After the discovery of the principle of molecular organization of a substance such as DNA in 1953, molecular biology began to develop. Further, in the process of research, scientists found out how DNA is recombined, its composition, and how our human genome is arranged.
Every day, at the molecular level, complex processes take place. How is the DNA molecule arranged, what does it consist of? What role do DNA molecules play in a cell? Let's talk in detail about all the processes occurring inside the double chain.
What is hereditary information?
So how did it all start? Back in 1868 found in the nuclei of bacteria. And in 1928, N. Koltsov put forward the theory that it is in DNA that all genetic information about a living organism is encrypted. Then J. Watson and F. Crick found a model for the now well-known DNA helix in 1953, for which they deserved recognition and an award - the Nobel Prize.
What is DNA anyway? This substance consists of 2 combined threads, more precisely spirals. A section of such a chain with certain information is called a gene.
DNA stores all the information about what kind of proteins will be formed and in what order. A DNA macromolecule is a material carrier of incredibly voluminous information, which is recorded in a strict sequence of individual building blocks - nucleotides. There are 4 nucleotides in total, they complement each other chemically and geometrically. This principle of complementation, or complementarity, in science will be described later. This rule plays a key role in encoding and decoding genetic information.
Since the DNA strand is incredibly long, there are no repetitions in this sequence. Every living being has its own unique DNA strand.
Functions of DNA
The functions include the storage of hereditary information and its transmission to offspring. Without this function, the genome of a species could not be preserved and developed over millennia. Organisms that have undergone major gene mutations are more likely to not survive or lose their ability to produce offspring. So there is a natural protection against the degeneration of the species.
Another essential function is the implementation of stored information. The cell cannot make any vital protein without the instructions that are stored in the double strand.
Composition of nucleic acids
Now it is already reliably known what the nucleotides themselves, the building blocks of DNA, consist of. They include 3 substances:
- Orthophosphoric acid.
- nitrogenous base. Pyrimidine bases - which have only one ring. These include thymine and cytosine. Purine bases containing 2 rings. These are guanine and adenine.
- Sucrose. DNA contains deoxyribose, RNA contains ribose.
The number of nucleotides is always equal to the number of nitrogenous bases. In special laboratories, a nucleotide is cleaved and a nitrogenous base is isolated from it. So they study the individual properties of these nucleotides and possible mutations in them.
Levels of organization of hereditary information
There are 3 levels of organization: gene, chromosomal and genomic. All the information needed for the synthesis of a new protein is contained in a small section of the chain - the gene. That is, the gene is considered the lowest and simplest level of encoding information.
Genes, in turn, are assembled into chromosomes. Thanks to such an organization of the carrier of hereditary material, groups of traits alternate according to certain laws and are transmitted from one generation to another. It should be noted that there are incredibly many genes in the body, but information is not lost, even when it is recombined many times.
There are several types of genes:
- according to their functional purpose, 2 types are distinguished: structural and regulatory sequences;
- according to the influence on the processes occurring in the cell, there are: supervital, lethal, conditionally lethal genes, as well as mutator and antimutator genes.
Genes are arranged along the chromosome in a linear order. In chromosomes, information is not randomly focused, there is a certain order. There is even a map showing positions, or gene loci. For example, it is known that data on the color of the eyes of a child is encrypted in chromosome number 18.
What is a genome? This is the name of the entire set of nucleotide sequences in the cell of the body. The genome characterizes the whole species, not a single individual.
What is the human genetic code?
The fact is that the whole huge potential of human development is laid down already in the period of conception. All hereditary information that is necessary for the development of the zygote and the growth of the child after birth is encrypted in the genes. Sections of DNA are the most basic carriers of hereditary information.
Humans have 46 chromosomes, or 22 somatic pairs plus one sex-determining chromosome from each parent. This diploid set of chromosomes encodes the entire physical appearance of a person, his mental and physical abilities and predisposition to diseases. Somatic chromosomes are outwardly indistinguishable, but they carry different information, since one of them is from the father, the other is from the mother.
The male code differs from the female code in the last pair of chromosomes - XY. The female diploid set is the last pair, XX. Males get one X chromosome from their biological mother, and then it is passed on to their daughters. The sex Y chromosome is passed on to sons.
Human chromosomes vary greatly in size. For example, the smallest pair of chromosomes is #17. And the biggest pair is 1 and 3.
The diameter of the double helix in humans is only 2 nm. The DNA is so tightly coiled that it fits in the small nucleus of the cell, although it will be up to 2 meters long if unwound. The length of the helix is hundreds of millions of nucleotides.
How is the genetic code transmitted?
So, what role do DNA molecules play in a cell during division? Genes - carriers of hereditary information - are inside every cell of the body. In order to pass on their code to a daughter organism, many creatures divide their DNA into 2 identical helices. This is called replication. In the process of replication, DNA unwinds and special "machines" complete each chain. After the genetic helix bifurcates, the nucleus and all organelles begin to divide, and then the whole cell.
But a person has a different process of gene transfer - sexual. The signs of the father and mother are mixed, the new genetic code contains information from both parents.
The storage and transmission of hereditary information is possible due to the complex organization of the DNA helix. After all, as we said, the structure of proteins is encrypted in genes. Once created at the time of conception, this code will copy itself throughout life. The karyotype (personal set of chromosomes) does not change during the renewal of organ cells. The transmission of information is carried out with the help of sex gametes - male and female.
Only viruses containing a single strand of RNA are unable to transmit their information to their offspring. Therefore, in order to reproduce, they need human or animal cells.
Implementation of hereditary information
Important processes are constantly taking place in the cell nucleus. All information recorded in chromosomes is used to build proteins from amino acids. But the DNA strand never leaves the nucleus, so another important compound, RNA, is needed here. Just RNA is able to penetrate the nuclear membrane and interact with the DNA chain.
Through the interaction of DNA and 3 types of RNA, all encoded information is realized. At what level is the implementation of hereditary information? All interactions occur at the nucleotide level. Messenger RNA copies a segment of the DNA chain and brings this copy to the ribosome. Here begins the synthesis of the nucleotides of a new molecule.
In order for the mRNA to copy the necessary part of the chain, the helix unfolds and then, upon completion of the recoding process, is restored again. Moreover, this process can occur simultaneously on 2 sides of 1 chromosome.
The principle of complementarity
They consist of 4 nucleotides - these are adenine (A), guanine (G), cytosine (C), thymine (T). They are connected by hydrogen bonds according to the rule of complementarity. The works of E. Chargaff helped to establish this rule, since the scientist noticed some patterns in the behavior of these substances. E. Chargaff discovered that the molar ratio of adenine to thymine is equal to one. And in the same way, the ratio of guanine to cytosine is always equal to one.
Based on his work, geneticists have formed a rule for the interaction of nucleotides. The rule of complementarity states that adenine combines only with thymine, and guanine with cytosine. During the decoding of the helix and the synthesis of a new protein in the ribosome, this alternation rule helps to quickly find the necessary amino acid that is attached to the transfer RNA.
RNA and its types
What is hereditary information? nucleotides in the DNA double strand. What is RNA? What is her job? RNA, or ribonucleic acid, helps to extract information from DNA, decode it, and, based on the principle of complementarity, create proteins necessary for cells.
In total, 3 types of RNA are isolated. Each of them performs strictly its function.
- Informational (mRNA), or it is also called matrix. It goes right into the center of the cell, into the nucleus. It finds in one of the chromosomes the necessary genetic material for building a protein and copies one of the sides of the double chain. Copying occurs again according to the principle of complementarity.
- Transport- This is a small molecule that has nucleotide decoders on one side, and amino acids corresponding to the main code on the other side. The task of tRNA is to deliver it to the "workshop", that is, to the ribosome, where it synthesizes the necessary amino acid.
- rRNA is ribosomal. It controls the amount of protein that is produced. Consists of 2 parts - amino acid and peptide site.
The only difference when decoding is that RNA does not have thymine. Instead of thymine, uracil is present here. But then, in the process of protein synthesis, with tRNA, it still correctly establishes all the amino acids. If there are any failures in the decoding of information, then a mutation occurs.
Repair of a damaged DNA molecule
The process of repairing a damaged double strand is called reparation. During the repair process, damaged genes are removed.
Then the required sequence of elements is exactly reproduced and crashes back into the same place on the chain from where it was extracted. All this happens thanks to special chemicals - enzymes.
Why do mutations occur?
Why do some genes begin to mutate and cease to fulfill their function - the storage of vital hereditary information? This is due to a decoding error. For example, if adenine is accidentally replaced with thymine.
There are also chromosomal and genomic mutations. Chromosomal mutations occur when pieces of hereditary information are missing, duplicated, or even transferred and integrated into another chromosome.
Genomic mutations are the most serious. Their cause is a change in the number of chromosomes. That is, when instead of a pair - a diploid set, a triploid set is present in the karyotype.
The most famous example of a triploid mutation is Down syndrome, in which the personal set of chromosomes is 47. In such children, 3 chromosomes are formed in place of the 21st pair.
There is also such a mutation as polyploidy. But polyploidy is found only in plants.
Genetic code- a way to record in a DNA molecule information about the number and order of amino acids in a protein.
Properties:
Tripletity - one amino acid is encoded by three nucleotides
Non-overlapping - the same nucleotide cannot be part of two or more triplets at the same time
Unambiguity (specificity) - a certain codon corresponds to only one
Universality - the genetic code works the same in organisms of different levels of complexity - from viruses to humans
Degeneracy (redundancy) - several codons can correspond to the same amino acid.
14. Stages of implementation of hereditary information in prokaryotes and eukaryotes.
Replication (synthesis) of DNA
DNA synthesis always starts at strictly defined points. The enzyme topoisomerase unwinds the helix. Helicase breaks hydrogen bonds between DNA strands and forms a replication fork. SSB proteins prevent the re-formation of hydrogen bonds.
RNA primase synthesizes short RNA fragments (primers) that are attached to the 3' end.
DNA polymerase starts from the primer and synthesizes a daughter chain (5 "3") -
The direction of synthesis of one strand of DNA coincides with the direction of movement of the replication fork, so this strand is synthesized continuously. Here the synthesis proceeds rapidly. The direction of synthesis of the second strand is opposite to that of the replication fork. Therefore, the synthesis of this chain occurs in the form of separate sections and proceeds slowly (Okazaki fragments).
DNA maturation: RNA primers are cleaved, missing nucleotides are completed, DNA fragments are joined using ligase. Topoisomerase unwinds the helix.
Stages of implementation of hereditary information (in eukaryotes)
1. Transcription
2.Processing
3. Translation
4. Post-translational changes
Broadcast- the synthesis of an RNA molecule based on a DNA molecule. The key enzyme is RNA polymerase.
RNA polymerase must recognize the promoter and interact with it. A promoter is a special section of DNA that is located before the informative part of the gene. Interaction with the promoter is necessary for the activation of RNA polymerase. Upon activation, RNA polymerase breaks hydrogen bonds between DNA strands.
RNA synthesis always occurs along a certain codogenic DNA strand. On this strand, the promoter is located closer to the 3' end.
Synthesis of RNA occurs according to the principles of complementarity and antiparallelism.
RNA polymerase reaches a stop codon (terminator or termination codon). This is a signal to stop synthesis. The enzyme is inactivated, separated from DNA, and a newly synthesized DNA molecule is released - the primary transcript - pro-RNA. The original DNA structure is restored.
Structural features of the eukaryotic gene:
In eukaryotes, genes include regions of various functions.
A) Introns - fragments of DNA (gene) that do not code for amino acids in the protein
B) Exons are sections of DNA that code for amino acids in a protein.
The discontinuous nature of the gene was discovered by Roberts and Sharpe (Nob. Prize 1903).
The number of introns and exons in different genes varies greatly.
Processing(maturation)
The primary transcript matures and a mature messenger RNA molecule is formed, which can participate in protein synthesis on ribosomes.
At the 5" end of the RNA, a special site (structure) is formed - a CEP or a cap. The CEP provides interaction with the small subunit of the ribosome.
At the 3" end of RNA, from 100 to 200 molecules of nucleotides carrying adenine (polyA) are attached. During protein synthesis, these nucleotides are gradually cleaved off, the destruction of polyA is a signal for the destruction of RNA molecules.
A CH 3 group is added to some RNA nucleotides - methylation. This increases the resistance of DNA to the action of cytoplasmic enzymes.
Splicing - introns are cut out and exons are stitched together. Restriction enzyme removes, ligase cross-links)
Mature messenger RNA includes:
The leader ensures the binding of messenger RNA to the ribosome subunit.
SC - start codon - the same for all messenger RNAs, codes for an amino acid
Coding region - codes for amino acids in a protein.
Stop codon - a signal to stop protein synthesis.
During processing, a strict selection into the cytoplasm occurs, about 10% of the molecules from the number of primary transcripts are released from the nucleus.
Alternative splicing
A person has 25-30 thousand genes.
However, about 100 thousand proteins have been isolated in humans.
Alternative splicing is a situation in which the same gene provides the synthesis of the same proRNA molecules in cells of different tissues. In different cells, the number and boundaries between exons and introns are determined differently. As a result, different mRNAs are obtained from the same primary transcripts and different proteins are synthesized.
Alternative splicing has been proven for about 50% of human genes.
Translation is the process of assembling a peptide chain on ribosomes according to the information contained in mRNA.
1. Initiation (beginning)
2. Elongation (elongation of the molecule)
3. Termination (end)
Initiation.
The matrRNA molecule contacts the small subunit of the ribosome with the help of CEP. The RNA leader binds to a subunit of the ribosome. The transpRNA, which carries the transport acid methionine, is attached to the start codon. Then the large subunit of the ribosome joins. In the whole ribosome, two active centers are formed: aminoacyl and peptidyl. Aminoacyl is free, and peptidyl is occupied by tRNA with methionine.
Elongation.
The aminoacyl center contains mRNA, the anticodon of which corresponds to the coding one.
After that, the ribosome shifts relative to mRNA by 1 codon. In this case, the aminoacyl center is released. The mRNA is located in the peptidyl center and binds to the second amino acid. The process is cyclically repeated.
3. Termination
A stop codon enters the aminoacyl center, which is recognized by a special protein, this is a signal to stop protein synthesis. The subunits of the ribosome are separated, releasing mRNA, and the polypeptide is synthesized again.
4. Posttranslational changes.
During translation, the primary structure of the polypeptide is formed. This is not enough to perform the functions of the protein, so the protein changes, which ensures its activity.
Formed:
A) secondary structure (hydrogen bonds)
B) globule - tertiary structure (disulfide bonds)
C) quaternary structure - hemoglobin
D) Glycosylation - attachment of sugar residues (antibodies) to the protein
E) cleavage of a large polypeptide into several fragments.
Differences in the implementation of hereditary information in prokaryotes and eukaryotes:
1. Prokaryotes lack exons and introns, so there are no stages of processing and splicing.
2. In prokaryotes, transcription and translation occur simultaneously, i.e. RNA synthesis is in progress and DNA synthesis is already beginning.
3. In eukaryotes, the synthesis of various types of RNA is controlled by various enzymes. In prokaryotes, all types of RNA are synthesized by one enzyme.
4. In eukaryotes, each gene has its own unique promoter; in prokaryotes, one promoter can control the work of several genes.
5. Only prokaryotes have an operon system
In the first quarter of the XX century. it was shown that elementary inherited traits are due to material units of heredity - genes localized in chromosomes, where they are located sequentially one after another in a linear order. On this basis, T. X. Morgan developed chromosome theory of heredity, for which he received the 1933 Nobel Prize in Physiology or Medicine "for his discoveries concerning the role of chromosomes in heredity."
Scientists also tried to determine the "products" of gene activity, that is, those molecules that are synthesized in cells under their control. In the works of Ephrussi, Beadle and Tatum, on the eve of World War II, the idea was put forward that genes produce proteins, but for this a gene must store information for the synthesis of a particular protein (enzyme). The complex mechanism for the realization of the information contained in DNA and its translation into the form of a protein was discovered only in the 60s of the last century.
GENETIC CODE.The idea that information about the primary structure of a protein is encoded in a gene was presented by F. Crick in his sequence hypothesis, according to which the sequence of the structural elements of the gene determines the sequence of amino acid residues in the synthesized polypeptide chain. The author of the hypothesis assumed that the code is most likely triplet, that the coding unit is represented by three pairs of DNA bases arranged in a certain sequence. Indeed, four base pairs of DNA: A-T, T-A, G-C, C-G - can encode only 4 amino acids, if we assume that each pair corresponds to one amino acid. Proteins are known to be made up of 20 basic amino acids. If we assume that each amino acid corresponds to two base pairs, then 16 amino acids (4 2) can be encoded. This is also not enough. With a triplet code of four base pairs, 64 codons (4 3) can be made, and this is more than enough to encode 20 amino acids. Experimental evidence that the genetic code is triplet was published in 1961 (F. Crick et al.). In the same year, at the V International Biochemical Congress in Moscow, M. Nirenberg and J. Mattei reported on the decoding of the first codon (UUU - the codon for phenylalanine) and, more importantly, proposed a method for determining the composition of codons in the cell-free system of protein synthesis.
Two questions immediately arose: is the code overlapping and is the code degenerate?
If the codons overlapped, then the replacement of one pair of bases would lead to the replacement of two or three amino acids at once in the synthesized protein. In reality, this does not happen, and the genetic code is considered non-overlapping.
The code is degenerate since almost every amino acid is associated with more than one codon, which determines their arrangement in the primary structure of the synthesized polypeptide chain. Only two amino acids - methionine and tryptophan - are associated with single codons - AUG and UGG, respectively. The arrangement of each of the three amino acids - arginine, leucine and serine - in the primary structure of the polypeptide chain is determined by six codons, etc. (see Table 3.2).
Among the features of the genetic code is also its versatility(it is basically the same for all living organisms). However, exceptions to this rule have also been found. In 1981, the determination of the complete nucleotide sequence of human mitochondrial DNA, containing 16,569 nucleotide pairs, was completed. The results obtained indicate that the mitochondrial genomes of higher and lower eukaryotes, encoding approximately the same set of functions, are characterized by differences in the semantic meaning of some codons, rules of anticodon-codon recognition, and general structural organization. So, it turned out that, unlike the usual universal code, the AUA codon encodes methionine instead of isoleucine, and the AGA and AGG triplets are not arginine codons, but termination signals. broadcasts; tryptophan is encoded by both the UGG triplet and the UGA triplet, which usually functions as a terminator codon.
In the genetic code, different codons of the same amino acid, i.e., synonymous codons, are almost always in the same square and differ from each other in the last of the three nucleotides (the only exceptions are the codons of arginine, serenium and leucine, which have six codons each , which cannot fit in one square, where only four codons fit). The genetic code has a linear reading order and is characterized by colinearity , i.e., the coincidence of the order of arrangement of codons in mRNA with the order of arrangement of amino acids of the synthesized half-dipeptide chain.
SYNTHESISPROTEIN IN A CAGE. The reproduction and action of genes are associated with matrix processes: the synthesis of macromolecules - DNA, RNA, proteins. Replication has already been considered above as a process that ensures the reproduction of genetic information. The modern theory of the gene, an achievement of molecular genetics, relies entirely on the success of biochemistry in the study of matrix processes. Conversely, the method of genetic analysis makes a significant contribution to the study of matrix processes, which are themselves under genetic control. Consider the action of a gene that provides transcription, or RNA synthesis, and broadcast, or protein synthesis.
TranscriptionDNA, This - transfer of genetic information encoded in a sequence of nucleotide pairs from a double-stranded DNA molecule to a single-stranded RNA molecule. The template for RNA synthesis is only one strand of DNA, called semantic.
In transcription, as in other matrix processes, there are three stages: initiation, elongation And termination. The enzyme that carries out this process is called DNA-dependent RNA polymerase, or simply RNA~polymerase; in this case, the polymerization of polyribonucleotide (RNA) occurs in the direction from the 5 "to the 3" end of the growing chain.
The synthesis of enzymes and other proteins necessary for the life and development of organisms occurs mainly at the first stage of the interphase, before the start of DNA replication.
As a result of transcription, the hereditary information recorded in the DNA of the gene is precisely transcribed(rewritten) into the nucleotide sequence of darkness. mRNA synthesis begins at the site of transcription initiation called promoter. The promoter is located in front of the gene and includes about 80 base pairs (in viruses and bacteria, this region corresponds to about one turn of the DNA helix and includes about 10 base pairs). Promoter nucleotide sequences often contain pairs of ATs, which is why they are also called TATA sequences.
Transcription is carried out with the help of RNA polymerase enzymes. In eukaryotes, three types of RNA polymerases are known: I - responsible for the synthesis of rRNA, II - for the synthesis of mRNA; III - for the synthesis of tRNA and low molecular weight rRNA - 5S RNA.
RNA polymerase binds strongly to the promoter and separates the nucleotides of complementary chains. Then this enzyme begins to move along the gene (DNA molecule) and, as the chains are disconnected, leads to the (sense) synthesis of mRNA on one of them, adding, according to the complementary principle, adenine to thymine, uracil to adenine, guanine to cytosine and cytosine to guanine. Those sections of DNA on which the polymerase formed mRNA are reconnected, and the synthesized mRNA molecule is gradually separated from the DNA. The end of mRNA synthesis is determined by the transcription stop site -- terminator. The nucleotide sequences of the promoter and terminator are recognized by special proteins that regulate the activity of RNA polymerase.
Before exiting the nucleus, a methylated guanine residue called the “cap” is added to the initial part of the mRNA (5 "end), and about 200 adenylic acid residues are added to the end of the mRNA (3" - end). In this form, the mature mRNA passes through the nuclear membrane into the cytoplasm to the ribosome and combines with it. It is believed that in eukaryotes, the “cap” of mRNA is involved in its binding to the small subunit of the ribosome.
Broadcast mRNA. This is protein synthesis on ribosomes directed by an mRNA template. In this case, the information is translated from the four-letter alphabet of nucleic acids to the twenty-letter alphabet of amino acid sequences of polypeptide chains.
There are three stages in this process.
Activation of free amino acids - formation aminoacyladenylates as a result of the interaction of amino acids with ATP under the control of enzymes specific for each amino acid. These enzymes are aminoacyltRNA synthase- participate in the next stage.
Aminoacylation of tRNA is the attachment of amino acid residues to tRNA by the interaction of tRNA and the aminoacyl-tRNA synthetase complex with aminoacyladenylates. In this case, each amino acid residue is attached to its specific class of tRNA.
Actually translation, or polymerization of amino acid residues with the formation of peptide bonds.
Thus, during translation, the sequence of nucleotides in mRNA is translated into the corresponding, strictly ordered sequence of amino acids in the synthesized protein molecule. The translation process involves mRNA, ribosomes, tRNA, aminoacyl-tRNA synthetases.
Signal broadcast initiation in pro- and eukaryotes, the OUT codon is used if it is located at the beginning of mRNA. In this case, it is “recognized” by a specialized initiating formylmethionine (in bacteria) or methionine (in eukaryotes) tRNA. In other cases, the AUG codon is "read" as methionine. The codon GUG can also serve as an initiation signal. This interaction occurs on the ribosome at its aminoacyl center (A-center), located predominantly on the small subunit of the ribosome.
The interaction of the AUG codon of messenger RNA, the small subunit of the ribosome, and formylmethionyl-tRNA forms initiation complex. The essence of this interaction is that it attaches its anticode to the AUG codon on mRNA.
UAC is a tRNA that has captured and carries a molecule of the amino acid methionine (in bacteria, the initiator is tRNA that carries formylmethionine). Then the large subunit of the ribosome (50S*) joins this complex, consisting of the small subunit of the ribosome (30S*), mRNA and tRNA. As a result, a fully assembled ribosome is formed, including one mRNA molecule and an initiator tRNA with an amino acid. The ribosome has aminoacyl And peptidyl centers.
The first amino acid (methionine) first enters the aminoacyl center. In the process of attaching a larger subunit of the ribosome, mRNA moves one codon, tRNA moves from the aminoacyl center to the peptidyl center. The next mRNA codon enters the aminoacyl center, which can connect with the anticodon of the next aminoacyl-tRNA. From this moment, the second stage of translation begins - elongation, during which the cycle of attachment of amino acid molecules to the growing polypeptide chain is repeated many times. So, in accordance with the codon of messenger RNA, the second tRNA molecule carrying the next amino acid enters the aminoacyl center of the ribosome. This tRNA binds with its anticodon to the complementary codon of the mRNA. Immediately, by means of pepticyltransferase, the preceding amino acid (methionine) is connected by its carboxyl group (COOH) to the amino group (NH 2) of the newly delivered amino acid. A peptide bond is formed between them. In this case, a water molecule is released:
As a result, the mRNA that delivered the methionine is released, and a dipeptide is already attached to the tRNA in the aminoacyl center. For further implementation of the elongation process, the aminoacyl center must be released, which happens.
As a result of the translation process, the dipeptidyl-tRNA complex moves from the aminoacyl center to the peptidyl one. This is due to the movement of the ribosome by one codon with the participation of the enzyme translocases and protein elongation factor. The released tRNA and the mRNA codon that was bound to it exit the ribosome. The next tRNA delivers an amino acid to the vacated aminoacyl center in accordance with the codon received there. This amino acid is linked to the previous amino acid by a peptide bond. In this case, the ribosome advances one more codon, and the process is repeated until one of the three termination codons (nonsense codons), i.e., UAA, UAG, or UGA, enters the aminoacyl center.
After the termination codon enters the aminoacyl center of the ribosome, the third stage of polypeptide synthesis begins - termination. It begins with the attachment of one of the protein termination factors to the mRNA termination codon, which leads to blocking of further chain elongation. Termination of synthesis leads to the release of the synthesized polypeptide chain and ribosome subunits, which, after release, dissociate and can take part in the synthesis of the next polypeptide chain,
The entire translation process is accompanied by the cleavage of GTP (guanosine triphosphate) molecules, and the participation of additional protein factors specific to the processes of initiation (initiation factors), elongation (elongation factors) and termination (termination factors) is necessary. These proteins are not an integral part of the ribosome, but are attached to it at certain stages of translation. In general terms, the process of translation is the same in all organisms.
The process of protein synthesis is very complex. In addition to those mentioned, many other enzymes provide its flow. At E. coli about 100 genes have been discovered that control the synthesis of polypeptides and the formation of various elements that make up the translation apparatus. Since the mRNA molecule is long enough, several ribosomes can join it. In each of the ribosomes associated with one mRNA molecule, the synthesis of the same protein molecules takes place, however, this synthesis is at different stages, which is determined by which of them earlier and which later entered into contact with the mRNA molecule. As the ribosome moves along the mRNA (from its 5"- to the Z "- end), the initiating site of the chain is released, the next active ribosome complex is assembled on it, and the synthesis of the polypeptide begins again on the same template. When several active ribosomes interact with one mRNA molecule, polyribosome, or polysome.
The polypeptide chains formed during protein synthesis undergo post-translational transformations and subsequently perform their specific functions. Primary Structure polypeptide is determined by the sequence of amino acids in it. Polypeptide chains spontaneously form a certain secondary structure, which is determined by the nature of the side groups of amino acid residues (α-helix, folded β-layer, random coil). All these and other structural features define some fixed three-dimensional configuration, which is called tertiary(or spatial) structure of the polypeptide, which essentially reflects the way the polypeptide chain is folded in three-dimensional space.
Proteins may be composed of one or more polypeptide chains. In the second case, they are called oligomeric proteins. They are characterized by a certain quaternary structure. This term refers to the general configuration of a protein that has arisen during the association of all its constituent polypeptide chains. In particular, the structural model of human hemoglobin includes two α-chains and two β-chains, which are interconnected and form a quaternary protein structure.
The accuracy of polypeptide synthesis depends on the correct formation of a system of hydrogen bonds between codons and anticodons. Before the closure of the next peptide bond with the help of ribosomes, the correctness of the formation of a codon-anticodon pair is checked. Direct evidence in favor of the active role of ribosomes in controlling the complementarity of the codon-anticodon bond is the detection of mutations that change ribosomal proteins and thus affect the accuracy of translation. Mutations will be discussed in Chapter 6.
SPECIALIZED TRANSFER OF GENETIC INFORMATION. RNA REPLICATION.Three types of processes are known within which specialized transfer of genetic information is carried out. One of them - the transfer of information from RNA to RNA - can only be fixed in cells infected with viruses, the genetic material of which is represented by RNA. These are, in particular, the tobacco mosaic virus and many other plant viruses, RNA-containing bacteriophages and some other animal viruses, such as polioviruses. These viral genomic RNAs, single-stranded or double-stranded, carry genes encoding specific RNA replicases that can synthesize complementary RNA molecules from the RNA template. They, in turn, can serve as templates for the synthesis of copies of parental RNA chains in a similar way. The transfer of genetic information from RNA to RNA is also based on the principle of complementary bases in the parent and daughter RNA strands.
Reverse transcription. This type of specialized transfer of genetic information not from DNA to RNA, but vice versa from RNA to DNA, was found in animal cells infected with certain types of viruses. This is a special type of RNA-containing viruses called retroviruses. It has now been established that another type of virus is the DNA-containing hepatitis virus. IN in its development also uses the transfer of information from RNA to DNA.
Retroviruses contain single-stranded RNA molecules, with each viral particle having two copies of the RNA genome, i.e., viruses of this type are the only known variety of diploid viruses. They were first discovered by their ability to cause tumor formation in animals. The first virus of this type was described in 1911. Pepton Rous, who discovered infectious sarcoma in chickens.
After retrovirus RNA enters the host cell, the viral genome undergoes reverse transcription. In this case, an RNA-DNA duplex is formed first, and then a double-stranded DNA. These steps precede the expression of viral genes at the protein level and the formation of RNA genomes.
The enzyme that catalyzes the complementary copying of RNA to form DNA is called reverse transcriptase. It is contained in retroviral particles (virions) and is activated after the virus enters the cell and destroys its lipid-glycoprotein shell.
There is more and more evidence that reverse transcription also occurs in a variety of eukaryotic cells, and reverse transcriptase plays an important role in the processes of genome rearrangement.
Retrovirus reverse transcriptases are essentially DNA polymerases that can be used in vitro as a DNA template. However, they work much more efficiently on RNA. Like all DNA polymerases, reverse transcriptases are unable to initiate the synthesis of new DNA strands. But if the synthesis is already initiated by primer RNA or the 3' end of DNA, then the enzyme efficiently performs the synthesis using the DNA strand as a template.
Retroviruses have proven to be a very useful tool in modern genetic engineering research. They serve as a source for obtaining practically pure reverse transcriptase, an enzyme that plays a major role in numerous studies based on the cloning of eukaryotic genes. Thus, a purified individual mRNA encoding a protein of interest to a researcher is, as a rule, much easier to isolate than a genome DNA fragment encoding this protein. A DNA copy of this mRNA can then be made using reverse transcriptase and inserted into a suitable plasmid for cloning and production of significant amounts of the desired DNA.
Translation of DNA. The third type of specialized transfer of genetic information from DNA directly to protein has been observed only in the laboratory in vitro. Under these conditions, some antibiotics, in particular streptomycin and neomycin, interacting with ribosomes can change their properties in such a way that ribosomes begin to use single-stranded DNA as a template instead of mRNA, from which the base sequence is directly translated into the amino acid sequence of the synthesized polypeptide.
1. Give definitions of concepts.
Genetic code
- a set of combinations of three nucleotides encoding 20 types of amino acids that make up the protein.
Triplet- three consecutive nucleotides.
Anticodon A region in tRNA consisting of three unpaired nucleotides that specifically binds to an mRNA codon.
Transcription
- the process of RNA synthesis using DNA as a template, occurring in all living cells.
Broadcast- the process of protein synthesis from amino acids on the mRNA (mRNA) matrix, carried out by the ribosome.
2. Compare the concepts of "genetic information" and "genetic code". What are their fundamental differences?
Genetic information - information about the structure of proteins, encoded using a sequence of nucleotides - the genetic code - in genes.
In other words, the genetic code is the principle of recording genetic information. Information is information, and code is how information is communicated.
3. Fill in the "Properties of the genetic code" cluster.
Properties: triplet, unambiguous, redundant, non-overlapping, polarity, universality.
4. What is the biological meaning of the redundancy of the genetic code?
Since there are 61 codons per 20 amino acids that make up proteins, some amino acids are encoded by more than one codon (the so-called code degeneracy).
This redundancy increases the reliability of the code and the whole mechanism of protein biosynthesis.
5. Explain what matrix synthesis reactions are. Why are they called that?
This is the synthesis of complex polymer molecules in living cells, occurring on the basis of the cell's genetic information encoded on the matrix (DNA molecule, RNA). Template synthesis occurs during DNA replication, transcription and translation. It underlies the process of reproducing one's own kind.
6. Sketch a tRNA molecule and label its main parts.
7. Fill in the table.
THE ROLE OF ORGANIC SUBSTANCES IN PROTEIN BIOSYNTHESIS
8. One of the DNA chains has the following nucleotide sequence:
C-T-T-A-A-C-A-C-C-C-C-T-G-A-C-G-T-G-A-C-G-C-G-G-C- C-G
Write the structure of the mRNA synthesized on this strand. What will be the amino acid composition of the protein fragment synthesized on the basis of this information in the ribosome?
mRNA
G-A-A-U-U-G-U-G-G-G-G-A-C-U-G-C-A-C-U-G-C-G-C-C-G- G-C-
Polypeptide chain
Glu-le-trp-gli-ley-gis-cis-ala-gli.
9. Sketch the process of protein synthesis.
10. Fill in the table.
STAGES OF IMPLEMENTATION OF HEREDITARY INFORMATION IN A CELL
11. Read § 2.10 and prepare an answer to the question: “Why is the decoding of the genetic code one of the most important scientific discoveries of our time?”
Deciphering the genetic code, that is, determining the "meaning" of each codon and the rules by which genetic information is read, is considered one of the most striking achievements of molecular biology.
It is proved that the code is universal for living. The discovery and decoding of the code can help find ways to treat various chromosomal and genomic diseases, study the mechanism of metabolic processes at the cellular and molecular level.
A huge amount of experimental data is rapidly accumulating. A new stage of DNA research has begun. Molecular biology has turned to much more complex supramolecular and cellular systems. It turned out to be possible to approach the problems associated with the molecular genetics of eukaryotes, with the phenomena of ontogeny.
12. Choose the correct answer.
Test 1
Protein synthesis cannot occur:
2) in the lysosome;
Test 2
Transcription is:
3) synthesis of mRNA on DNA;
Test 3
All amino acids that make up a protein are coded for:
4) 64 triplets.
Test 4
If for protein synthesis we take the ribosomes of sea bass, enzymes and amino acids of the gray crow, ATP of the quick lizard, wild rabbit mRNA, then protein will be synthesized:
4) wild rabbit.
13. Establish a correspondence between the properties of the genetic code and their characteristics.
Properties of the genetic code
1. Tripletity
3. Uniqueness
4. Versatility
5. Non-overlapping
6. Polarity
Characteristic
A. Each nucleotide is part of only one triplet
B. The genetic code is the same for all living organisms on Earth
B. One amino acid is encoded by three consecutive nucleotides
D. Some triplets define the start and end of a translation
E. Each triplet encodes only one specific amino acid.
E. An amino acid may be defined by more than one triplet.
14. Insert the missing element.
Nucleotide - Letter
Triplet - Word
Gene - Suggestion
15. Explain the origin and general meaning of the word (term), based on the meaning of the roots that make it up.
16. Choose a term and explain how its modern meaning corresponds to the original meaning of its roots.
The chosen term is transcription.
Correspondence - the term corresponds to its original meaning, as there is a transfer of genetic information from DNA to RNA.
17. Formulate and write down the main ideas of § 2.10.
Genetic information in living organisms is recorded using the genetic code. A code is a set of combinations of three nucleotides (triplets) encoding 20 types of amino acids that make up a protein. The code has the properties:
1. Tripletity
2. Degeneracy (redundancy)
3. Uniqueness
4. Versatility
5. Non-overlapping
6. Polarity.
The processes by which complex polymer molecules are synthesized in living cells occur on the basis of the genetic information of the cell encoded on the matrix (DNA molecule, RNA). Matrix synthesis is DNA replication, transcription and translation.
Remember!
What is the structure of proteins and nucleic acids?
Long protein chains are built from only 20 different types of amino acids that have a common structural plan, but differ from each other in the structure of the radical. Connecting, amino acid molecules form so-called peptide bonds. Twisting in the form of a spiral, the protein thread acquires a higher level of organization - a secondary structure. Finally, the polypeptide coils up to form a coil (globule). It is this tertiary structure of the protein that is its biologically active form, which has individual specificity. However, for a number of proteins, the tertiary structure is not final. The secondary structure is a polypeptide chain twisted into a helix. For a stronger interaction in the secondary structure, an intramolecular interaction occurs with the help of –S–S– sulfide bridges between the turns of the helix. This ensures the strength of this structure. The tertiary structure is a secondary spiral structure twisted into globules - compact lumps. These structures provide maximum strength and greater abundance in cells compared to other organic molecules.
DNA is a double helix, RNA is a single strand of nucleotides.
What types of RNA do you know?
i-RNA, t-RNA, r-RNA.
i-RNA - synthesized in the nucleus on the DNA template, is the basis for protein synthesis.
tRNA is the transport of amino acids to the site of protein synthesis - to ribosomes.
Where are ribosome subunits formed?
rRNA - synthesized in the nucleoli of the nucleus, and forms the ribosomes themselves of the cell.
What is the function of ribosomes in a cell?
Protein biosynthesis - the assembly of a protein molecule
Review questions and assignments
1. Remember the full definition of the concept of "life".
F. Engels “Life is a way of existence of protein bodies, the essential point of which is the constant exchange of substances with the external nature surrounding them, and with the cessation of this metabolism, life also stops, which leads to protein decomposition. And in inorganic bodies a similar exchange of substances can take place, which takes place everywhere in the course of time, since chemical actions take place everywhere, even if very slowly. But the difference lies in the fact that in the case of inorganic bodies metabolism destroys them, while in the case of organic bodies it is a necessary condition for their existence.
2. Name the main properties of the genetic code and explain their meaning.
The code is triplet and redundant - from 4 nucleotides you can create 64 different triplets, i.e. code for 64 amino acids, but only 20 are used in the living.
The code is unambiguous - each triplet encrypts only one amino acid.
There are punctuation marks between the genes - the marks are necessary for the correct grouping into triplets of a monotonous sequence of nucleotides, since there are no division marks between the triplets. The role of gene marking is performed by three triplets that do not code for any amino acids - UAA, UAG, UGA. They signify the end of a protein molecule, like a dot in a sentence.
There are no punctuation marks inside the gene - because the genecode is like a language; Let's look at this property using the phrase as an example:
THE CAT WAS QUIET, THAT CAT WAS CUTE TO ME
The gene is stored like this:
ZHILBYLKOTTIKHBYLSERMILMNETOTKOT
Meaning will be restored if the triples are correctly grouped, even if there are no punctuation marks. If we start the grouping from the second letter (second nucleotide), we get the following sequence:
ILB YLK OTT IHB YLS ERM ILM NO OTK FROM
This sequence no longer has a biological meaning, and if it is implemented, then a substance alien to this organism will be obtained. Therefore, the gene in the DNA chain has a strictly fixed beginning of reading and completion.
The code is universal - it is the same for all creatures living on Earth: in bacteria, fungi, humans, the same triplets encode the same amino acids.
3. What processes underlie the transmission of hereditary information from generation to generation and from the nucleus to the cytoplasm, to the site of protein synthesis?
Meiosis is the basis for the transmission of hereditary information from generation to generation. Transcription (from Latin transcription - rewriting). Information about the structure of proteins is stored in the form of DNA in the cell nucleus, and protein synthesis occurs on ribosomes in the cytoplasm. Messenger RNA acts as an intermediary that transmits information about the structure of a certain protein molecule to the site of its synthesis. Broadcast (from lat. trans lation - transmission). The mRNA molecules exit through the nuclear pores into the cytoplasm, where the second stage of the implementation of hereditary information begins - the translation of information from the "language" of RNA to the "language" of protein.
4. Where are all types of ribonucleic acids synthesized?
All types of RNA are synthesized on a DNA template.
5. Tell where protein synthesis occurs and how it is carried out.
Stages of protein biosynthesis:
– Transcription (from Latin rewriting): the process of i-RNA synthesis on a DNA template, this is the transfer of genetic information from DNA to RNA, transcription is catalyzed by the enzyme RNA polymerase. 1) RNA polymerase movements - unwinding and restoration of the DNA double helix, 2) Information from the DNA gene - to i-RNA according to the principle of complementarity.
– Connection of amino acids with t-RNA: The structure of t-RNA: 1) an amino acid is covalently attached to t-RNA using the enzyme t-RNA synthetase, corresponding to an anticodon, 2) A certain amino acid is attached to the leaf petiole of t-RNA
– Translation: ribosomal protein synthesis from amino acids to mRNA, occurring in the cytoplasm. 1) Initiation - the beginning of synthesis. 2) Elongation - the actual protein synthesis. 3) Termination - recognition of a stop codon - the end of the synthesis.
6. Consider fig. 45. Determine in which direction - from right to left or from left to right - the ribosome shown in the figure moves relative to mRNA. Prove your point.
i-RNA moves to the right, the ribosome always moves in the opposite direction so as not to interfere with processes, since several ribosomes (polysome) can sit on one strand of i-RNA at the same time. It also shows in which direction tRNAs move - from right to left, like the ribosome.
Think! Remember!
1. Why can't carbohydrates perform the function of storing information?
There is no principle of complementarity in carbohydrates, it is impossible to create genetic copies.
2. How is hereditary information about the structure and functions of non-protein molecules synthesized in the cell realized?
The formation in cells of other organic molecules, such as fats, carbohydrates, vitamins, etc., is associated with the action of catalytic proteins (enzymes). For example, enzymes that ensure the synthesis of fats in humans "make" human lipids, and similar catalysts in sunflower - sunflower oil. Enzymes of carbohydrate metabolism in animals form a reserve substance glycogen, and in plants with an excess of glucose, starch is synthesized.
3. Under what structural state can DNA molecules be sources of genetic information?
In a state of spiralization, since in this state DNA is part of the chromosomes.
4. What structural features of RNA molecules ensure their function of transferring information about the protein structure from chromosomes to the site of its synthesis?
i-RNA - synthesized in the nucleus on the DNA template, is the basis for protein synthesis. The composition of RNA - nucleotides complementary to DNA nucleotides, small in size compared to DNA (which provides an exit from the nuclear pores).
5. Explain why the DNA molecule could not be built from three types of nucleotides.
The code is triplet and redundant - from 4 nucleotides you can create 64 different triplets (43), i.e. encode 64 amino acids, but only 20 are used in living things. This is necessary to replace any nucleotide, if suddenly it is not in the cell, then the nucleotide will be automatically replaced with a similar one encoding the same amino acid. If there were three nucleotides, then 33 would be only 9 amino acids, which is impossible, since 20 amino acids are needed for any organism.
6. Give examples of technological processes based on matrix synthesis.
Matrix printer,
Nanotechnology,
Camera matrix
Laptop screen matrix
LCD Matrix
7. Imagine that in the course of some experiment, tRNA from crocodile cells, monkey amino acids, thrush ATP, polar bear mRNA, essential tree frog enzymes and pike ribosomes were taken for protein synthesis. Whose protein was eventually synthesized? Explain your point of view.
The genetic code is encrypted in i-RNA, which means - a polar bear.
