Proteins play a very important role in the life of organisms, performing protective, structural, hormonal, and energy functions. Provides muscle growth and bone tissue. Proteins inform about the structure of the cell, its functions and biochemical properties, and are part of valuable, useful to the body food products (eggs, dairy products, fish, nuts, legumes, rye and wheat). The digestibility of such food is explained biological value. With an equal amount of protein, the product whose value is higher will be easier to digest. Defective polymers must be removed from the body and replaced with new ones. This process occurs during the synthesis of proteins in cells.
What are proteins?
Substances consisting only of amino acid residues are called simple proteins (proteins). If necessary, their energetic property is used, so people leading healthy image life, often additional protein intake is needed. Complex proteins, proteids, contain a simple protein and a non-protein part. Ten amino acids in protein are essential, which means that the body cannot synthesize them on its own, they come from food, while the other ten are replaceable, that is, they can be created from other amino acids. This is how a vital process for all organisms begins.
The main stages of biosynthesis: where do proteins come from?
New molecules are made through biosynthesis, a chemical reaction of a compound. There are two main stages of protein synthesis in a cell. This is transcription and broadcast. Transcription occurs in the nucleus. This is a reading from DNA (deoxyribonucleic acid), which carries information about the future protein, to RNA (ribonucleic acid), which transfers this information from DNA to the cytoplasm. This happens due to the fact that DNA does not directly participate in biosynthesis; it only carries information, not having the ability to enter the cytoplasm where protein is synthesized, and performing only the function of a carrier of genetic information. Transcription allows you to read data from a DNA template into RNA according to the principle of complementarity.
The role of RNA and DNA in the process
So, the synthesis of proteins in cells is triggered by a DNA chain that carries information about a specific protein and is called a gene. The DNA chain unwinds during transcription, that is, its helix begins to disintegrate into a linear molecule. From DNA, the information must be converted to RNA. In this process, adenine should become opposite thymine. Cytosine has a guanine pair, just like DNA. Opposite to adenine, RNA becomes uracil, because in RNA such a nucleotide as thymine does not exist, it is simply replaced by uracil nucleotide. Cytosine is adjacent to guanine. Opposite adenine is uracil, and paired with thymine is adenine. These RNA molecules that are reversed are called messenger RNAs (mRNAs). They are capable of exiting the nucleus through pores into the cytoplasm and ribosomes, which, in fact, perform the function of protein synthesis in cells.
About the complex in simple words
Now the polypeptide chain of the protein is assembled from the amino acid sequences. Transcription can be called reading information about the future protein from a DNA template onto RNA. This can be defined as the first stage. After RNA leaves the nucleus, it must travel to the ribosomes, where a second step occurs, called translation.
Translation is already a transition of RNA, that is, the transfer of information from nucleotides to a protein molecule, when RNA tells what sequence of amino acids should be in the substance. In this order, messenger RNA enters the cytoplasm to the ribosomes, which carry out the synthesis of proteins in the cell: A (adenine) - G (guanine) - U (uracil) - C (cytosine) - U (uracil) - A (adenine).
Why are ribosomes needed?
In order for translation to occur and, as a result, a protein to be formed, components such as messenger RNA itself, transfer RNA, and ribosomes as a “factory” in which the protein is produced are needed. In this case, two types of RNA function: informational, which was formed in the nucleus with DNA, and transport. The second acid molecule has the appearance of clover. This “clover” attaches an amino acid to itself and carries it to the ribosomes. That is, it transports organic compounds directly to the “factory” for their formation.
How rRNA works
There are also ribosomal RNAs, which are part of the ribosome itself and perform protein synthesis in the cell. It turns out that ribosomes are non-membrane structures; they do not have membranes, like, for example, a nucleus or the endoplasmic reticulum, but consist simply of proteins and ribosomal RNA. What happens when a sequence of nucleotides, that is, messenger RNA, gets to the ribosomes?
Transfer RNA, which is located in the cytoplasm, pulls amino acids towards itself. Where do amino acids come from in the cell? And they are formed as a result of the breakdown of proteins that are ingested with food. These compounds are transported by the bloodstream to the cells, where the proteins necessary for the body are produced.
The final stage of protein synthesis in cells
Amino acids float in the cytoplasm just like transfer RNAs, and when the polypeptide chain is assembled directly, these transfer RNAs begin to combine with them. However, not in every sequence and not every transfer RNA can combine with all types of amino acids. There is a specific site to which the required amino acid is attached. The second section of transfer RNA is called an anticodon. This element consists of three nucleotides that are complementary to the nucleotide sequence in the messenger RNA. One amino acid requires three nucleotides. For example, for simplicity, a certain protein consists of only two amino acids. It is obvious that proteins generally have a very long structure and consist of many amino acids. The A - G - U chain is called a triplet, or codon, and transfer RNA in the form of a clover will be attached to it, at the end of which there will be a certain amino acid. The next triplet C - U - A will be joined by another tRNA, which will contain a completely different amino acid, complementary to this sequence. In this order, further assembly of the polypeptide chain will occur.
Biological significance of synthesis
A peptide bond is formed between the two amino acids located at the ends of the clovers of each triplet. At this stage, the transfer RNA enters the cytoplasm. The triplets are then joined by the next transfer RNA with another amino acid, which forms a polypeptide chain with the previous two. This process is repeated until the required amino acid sequence is reached. In this way, protein synthesis occurs in the cell, and enzymes, hormones, blood substances, etc. are formed. Not every cell produces any protein. Each cell can make a specific protein. For example, hemoglobin will be formed in red blood cells, and the cells of the pancreas will synthesize hormones and various enzymes that break down food that enters the body.
The proteins actin and myosin will be formed in the muscles. As you can see, the process of protein synthesis in cells is multi-stage and complex, which indicates its importance and necessity for all living things.
The most important functions of the body - metabolism, growth, development, transmission of heredity, movement, etc. - are carried out as a result of many chemical reactions involving proteins, nucleic acids and other biologically active substances. At the same time, various compounds are continuously synthesized in cells: building proteins, enzyme proteins, hormones. During metabolism, these substances are worn out and destroyed, and new ones are formed in their place. Since proteins create the material basis of life and accelerate all metabolic reactions, the vital activity of the cell and the organism as a whole is determined by the ability of cells to synthesize specific proteins. Their primary structure is predetermined by the genetic code in the DNA molecule.
Protein molecules consist of tens and hundreds of amino acids (more precisely, amino acid residues). For example, there are about 600 of them in a hemoglobin molecule, and they are distributed into four polypeptide chains; in the ribonuclease molecule there are 124 such amino acids, etc.
The main role in determining the primary structure of a protein belongs to molecules DNA. Its different sections encode the synthesis of different proteins; therefore, one DNA molecule is involved in the synthesis of many individual proteins. The properties of proteins depend on the sequence of amino acids in the polypeptide chain. In turn, the alternation of amino acids is determined by the sequence of nucleotides in DNA, and each amino acid corresponds to a specific triplet. It has been experimentally proven that, for example, a DNA section with an AAC triplet corresponds to the amino acid leucine, an ACC triplet to tryptophan, an ACA triplet to cysteine, etc. By dividing the DNA molecule into triplets, you can imagine which amino acids and in what sequence will be located in the protein molecule. A set of triplets constitutes the material basis of genes, and each gene contains information about the structure of a specific protein (a gene is the basic biological unit of heredity; chemically, a gene is a section of DNA that includes several hundred nucleotide pairs).
Genetic code - the historically established organization of DNA and RNA molecules, in which the sequence of nucleotides in them carries information about the sequence of amino acids in protein molecules. Code properties: triplet (codon), non-overlapping (codons follow each other), specificity (one codon can determine only one amino acid in a polypeptide chain), universality (in all living organisms the same codon determines the inclusion of the same amino acid in the polypeptide), redundancy (for most amino acids there are several codons). Triplets that do not carry information about amino acids are stop triplets, indicating the start site of synthesis i-RNA.(V.B. Zakharov. Biology. Reference materials. M., 1997)
Since DNA is located in the cell nucleus, and protein synthesis occurs in the cytoplasm, there is an intermediary that transfers information from DNA to ribosomes. RNA serves as such an intermediary, onto which the nucleotide sequence is rewritten, in exact accordance with that on DNA - according to the principle of complementarity. This process is called transcriptions and proceeds as a matrix synthesis reaction. It is characteristic only of living structures and underlies the most important property of living things - self-reproduction. Protein biosynthesis is preceded by template synthesis of mRNA on a DNA strand. The resulting mRNA leaves the cell nucleus into the cytoplasm, where ribosomes are strung on it, and amino acids are delivered here with the help of RNA.
Protein synthesis is a complex multi-step process that involves DNA, mRNA, tRNA, ribosomes, ATP and various enzymes. First, amino acids in the cytoplasm are activated by enzymes and attached to tRNA (to the site where the CCA nucleotide is located). At the next stage, amino acids are combined in the order in which the alternation of nucleotides from DNA is transferred to mRNA. This stage is called broadcast. On an mRNA strand there is not one ribosome, but a group of them - such a complex is called a polysome (N.E. Kovalev, L.D. Shevchuk, O.I. Shchurenko. Biology for preparatory departments of medical institutes).
Scheme Protein biosynthesis
Protein synthesis consists of two stages - transcription and translation.
I. Transcription (rewriting) - biosynthesis of RNA molecules, carried out in chromosomes on DNA molecules according to the principle of template synthesis. With the help of enzymes, all types of RNA (mRNA, rRNA, tRNA) are synthesized in the corresponding sections of the DNA molecule (genes). 20 varieties of tRNA are synthesized, since 20 amino acids take part in protein biosynthesis. Then mRNA and tRNA are released into the cytoplasm, rRNA is integrated into ribosomal subunits, which also exit into the cytoplasm.
II. Translation (transfer) is the synthesis of polypeptide chains of proteins, carried out in ribosomes. It is accompanied by the following events:
1. Formation of the functional center of the ribosome - FCR, consisting of mRNA and two ribosomal subunits. In the FCR there are always two triplets (six nucleotides) of mRNA, forming two active centers: A (amino acid) - the center for recognizing the amino acid and P (peptide) - the center for attaching the amino acid to the peptide chain.
2. Transport of amino acids attached to tRNA from the cytoplasm to the FCR. In the active center A, the anticodon of the tRNA is read with the codon of the mRNA; in the case of complementarity, a bond is formed, which serves as a signal for advancement (jump) along the ribosomal mRNA by one triplet. As a result of this, the complex “rRNA codon and tRNA with amino acid” moves to the active center of P, where the amino acid is added to the peptide chain (protein molecule). The tRNA then leaves the ribosome.
3. The peptide chain lengthens until translation ends and the ribosome jumps off the mRNA. One mRNA can contain several ribosomes at the same time (polysome). The polypeptide chain is immersed in the channel of the endoplasmic reticulum and there acquires a secondary, tertiary or quaternary structure. The assembly speed of one protein molecule, consisting of 200-300 amino acids, is 1-2 minutes. Formula for protein biosynthesis: DNA (transcription) --> RNA (translation) --> protein.
Having completed one cycle, polysomes can take part in the synthesis of new protein molecules.
The protein molecule separated from the ribosome has the form of a thread that is biologically inactive. It becomes biologically functional after the molecule acquires a secondary, tertiary and quaternary structure, that is, a certain spatially specific configuration. The secondary and subsequent structures of the protein molecule are predetermined in the information contained in the alternation of amino acids, i.e., in the primary structure of the protein. In other words, the program for the formation of a globule, its unique configuration, are determined by the primary structure of the molecule, which in turn is built under the control of the corresponding gene.
The rate of protein synthesis is determined by many factors: the temperature of the environment, the concentration of hydrogen ions, the amount of the final product of synthesis, the presence of free amino acids, magnesium ions, the state of ribosomes, etc.
protein biosynthesis happens differently. Nucleic acids - DNA and RNA - take an active part in protein biosynthesis, and energy is used as the necessary energy chemical bonds high-molecular organic compounds present in the cell, mainly ATP.The sequence of DNA nucleotides (i.e. genes), or the genetic code, is a system for recording information about the sequence of amino acids in proteins and is actually a code that ensures protein biosynthesis.
Genetic information, in accordance with the genetic code, at some point is rewritten from DNA, as from a matrix, into the nucleotide sequence of the thread informational RNA (mRNA). It then determines the sequence of amino acid assembly of the corresponding protein molecule.
It is important to note that the genetic code is universal for all organisms existing on Earth. This property of the universality of the code allows us to draw an important ideological conclusion about the unity of origin of all living organisms - prokaryotes, eukaryotes and viruses.
Currently, triplets for all 20 amino acids that make up 8 natural proteins have been deciphered. The genetic code was deciphered in the 60s. XX century This was done by biochemists X. Koran, M. Nirenberg And R. Holly. For deciphering the genetic code and its role in protein synthesis, these scientists were awarded the Nobel Prize in 1968.
Many structural components of the cell take an active part in biosynthesis: various molecules RNA, ribosomes and molecules of various amino acids from which the polymer protein molecule is built. Although the protein structure plan is encoded in DNA, it itself does not take part in the synthesis of protein molecules, but only serves matrix for the synthesis of messenger RNA (mRNA). Therefore, the process of protein synthesis consists of two stages: creation of mRNA And assembly of a protein molecule based on information in this mRNA molecule.
The synthesis of protein molecules occurs continuously. It proceeds at high speed: from 50 to 60 thousand peptide bonds are formed in 1 minute. The synthesis of one molecule usually lasts 3-4 seconds. The average lifespan of proteins is about two days, although individual proteins do not degrade for several months. As a result, half of the human body’s proteins (in total, about 17 kg of protein) are renewed in about 80 days. Material from the site
The process of biosynthesis at all its stages occurs with the participation of many enzymes and with the inevitable consumption large quantity energy.
The clear sequence of ongoing processes, their matrix organization and distribution of functions between all involved components lead to the conclusion that protein biosynthesis is an integral molecular system for performing complex reactions that ensures the creation of substances necessary for life.
Protein biosynthesis is a plastic part of cell metabolism. It is characterized by the matrix basis for the assembly of protein molecules. Synthesis occurs in ribosomes with the direct participation of mRNA, tRNA, rRNA and monomers - amino acids. Unlike photosynthesis, protein biosynthesis occurs under the strict control of genetic information copied by mRNA from the DNA genetic code. The process of biosynthesis of a protein molecule is determined by two stages: transcription (write-off) and translation (transmission).
In all living cells, proteins are synthesized by ribosomes . The ribosome is a large macromolecule with a complex asymmetric quaternary structure, built from ribonucleic acids (ribosomal RNA) and proteins. In order to synthesize protein, the ribosome must be equipped with:
1. A program that specifies the order of alternation of amino acid residues in the polypeptide chain of a protein.
2. Amino acid material from which protein is to be built.
3. Energy.
The ribosome itself has a catalytic (enzymatic) function, responsible for the formation of peptide bonds and, accordingly, the polymerization of amino acid residues into the polypeptide chain of a protein.
The program that sets the order of alternation of amino acid residues in the polypeptide chain of a protein comes from deoxyribonucleic acid (DNA), i.e., from the cell genome. Individual sections of double-stranded DNA, called genes, are templates for the synthesis of single-stranded RNA chains on them. The synthesized RNA strands are complementary to one of the DNA strands and thus exactly reproduce the deoxyribonucleotide sequence of the other DNA strand in their ribonucleotide sequence. The process of such gene copying, carried out by the enzyme RNA polymerase, is called transcription. RNA during and after synthesis, especially in eukaryotic cells, can undergo a number of additional changes called processing, during which certain pieces of the nucleotide sequence can be cut out of it. The resulting RNA then enters the ribosomes as a program that determines the amino acid sequence in the synthesized protein. It is called messenger RNA (mRNA). Thus, it is gene transcription and the formation of mRNA that ensures the flow of information from DNA to ribosomes.
The starting material from which proteins are built are amino acids. However, free amino acids are not used by the ribosome. In order to serve as a substrate for the ribosome, the amino acid must be activated by coupled cleavage of ATP and accepted (covalently attached) by a special RNA molecule called transfer or transfer RNA (tRNA), using the enzyme aminoacyl-tRNA- syntheses. The resulting aminoacyl-tRNA enters the ribosome as a substrate for protein synthesis. In addition, the energy of the chemical bond between the amino acid residue and tRNA is used for the formation of a peptide bond in the ribosome. Thus, the activation of amino acids and the formation of aminoacyl-tRNA provide the flow of both material and energy for ribosomal protein synthesis.
These three streams (information, material and energy) meet in the ribosome. Perceiving them, the ribosome translates, or translates, genetic information from the language of the nucleotide sequence of mRNA into the language of the amino acid sequence of the synthesized polypeptide chain of the protein. If we imagine this in molecular terms, then the ribosome sequentially scans the mRNA chain (moves along it) and also sequentially selects aminoacyl-tRNA from the environment, as a result of which the specificity of the aminoacyl residue of the aminoacyl-tRNA selected by the ribosome is determined each time by the specificity of the combination of nucleotides read in this moment ribosome of a piece of mRNA. Thus, the problem of the genetic code arises: what combinations of nucleotides determine, i.e. encode, each of the 20 amino acids from which protein molecules are built?
The movement of the ribosome along the mRNA chain (or, in other words, the passage of the mRNA chain through the ribosome) sets a strict temporal order for the entry of different aminoacyl-tRNAs into the ribosome in accordance with the order of the coding nucleotide combinations along the mRNA. The aminoacyl residue of the selected aminoacyl-tRNA is each time covalently attached by the ribosome to the growing polypeptide chain. The deacylated tRNA is released from the ribosome into solution. This is how the polypeptide chain of a protein is built sequentially, step by step (see Scheme 1).
Protein biosynthesis and genetic code
Definition 1
Protein biosynthesis– enzymatic process of protein synthesis in a cell. It involves three structural elements cells - nucleus, cytoplasm, ribosomes.
In the cell nucleus, DNA molecules store information about all the proteins that are synthesized in it, encrypted using a four-letter code.
Definition 2
Genetic code is the sequence of nucleotides in a DNA molecule, which determines the sequence of amino acids in a protein molecule.
The properties of the genetic code are as follows:
The genetic code is triplet, that is, each amino acid has its own code triplet ( codon), consisting of three adjacent nucleotides.
Example 1
The amino acid cysteine is encoded by the triplet A-C-A, valine - by the triplet C-A-A.
The code does not overlap, that is, the nucleotide cannot be part of two neighboring triplets.
The code is degenerate, that is, one amino acid can be encoded by several triplets.
Example 2
The amino acid tyrosine is encoded by two triplets.
The code does not have commas (separating marks), information is read in triplets of nucleotides.
Definition 3
Gene – a section of a DNA molecule that is characterized by a specific sequence of nucleotides and determines the synthesis of one polypeptide chain.
The code is universal, that is, the same for all living organisms - from bacteria to humans. All organisms have the same 20 amino acids, which are encoded by the same triplets.
Stages of protein biosynthesis: transcription and translation
The structure of any protein molecule is encoded in DNA, which is not directly involved in its synthesis. It serves only as a template for RNA synthesis.
The process of protein biosynthesis occurs on ribosomes, which are located primarily in the cytoplasm. This means that in order to transfer genetic information from DNA to the site of protein synthesis, an intermediary is needed. This function is performed by mRNA.
Definition 4
The process of synthesis of an mRNA molecule on one strand of a DNA molecule based on the principle of complementarity is called transcription, or rewriting.
Transcription occurs in the cell nucleus.
The transcription process is carried out simultaneously not on the entire DNA molecule, but only on it small area, which corresponds to a specific gene. In this case, part of the DNA double helix unwinds and a short section of one of the chains is exposed - now it will serve as a template for mRNA synthesis.
Then the enzyme RNA polymerase moves along this chain, connecting nucleotides into an mRNA chain, which elongates.
Note 2
Transcription can simultaneously occur on several genes on the same chromosome and on genes on different chromosomes.
The resulting mRNA contains a nucleotide sequence that is an exact copy of the nucleotide sequence on the template.
Note 3
If the DNA molecule contains the nitrogenous base cytosine, then the mRNA contains guanine and vice versa. The complementary pair in DNA is adenine - thymine, and RNA contains uracil instead of thymine.
Two other types of RNA are also synthesized on special genes - tRNA and rRNA.
The beginning and end of the synthesis of all types of RNA on the DNA template are strictly fixed by special triplets that control the start (initiating) and stopping (terminal) of synthesis. They act as “dividing marks” between genes.
The combination of tRNA with amino acids occurs in the cytoplasm. The tRNA molecule is shaped like a clover leaf, with a anticodon– a triplet of nucleotides that encodes the amino acid that this tRNA carries.
There are as many types of amino acids as there are tRNAs.
Note 4
Since many amino acids can be encoded by several triplets, the number of tRNAs is more than 20 (about 60 tRNAs are known).
The connection of tRNA with amino acids occurs with the participation of enzymes. tRNA molecules transport amino acids to ribosomes.
Definition 5
Broadcast is a process by which information about the structure of a protein, recorded in mRNA as a sequence of nucleotides, is implemented as a sequence of amino acids in the protein molecule that is synthesized.
This process takes place in ribosomes.
First, the mRNA attaches to the ribosome. The first ribosome, which synthesizes protein, is “strung” on the mRNA. As the ribosome moves to the end of the mRNA that has become free, a new ribosome is “strung” on. One mRNA can simultaneously contain more than 80 ribosomes that synthesize the same protein. Such a group of ribosomes connected to one mRNA is called polyribosome, or polysome. The type of protein that is synthesized is determined not by the ribosome, but by the information recorded on the mRNA. The same ribosome is capable of synthesizing different proteins. After protein synthesis is completed, the ribosome is separated from the mRNA, and the protein enters the endoplasmic reticulum.
Each ribosome consists of two subunits - small and large. The mRNA molecule attaches to the small subunit. At the site of contact between the ribosome and iRNA there are 6 nucleotides (2 triplets). One of them is constantly approached from the cytoplasm by tRNAs with different amino acids and touched with the anticodon of the mRNA codon. If the codon and anticodon triplets turn out to be complementary, a peptide bond occurs between the amino acid of the already synthesized part of the protein and the amino acid that is delivered by the tRNA. The combination of amino acids into a protein molecule is carried out with the participation of the enzyme synthetase. The tRNA molecule gives up the amino acid and moves into the cytoplasm, and the ribosome moves one triplet of nucleotides. This is how the polypeptide chain is sequentially synthesized. All this continues until the ribosome reaches one of three stop codons: UAA, UAG or UGA. After this, protein synthesis stops.
Note 5
Thus, the sequence of mRNA codons determines the sequence of inclusion of amino acids in the protein chain. The synthesized proteins enter the channels of the endoplasmic reticulum. One protein molecule in a cell is synthesized in 1 - 2 minutes.