Transcription
The process of rewriting information in the cell nucleus from a portion of a DNA molecule to an mRNA molecule (mRNA) is called transcription(lat. Transcriptio - rewriting). The primary product of the gene, mRNA, is synthesized. This is the first step in protein synthesis. On the corresponding section of DNA, the RNA polymerase enzyme recognizes the sign of the start of transcription - preview The starting point is considered to be the first DNA nucleotide, which is included by the enzyme in the RNA transcript. As a rule, coding regions begin with the codon AUG, sometimes GUG is used in bacteria. When RNA polymerase binds to the promoter, the DNA double helix is locally untwisted and one of the strands is copied according to the principle of complementarity. mRNA is synthesized, its assembly speed reaches 50 nucleotides per second. As the RNA polymerase moves, the mRNA chain grows, and when the enzyme reaches the end of the copying site - terminator, the mRNA moves away from the template. The DNA double helix behind the enzyme is repaired.
Transcription of prokaryotes takes place in the cytoplasm. Due to the fact that DNA consists entirely of coding nucleotide sequences, therefore, the synthesized mRNA immediately acts as a template for translation (see above).
Transcription of mRNA in eukaryotes occurs in the nucleus. It begins with the synthesis of large molecules - precursors (pro-mRNA), called immature, or nuclear RNA. The primary product of the gene - pro-mRNA is an exact copy of the transcribed DNA region, includes exons and introns. The process of formation of mature RNA molecules from precursors is called processing. mRNA maturation occurs by splicing are cuttings by enzymes restrictase introns and connection of sites with transcribed exon sequences by ligase enzymes. (Fig.). Mature mRNA is much shorter than pro-mRNA precursor molecules, the size of introns in them varies from 100 to 1000 nucleotides or more. Introns account for about 80% of all immature mRNA.
It has now been shown that it is possible alternative splicing, in which nucleotide sequences can be deleted from one primary transcript in its different regions and several mature mRNAs will be formed. This type of splicing is typical in the immunoglobulin gene system in mammals, which makes it possible to form mRNA based on a single transcript. different types antibodies.
Upon completion of processing, the mature mRNA is selected before leaving the nucleus. It has been established that only 5% of mature mRNA enters the cytoplasm, and the rest is cleaved in the nucleus.
Broadcast
Translation (lat. Translatio - transfer, transfer) - translation of information contained in the nucleotide sequence of the mRNA molecule into the amino acid sequence of the polypeptide chain (Fig. 10). This is the second stage of protein synthesis. The transfer of mature mRNA through the pores of the nuclear envelope produces special proteins that form a complex with the RNA molecule. In addition to mRNA transport, these proteins protect mRNA from the damaging effects of cytoplasmic enzymes. In the process of translation, tRNAs play a central role; they ensure the exact correspondence of the amino acid to the code of the mRNA triplet. The process of translation-decoding occurs in ribosomes and is carried out in the direction from 5 to 3. The complex of mRNA and ribosomes is called a polysome.
Translation can be divided into three phases: initiation, elongation, and termination.
Initiation.
At this stage, the entire complex involved in the synthesis of the protein molecule is assembled. There is a union of two ribosome subunits at a certain site of mRNA, the first aminoacyl - tRNA is attached to it, and this sets the frame for reading information. Any mRNA molecule contains a site that is complementary to the rRNA of the small subunit of the ribosome and specifically controlled by it. Next to it is the initiating start codon AUG, which encodes the amino acid methionine.
Elongation
- it includes all reactions from the moment of formation of the first peptide bond to the attachment of the last amino acid. The ribosome has two sites for the binding of two tRNA molecules. The first t-RNA with the amino acid methionine is located in one section, peptidyl (P), and the synthesis of any protein molecule begins from it. The second t-RNA molecule enters the second site of the ribosome - aminoacyl (A) and attaches to its codon. A peptide bond is formed between methionine and the second amino acid. The second tRNA moves along with its mRNA codon to the peptidyl center. The movement of tRNA with the polypeptide chain from the aminoacyl center to the peptidyl center is accompanied by the advancement of the ribosome along the mRNA by a step corresponding to one codon. The tRNA that delivered the methionine returns to the cytoplasm, and the amnoacyl center is released. It receives a new t-RNA with an amino acid encrypted by the next codon. A peptide bond is formed between the third and second amino acids, and the third tRNA, together with the mRNA codon, moves to the peptidyl center. The process of elongation, elongation of the protein chain. It continues until one of the three codons that do not code for amino acids enters the ribosome. This is a terminator codon and there is no corresponding tRNA for it, so none of the tRNAs can take a place in the aminoacyl center.
Termination
- completion of polypeptide synthesis. It is associated with the recognition by a specific ribosomal protein of one of the termination codons (UAA, UAG, UGA) when it enters the aminoacyl center. A special termination factor is attached to the ribosome, which promotes the separation of ribosome subunits and the release of the synthesized protein molecule. Water is attached to the last amino acid of the peptide and its carboxyl end is separated from the tRNA.
The assembly of the peptide chain is carried out at a high speed. In bacteria at a temperature of 37°C, it is expressed in the addition of 12 to 17 amino acids per second to the polypeptide. In eukaryotic cells, two amino acids are added to a polypeptide in one second.
The synthesized polypeptide chain then enters the Golgi complex, where the construction of the protein molecule is completed (second, third, fourth structures appear in succession). Here there is a complexation of protein molecules with fats and carbohydrates.
The whole process of protein biosynthesis is presented in the form of a scheme: DNA ® pro mRNA ® mRNA ® polypeptide chain ® protein ® protein complexing and their transformation into functionally active molecules.
The stages of the implementation of hereditary information also proceed in a similar way: first, it is transcribed into the nucleotide sequence of mRNA, and then translated into the amino acid sequence of the polypeptide on ribosomes with the participation of tRNA.
Transcription of eukaryotes is carried out under the action of three nuclear RNA polymerases. RNA polymerase 1 is located in the nucleolus and is responsible for the transcription of rRNA genes. RNA polymerase 2 is found in the nuclear sap and is responsible for the synthesis of the mRNA precursor. RNA polymerase 3 is a small fraction in the nuclear sap that synthesizes small rRNAs and tRNAs. RNA polymerases specifically recognize the nucleotide sequence of the transcription promoter. Eukaryotic mRNA is first synthesized as a precursor (pro-mRNA), information from exons and introns is written off to it. The synthesized mRNA is larger than necessary for translation and is less stable.
In the process of maturation of the mRNA molecule, introns are cut out with the help of restriction enzymes, and exons are sewn together with the help of ligase enzymes. The maturation of mRNA is called processing, and the joining of exons is called splicing. Thus, mature mRNA contains only exons and is much shorter than its predecessor, pro-mRNA. Intron sizes vary from 100 to 10,000 nucleotides or more. Intons account for about 80% of all immature mRNA. At present, the possibility of alternative splicing has been proven, in which nucleotide sequences can be deleted from one primary transcript in its different regions and several mature mRNAs will be formed. This type of splicing is characteristic of the immunoglobulin gene system in mammals, which makes it possible to form different types of antibodies based on a single mRNA transcript. Upon completion of processing, the mature mRNA is selected before being released into the cytoplasm from the nucleus. It has been established that only 5% of the mature mRNA enters, and the rest is cleaved in the nucleus. The transformation of the primary transcriptons of eukaryotic genes, associated with their exon-intron organization, and in connection with the transition of mature mRNA from the nucleus to the cytoplasm, determines the features of the realization of the genetic information of eukaryotes. Therefore, the eukaryotic mosaic gene is not a cistronome gene, since not all of the DNA sequence is used for protein synthesis.
The main question of genetics is the question of protein synthesis. Summarizing data on the structure and synthesis of DNA and RNA, Crick in 1960. proposed a matrix theory of protein synthesis based on 3 provisions:
1. Complementarity of nitrogenous bases of DNA and RNA.
2. The linear sequence of the location of genes in a DNA molecule.
3. The transfer of hereditary information can only occur from nucleic acid to nucleic acid or to protein.
From protein to protein, the transfer of hereditary information is impossible. Thus, only nucleic acids can be a template for protein synthesis.
Protein synthesis requires:
1. DNA (genes) on which molecules are synthesized.
2. RNA - (i-RNA) or (m-RNA), r-RNA, t-RNA
In the process of protein synthesis, the stages are distinguished: transcription and translation.
Transcription- census (rewriting) of information about the nucleic structure from DNA to RNA (t-RNA, and RNA, r-RNA).
Reading of hereditary information begins with a certain section of DNA, which is called a promoter. The promoter is located before the gene and includes about 80 nucleotides.
On the outer chain of the DNA molecule, i-RNA (intermediate) is synthesized, which serves as a matrix for protein synthesis and is therefore called matrix. It is an exact copy of the sequence of nucleotides on the DNA chain.
There are regions in DNA that do not contain genetic information (introns). The sections of DNA that contain information are called exons.
There are special enzymes in the nucleus that cut out introns, and exon fragments are “spliced” together in a strict order into a common thread, this process is called “splicing”. During splicing, mature mRNA is formed, which contains the information necessary for protein synthesis. Mature mRNA (matrix RNA) passes through the pores of the nuclear membrane and enters the channels of the endoplasmic reticulum (cytoplasm) and here it combines with ribosomes.
Broadcast- the sequence of nucleotides in i-RNA is translated into a strictly ordered sequence of amino acids in the synthesized protein molecule.
The translation process includes 2 stages: the activation of amino acids and the direct synthesis of a protein molecule.
One mRNA molecule binds to 5-6 ribosomes to form polysomes. Protein synthesis occurs on the mRNA molecule, with ribosomes moving along it. During this period, amino acids in the cytoplasm are activated by special enzymes secreted by enzymes secreted by mitochondria, each of them with its own specific enzyme.
Almost instantly, amino acids bind to another type of RNA - a low molecular weight soluble RNA that acts as an amino acid carrier to the mRNA molecule and is called transport (t-RNA). tRNA carries amino acids to the ribosomes to a certain place, where by this time the mRNA molecule is located. The amino acids are then joined together by peptide bonds to form a protein molecule. By the end of protein synthesis, the molecule is gradually shedding from mRNA.
On one mRNA molecule, 10-20 protein molecules are formed, and in some cases much more.
The most obscure question in protein synthesis is how tRNA finds the appropriate mRNA site to which the amino acid it brings must be attached.
The sequence of arrangement of nitrogenous bases in DNA, which determines the arrangement of amino acids in the synthesized protein, is the genetic code.
Since the same hereditary information is "recorded" in nucleic acids four characters (nitrogenous bases), and in proteins - twenty (amino acids). The problem of the genetic code is reduced to establishing a correspondence between them. Geneticists, physicists, and chemists played an important role in deciphering the genetic code.
To decipher the genetic code, first of all, it was necessary to find out what is the minimum number of nucleotides that can determine (encode) the formation of one amino acid. If each of the 20 amino acids were encoded by one base, then DNA would have to have 20 different bases, but in fact there are only 4. Obviously, the combination of two nucleotides is also not enough to code for 20 amino acids. It can only code for 16 amino acids 4 2 = 16.
Then it was proposed that the code includes 3 nucleotides 4 3 = 64 combinations and, therefore, is able to encode more than enough amino acids to form any proteins. This combination of three nucleotides is called a triplet code.
The code has the following properties:
1. The genetic code is triplet(each amino acid is encoded by three nucleotides).
2. Degeneracy- one amino acid can be encoded by several triplets, the exception is tryptophan and methionine.
3. In codons for one amino acid, the first two nucleotides are the same, and the third one changes.
4.Non-overlapping– triplets do not overlap each other. One triplet cannot be part of another; each of them independently encodes its own amino acid. Therefore, any two amino acids can be nearby in the polypeptide chain and any combination of them is possible, i.e. in the base sequence ABCDEFGHI, the first three bases code for 1 amino acid (ABC-1), (DEF-2), etc.
5.Universal, those. in all organisms, the codons for certain amino acids are the same (from chamomile to humans). The universality of the code testifies to the unity of life on earth.
6. Kneeling- the coincidence of the arrangement of codons in mRNA with the order of amino acids in the synthesized polypeptide chain.
A codon is a triplet of nucleotides that codes for 1 amino acid.
7. Pointless It does not code for any amino acid. Protein synthesis at this site is interrupted.
IN last years it turned out that the universality of the genetic code is violated in mitochondria, four codons in mitochondria have changed their meaning, for example, the codon UGA - answers to tryptophan instead of "STOP" - the cessation of protein synthesis. AUA - corresponds to methionine - instead of "isoleucine".
The discovery of new codons in mitochondria may serve as evidence that the code evolved and that it did not immediately become so.
Let hereditary information from a gene to a protein molecule can be expressed schematically.
DNA - RNA - protein
Studying chemical composition cells showed that different tissues of the same organism contain a different set of protein molecules, although they have the same number of chromosomes and the same genetic hereditary information.
We note the following circumstance: despite the presence in each cell of all the genes of the whole organism, very few genes work in a single cell - from tenths to several percent of the total number. The rest of the areas are "silent", they are blocked by special proteins. This is understandable, why, for example, hemoglobin genes work in a nerve cell? Just as the cell dictates which genes to be silent and which to work, it should be assumed that the cell has some kind of perfect mechanism that regulates the activity of genes, which determines which genes should be active at a given moment and which should be in an inactive (repressive) state. Such a mechanism, according to the French scientists F. Jacobo and J. Monod, was called induction and repression.
Induction- stimulation of protein synthesis.
Repression- inhibition of protein synthesis.
Induction ensures the work of those genes that synthesize a protein or enzyme, and which is necessary at this stage of the cell's life.
In animals, cell membrane hormones play an important role in the process of gene regulation; in plants, environmental conditions and other highly specialized inductors.
Example: when thyroid hormone is added to the medium, a rapid transformation of tadpoles into frogs takes place.
Milk sugar (lactose) is necessary for the normal functioning of the E (Coli) bacterium. If the environment in which the bacteria are located does not contain lactose, these genes are in a repressive state (i.e. they do not function). The lactose introduced into the medium is an inductor, including the genes responsible for the synthesis of enzymes. After the removal of lactose from the medium, the synthesis of these enzymes stops. Thus, the role of a repressor can be played by a substance that is synthesized in the cell, and if its content exceeds the norm or it is used up.
Different types of genes are involved in protein or enzyme synthesis.
All genes are in the DNA molecule.
Their functions are not the same:
- structural - genes that affect the synthesis of an enzyme or protein are located in the DNA molecule sequentially one after another in the order of their influence on the course of the synthesis reaction, or you can also say structural genes - these are genes that carry information about the amino acid sequence.
- acceptor- genes do not carry hereditary information about the structure of the protein, they regulate the work of structural genes.
Before a group of structural genes is a common gene for them - operator, and in front of him promoter. In general, this functional group is called feathered.
The entire group of genes of one operon is included in the synthesis process and is switched off from it simultaneously. Turning on and off structural genes is the essence of the entire process of regulation.
The function of switching on and off is performed by a special section of the DNA molecule - gene operator. The gene operator is the starting point of protein synthesis or, as they say, "reading" of genetic information. further in the same molecule at some distance is a gene - a regulator, under the control of which a protein called a repressor is produced.
From all of the above, it can be seen that protein synthesis is very difficult. The cell genetic system, using the mechanisms of repression and induction, can receive signals about the need to start and end the synthesis of a particular enzyme and carry out this process at a given rate.
The problem of regulating the action of genes in higher organisms is of great practical importance in animal husbandry and medicine. Establishment of the factors regulating protein synthesis would open up wide possibilities for controlling ontogeny, creating highly productive animals, as well as animals resistant to hereditary diseases.
Control questions:
1. Name the properties of genes.
2. What is a gene?
3. What is the biological significance of DNA, RNA.
4. Name the stages of protein synthesis
5. List the properties of the genetic code.