Abstract: The modern concept of the gene. Modern understanding of the gene Modern understanding of the structure of the gene
“The modern idea of the gene and genome” Presentation on the topic: Gene is a structural and functional unit of heredity in living organisms. A gene is a section of DNA that specifies the sequence of a particular polypeptide or functional RNA. Genes (more precisely, alleles of genes) determine the hereditary characteristics of organisms that are transmitted from parents to offspring during reproduction. Among some organisms, mostly unicellular, there is horizontal gene transfer that is not associated with reproduction. History of the term The term "gene" was coined in 1909 by the Danish botanist Wilhelm Johansen three years after William Batson coined the term "genetics". The study of genes The study of genes is the science of genetics, the founder of which is Gregor Mendel, who in 1865 published the results of his research on the transmission of traits by inheritance when crossing peas. The regularities formulated by him were later called Mendel's laws. Gene properties
- discreteness - immiscibility of genes;
- stability - the ability to maintain a structure;
- lability - the ability to repeatedly mutate;
- multiple allelism - many genes exist in a population in a variety of molecular forms;
- allelism - in the genotype of diploid organisms, only two forms of the gene;
- specificity - each gene encodes its own trait;
- pleiotropy - multiple effect of a gene;
- expressivity - the degree of expression of a gene in a trait;
- penetrance - the frequency of manifestation of a gene in the phenotype;
- amplification - an increase in the number of copies of a gene.
- Structural genes - genes encoding protein synthesis. The arrangement of nucleotide triplets in structural genes is collinear with the amino acid sequence in the polypeptide chain encoded by this gene
- Functional genes are genes that control and direct the activity of structural genes.
- Small, compact genomes, typically no larger than 10 million base pairs, with a strict correspondence between genome size and number of genes.
- Large genomes larger than 100 million base pairs that do not have a clear relationship between genome size and number of genes.
The history of views on the units of heredity (genes) discovered by Mendel can be conditionally divided into several periods. In accordance with the "classical" point of view, which prevailed in the 30s. XX century, the gene was considered as an indivisible unit of genetic transmission, function, mutation and recombination. Since the 1940s, in connection with the establishment of the genetic role of DNA, a “neoclassical” concept has been formed, according to which a gene (cistron) is a segment of a DNA molecule with a specific nucleotide sequence that determines the primary structure of the synthesized mRNA molecule and the corresponding polypeptide or a single molecule tRNA or rRNA. In this case, the gene is subdivided into its constituent parts in the form of elementary units of mutation (mutons) and recombination (recons), which can be identified as certain sections of the polynucleotide. The genes that determine the structure of polypeptides and RNA molecules are called structural genes. The modern period of gene understanding, which began in the 1970s, is associated with the emergence of new knowledge about the discontinuous ("mosaic") structure of eukaryotic genes and a number of other features of the genetic organization of various organisms (overlapping genes, repetitive genes, pseudogenes, mobile genes, etc.). ).
Within the framework of classical (formal) genetics, it is customary to consider a gene as a structural unit that determines an elementary trait (phene) of an organism. The totality of all the genes of an individual organism (individual) is called it genotype and the set of features phenotype. term genome it is customary to denote the totality of all genetic elements (DNA of chromosomes, mitochondria, plasmids, etc.) that are constant for organisms of a given species. It should be noted that the sizes of genomes (the amount of genomic DNA or RNA in the corresponding viruses) have significant differences in organisms belonging to different levels of organization of living matter (viruses, bacteria, eukaryotes).
In accordance with modern concepts, most of the structural genes of prokaryotes (bacteria) are represented by continuous sections of the DNA molecule, all the information of which is used in the synthesis of encoded polypeptide chains. Consequently, the genetic information of the prokaryotic gene is fully realized. In some small viruses, an unusual structural and functional organization of the genetic material in the form of overlapping genes (according to the “gene in gene” principle) was found, which allows for even more economical use of the very limited information capabilities of the genome. Thus, some DNA segments of one of the smallest fX174 bacteriophages contain information from not one, but simultaneously two different genes, which allows the genome of such a small size to encode at least nine different protein molecules. Reading information of overlapping genes starts from different starting points of the same nucleotide sequence, i.e. there are different reading frames for this sequence.
In contrast to prokaryotes, for eukaryotes, the discontinuous nature of the structural and functional organization of genes is typical. The information of such a gene about the structure of the synthesized polypeptide does not exist in the form of a continuous nucleotide sequence of a certain section of the DNA molecule, but in the form of coding fragments ( exons) that are interrupted (separated) by "uninformative" nucleotide sequences ( introns) that are not directly involved in the coding of this polypeptide. Consequently, the genes of various eukaryotic organisms are a mosaic of several exons and introns alternating in a certain order. The sizes of introns in such genes range from ten to more than 1000 base pairs. It is hypothesized that introns may play a role in the regulation of RNA processing, which will be discussed further. There is evidence to suggest that they probably significantly affect the processes of recombination between homologous genes. There is also a hypothesis that genes of different proteins or genes that determine proteins of the same family but have accumulated different mutations can relatively easily and often recombine along intron regions. It can be assumed that such properties of nitrons should accelerate the evolution of protein molecules, facilitating the evolution of eukaryotes in general, which gives them significant advantages over prokaryotes. As an "evolutionary reserve" of eukaryotes, it is possible to consider, probably, those found in their genomes pseudogenes, which are DNA nucleotide sequences that are homologous to the sequences of known (functioning) genes, but for one reason or another do not show informational activity, i.e. not giving the final mature product.
One of the features of the genetic organization of eukaryotes is also the presence in their genomes of a significant number of repetitive genes encoding the primary structure of tRNA, rRNA, histone proteins, etc., as well as other (less extended and not always identified in terms of functional significance) repetitive DNA sequences, the number of copies of which can vary from a few to several thousand or more. For example, in the haploid human genome, which contains about 3 x 10 9 base pairs, repetitive DNA sequences make up about 30%, while the remaining 70% of the genome is represented by "unique" sequences that exist in single copies.
Mobile (transposable) genes have also been found in the genomes of various organisms (prokaryotes and eukaryotes), the role of which will be described below.
TASKS FOR INDEPENDENT WORK
- 1. Calculate the linear dimensions (in nucleotide pairs and length units) of a bacterial gene encoding a polypeptide consisting of 100 amino acid residues.
- 2. Explain the reason for the situation in which the gene of a eukaryotic cell, which occupies a DNA region of 2400 base pairs, encodes a polypeptide consisting of 180 amino acid residues.
- 3. Draw a diagram of the discontinuous structure of a hypothetical gene consisting of five exons and four nitrons and encoding a polypeptide that includes 300 amino acid residues (the relative sizes of individual exons and nitrons can be chosen arbitrary).
Question 1. What is a genome?
The genome is a set of genes characteristic of the haploid set of chromosomes of a given biological species. The genome, in contrast to the genotype, is a characteristic of the species, and not of an individual, since it describes a set of genes characteristic of a given species, and not their alleles, which determine the individual differences of individual organisms. The degree of similarity of the genomes of different species reflects their evolutionary relationship.
Question 2. What determines the existing specialization of cells?
The specialization of body cells is determined by the selective functioning of genes. In each cell, genes work that are characteristic of this particular type of tissue and organ: in muscle cells - genes for muscle proteins, in the cells of the walls of the stomach - genes for digestive enzymes, etc. Most of the other genes are blocked , and their activation can lead to the development of serious diseases (for example, to the appearance of a cancerous tumor).
Question 3. What are the mandatory elements that make up the gene of a eukaryotic cell?
Mandatory elements of the eukaryotic gene are:
- regulatory regions located at the beginning and end of the gene, and sometimes outside the gene (at some distance from it). They determine when, under what circumstances and in what types of tissues this gene will work;
- a structural part that contains information about the primary structure of the encoded protein; usually the structural part is less than the regulatory one.
Question 4. Give examples of gene interaction.material from the site
An example of the interaction of genes is the pigmentation (coloration) of wool in a rabbit. The formation of a certain color is regulated by two genes. One of them (let's call it A) is responsible for the presence of pigment, and if the work of this gene is disrupted (recessive allele), the rabbit's coat will be white (genotype aa). The second gene (let's call it B) is responsible for the uneven coloring of wool. In the case of normal functioning of this gene (dominant allele), the synthesized pigment accumulates at the base of the hair, and the rabbit has a gray color (genotypes AaBb, AABb, AaBB, AABB). If the second gene is represented only by recessive alleles, then the synthesized pigment is distributed evenly. These rabbits have black hair (genotypes Aavv, AAvv).
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On this page, material on the topics:
- modern understanding of the gene and genome
- Modern ideas about the gene, genotype, genome
- modern understanding of the gene and genome
- modern understanding of the gene and genome
- what are the required elements in the gene
MODERN METHODS OF DNA STUDY.
Genetic dogma: information is recorded in DNA and transmitted to daughter DNA molecules
from generation to generation through the process of replication.
DNA ® RNA ® protein
REPLICATION is the process of DNA duplication. This process became fully understood only after WATSON and Crick proposed the structure of DNA in the form of a double helix, the polynucleotide chains of which are connected by complementary, nitrogenous bases (A:::T, G:::C). If the nitrogenous bases are complementary to each other, then the polynucleotide chains are also complementary. The replication mechanism is based on the principle of complementarity. The mechanism of replication includes matrix biosynthesis. DNA replication proceeds in a semi-conservative way: a daughter chain is synthesized on each parent polynucleotide chain.
Conditions required for replication:
Matrix - strands of DNA. The splitting of the strand is called the REPLICATIVE FORK. She
It can form within the DNA molecule. They're moving in different directions
Forming a REPLICATIVE EYE. Such eyes in the EUKARYOT DNA molecule
Several, each has two forks
2. Substrate. The plastic material is DEOXYNUCLEOTIDE TRIFOSPHATES:
dATP, dGTP, dCTP, dTTP. Then they decompose to DEOXYNUCLEOTIDE MONOPOSPHATES, two molecules of inorganic phosphate with the release of energy, i.e. they are simultaneously a source of both energy and plastic material.
D-ATP® D-AMP + FF + E.
Magnesium ions.
Replicative Enzyme Complex:
A) DNA unwinding proteins:
DNA-A (causes strand separation)
Helicases (cleave the DNA strand)
TOPOISOMERASES 1 and 2 (unwind over the helix). Break (3",5") -
Phosphodiester bonds. TOPOISOMERASE 2 is called GIRASE in PROKARYOTES.
B) Proteins that prevent the connection of DNA strands (SSB proteins)
C) DNA POLYMERASE (catalyses the formation of phosphodiester bonds). DNA-
POLYMERASE only lengthens the already existing thread, but cannot connect two free NUCLEOTIDES.
D) PRIMASE (catalyses the formation of a "seed" for synthesis). It is in its structure RNA POLYMERASE, which connects single NUCLEOTIDES.
E) DNA LIGASE.
PRIMERS - "seed" for replication. This is a short fragment consisting of RIBONUCLEOTIDE TRIFOSPHATES (2 - 10). The formation of PRIMERS is catalyzed by PRIMASE.
The main stages of replication.
INITIATE replication.
Occurs under the influence of external stimuli (growth factors). Proteins bind to receptors on the plasma membrane and induce replication into the synthetic phase of the cell cycle. The meaning of initiation is to attach DNA-A to the replication point, which stimulates the divergence of the double helix. HELIKAZA also takes part in this. There are enzymes (TOPOISOMERASES) that cause unwinding over the helix. SSB proteins prevent daughter chains from joining.
Functions of genetics
Representatives of any biological species reproduce creatures similar to themselves. This property of descendants to be similar to their ancestors is called heredity. Despite the enormous influence of heredity in shaping the phenotype of a living organism, related individuals differ to a greater or lesser extent from their parents. This property of descendants is called variability. It is the phenomena of heredity and variability that determine the subject of study of genetics.
Genetics as a science solves the following main tasks:
III studies ways of storing genetic information in different organisms (viruses, bacteria, plants, animals and humans) and its material carriers;
III analyzes the methods of transmission of hereditary information from one generation of organisms to another;
Ш reveals the mechanisms and patterns of implementation of genetic information in the process of individual development and the impact on their environmental conditions;
SH studies the regularities and mechanisms of variability and its role in adaptive reactions and in the evolutionary process;
Sh is looking for ways to fix damaged genetic information.
Modern ideas about the gene
Just as in physics the elementary units of matter are atoms, in genetics the elementary discrete units of heredity and variability are genes.
In the spring of 1953, researchers American D. Watson and Englishman F. Crick deciphered the "holy of holies" of heredity - its genetic code. It was from that time that the word "DNA" - deoxyribonucleic acid - became known not only to a narrow circle of scientists, but to every educated person all over the world. At the beginning of 2001, it was solemnly announced the fundamental decoding of the entire human genome - DNA, which is part of all 23 pairs of chromosomes of the cell nucleus. These biotechnological advances have been compared to going into space.
According to modern concepts, the gene encoding the synthesis of a certain protein in eukaryotes consists of several mandatory elements. First of all, this is an extensive regulatory zone that has a strong influence on the activity of a gene in a particular tissue of the body at a certain stage of its individual development. Next is a promoter directly adjacent to the coding elements of the gene - a DNA sequence up to 80-100 base pairs long, responsible for binding the RNA polymerase that transcribes this gene. Following the promoter lies the structural part of the gene, which contains information about the primary structure of the corresponding protein. This region for most eukaryotic genes is significantly shorter than the regulatory zone, but its length can be measured in thousands of base pairs.
An important feature of eukaryotic genes is their discontinuity. This means that the region of the gene encoding the protein consists of two types of nucleotide sequences. Some - exons - sections of DNA that carry information about the structure of the protein and are part of the corresponding RNA and protein. Others - introns - do not encode the structure of the protein and are not included in the composition of the mature mRNA molecule.
Nucleic acids, like proteins, are essential for life. They represent the genetic material of all living organisms, down to the simplest viruses. Nucleic acids are made up of monomeric units called nucleotides. Long molecules are built from nucleotides - polynucleotides. A nucleotide molecule consists of three parts: a five-carbon sugar, a nitrogenous base, and phosphoric acid. The sugar in nucleotides is a pentose.
There are two types of nucleic acids - ribonucleic (RNA) and deoxyribonucleic (DNA). Both types of nucleic acids contain bases of four different types: two of them belong to the class of purines, others to the class of pyrimidines. The nitrogen contained in the rings gives the molecules their basic properties. Purines are adenine (A) and guanine (G), and pyrimidines are cytosine (C) and thymine (T) or uracil (U). Purines have two rings, while pyrimidines have one. RNA contains uracil instead of thymine.
Deoxyribonucleic acid, or DNA, was first isolated from cell nuclei. Therefore, it was called nucleic (Greek nucleus - core). DNA consists of a chain of nucleotides with four different bases: adenine (A), guanine (G), cytosine (C), and thymine (T). DNA almost always exists in the form of a double helix, that is, it consists of two nucleotide chains that make up a pair. What holds them together is what is known as base pair complementarity. "Complementary" means that when A and T are opposite each other in two strands of DNA, a bond is spontaneously formed between them. Similarly, a complementary pair is formed by G and C.
Human cells contain 46 chromosomes. The length of the human genome (all the DNA in the chromosomes) can reach two meters and consists of three billion nucleotide pairs. A gene is a unit of heredity. It is part of a DNA molecule and contains encoded information about the amino acid sequence of a single protein or ribonucleic acid (RNA).
The expression of genes encoding proteins or nucleic acids should result in the formation of functionally complete macromolecules, accompanied by the formation of a certain phenotype of the organism. In accordance with the basic postulate of molecular biology, genetic information is transmitted unidirectionally from nucleic acids to proteins according to the scheme: DNA<->RNA -> protein, i.e. in some cases, it is possible to transfer genetic information from RNA to DNA using the mechanism of reverse transcription. The transfer of genetic information from proteins to nucleic acids has not been found.
At the first stage of gene expression, the genetic information is rewritten into matrix (messenger) RNA (mRNA), which are the place of intermediate storage of information. In some cases, the RNA itself is the end result of gene expression, and after a series of enzymatic modifications, they are directly used in cellular processes. This applies primarily to ribosomal and transfer RNA (rRNA and tRNA). Such RNAs include small nuclear RNAs (snRNAs) involved in the processing of eukaryotic mRNA precursors, RNAs that are part of enzymes, and natural antisense RNAs.
RNA synthesis occurs as a result of a complex sequence of biochemical reactions called transcription. At the second stage of the implementation of genetic information, called translation, the mRNA nucleotide sequence determines the sequence of amino acid residues of synthesized proteins.
Thus, gene expression is determined by two global molecular genetic mechanisms: gene transcription and translation of synthesized mRNA by ribosomes, which ends with the formation of polypeptide chains encoded by genes. However, the process of gene expression is not limited to their transcription and translation.
The essential moments of gene expression are post-transcriptional and post-translational modifications of mRNA and proteins, which include the processing of their precursors (removal of redundant sequences and other covalent modifications of RNA and protein sequences).
