The word gene is a transliteration of English "gsne", which means "start" and "bear". Originated from Indo-European language family, it later became gM (clan) in Latin, and many words in modern English such as genus (species), genius (genius) and Genius (reproduction). 1909, Danish scholar Johnson put forward the word gene, which is used to refer to the genetic factor that controls any genetic trait in any organism and its genetic law conforms to Mendel's law.
Before Mendel's law was discovered, people had put forward many views on biological inheritance. For example, the popular theory of fusion genetics holds that parents' genetic material is mixed, diluted and inseparable like blood in their offspring, but Mendel's experimental results are the opposite. Modern recessive genes have not disappeared in hybrid offspring, and the traits it determines can still appear in the second generation. Based on this, Mendel put forward the theory of "genetic particles". Mendel's theory was further verified in many animals and plants in the early 20th century. The most representative is that in 19 10, American scientist Morgan discovered the sex-linked genetic phenomenon of supercilious look in Drosophila, that is, supercilious look always appeared in male Drosophila, which first located a specific gene on a specific chromosome (sex-determined chromosome), and finally reached the same goal in genetics and cytology. Someone once made an image metaphor for this: if Mendel's theory is compared to the separation of seven notes from a magnificent symphony of biology, then Morgan's chromosome inheritance theory not only proves the existence of six strings on the lyre, but also proves that these seven notes are emitted from this big stringed instrument.
Both Mendel's theory and Morgan's theory of genes regard genes as an independent genetic unit with clear boundaries. Even in the early 1950s, after people had a clear understanding of the chemical nature of genes (nucleic acids) and the double helix structure of DNA, they still thought that genes were the inseparable basic genetic units, just as people thought that molecules were the basic particles of matter. It was not until 1957 that this concept was corrected. Bensel, a famous geneticist, put forward a brand-new concept of gene after 10 years' efforts, and made three discoveries, thus completely breaking through the classic concept of gene inseparability. He thinks: (1), as a unit of gene, can be accurate to the level of a single nucleotide or base, and is called a mutant. (2) As an exchange unit, just like a mutant unit, the basic unit is still a single nucleic acid count, which is called an interchanger. (3) As a functional unit, genes are also separable. Bensel's contribution lies not only in putting forward a brand-new concept of gene, but also in introducing "gene" as a concept into gene experiments. Bensel arranged the mutants into commutators on the gene map like a chromosome map, which is a leap from macro to micro in genetics.
In 1969, Shapiro et al. isolated the lactose operon from E.coli and transcribed it in vitro. It is proved that a gene can function independently without chromosomes. 1970, Timin discovered retrovirus with RNA as its genetic material, and proposed that the genetic material is not only DNA, but also RNA, thus expanding the content of the central rule.
After 20 years, 1977, people were infected with simian virus (SV. ) and adenovirus (AdV) found that there were internal spacer regions in some genes, and the order of spacer regions had nothing to do with the protein sequence determined by genes, which surprised scientists. Subsequently, the separable discontinuity of this gene was also confirmed in yeast tRNA gene, fruit fly n3NA gene and human collagen gene. In this way, the concept of gene has a new content: the structure of gene is discontinuous. Because this is a common phenomenon in biology, especially in eukaryotes, for convenience, people call the part of this split gene that can express genetic information exon, while the unexpressed part is called intron.
1980, French scientist Slonim muskie confirmed in the study of yeast mitochondrial DNA that the intron of one gene may be the exon of another gene, that is, the intron may also be functional, the splicing enzyme did not kill them, and all DNA members in the biological world may have no waste.
Contrary to the concept of gene division or discontinuity, it is the overlap of genes. In 1977, Sanger et al. found that several genes in phage A 174DNA were the same as those in Fils et al. SV40DNA.
Although this phenomenon is not common in nature, it at least shows that genes do have overlapping reading frames, which embodies the principle of "saving" in biology.
The challenges to classical, modern and even modern gene concepts are not limited to these. For example, the hypothesis of one gene and one polypeptide has long been proved to be correct, but in recent years, it has been found that some genes never produce any protein or polypeptide, but only RNA, such as various tRNA and rRNA genes. Therefore, people have to add it:
The function of genes is to determine protein or nucleic acid. But this still doesn't explain some facts: there are some fragments in DNA that don't produce any substance at all, but only work in position or structure. For example, the control area and the startup area only play the role of identifying protein, which leads to the opening or closing of its "subordinate" activities. Other genes, such as pseudogenes, don't even see any effect at present. This makes it difficult to give a unified definition of genes from products.
At the end of this century, a strange phenomenon was found in Escherichia coli, where genes can "fly" back and forth between chromosomes and extrachromosomal DNA. In fact, this kind of gene jump phenomenon was discovered by a female scientist, mcclintock, when she was studying the phenomenon of maize tissue differentiation in the early 1950s, but her discovery did not attract people's general attention at that time. Soon after, the phenomenon of gene jumping was confirmed in human immunoglobulin genes, so people fully realized that the stability of genes is relative. Medical scientists further hypothesize that this instability of genes may also have a lot to do with cancer and infectious diseases. As the first person to discover gene instability, mcclintock won the Nobel Prize in Physiology and Medicine with 1983. (Lai) According to
2.2 Chromosomes are the carriers of genes.
/kloc-In the second half of the 20th century, inspired by the cell theory, people realized that studying the structure and physiology of cells is a shortcut to clarify life phenomena, including reproduction and heredity. In addition, with the development of physics and chemistry, there were excellent microscopes, slicers and various chemical dyes at that time, which provided very favorable conditions for cytological research. Therefore, biologists have discovered and described the cell mitosis and meiosis of germ cells during maturation. These findings focused people's attention on chromosomes. As early as 1882, German cytologist W. Fleming (1843- 19 15) found that a part of the nucleus was easy to stain, and called it chromatin. Later, in 1888, German anatomist W. Waldewr (1836-1921) officially called the chromatin discovered by froman a chromosome. Since then, research reports on chromosomes have emerged one after another. It is found that all individuals of the same species have the same and stable chromosome number, and the size and morphology of different chromosome pairs in the same nucleus of many organisms are obviously different, thus putting forward the hypothesis of chromosome individuality and continuity. In particular, the behavior of chromosomes in the process of cell division is more noticeable. It reminds people that the changes of genetic genes are parallel or consistent with the chromosome behavior of higher animals and plants during sexual reproduction. For example, genes are paired in somatic cells and chromosomes are also paired in somatic cells; Genes are haploid in germ cells and chromosomes are haploid in germ cells; Different pairs of genes can be combined freely in the process of separation, and homologous chromosomes are randomly distributed during meiosis. In other words, the separation and distribution of genes correspond to the distribution of chromosomes and the formation of tetrads in germ cells during meiosis. According to this understanding, hybrid offspring (f; ) When gametes are formed, homologous chromosomes are separated, resulting in two types of gametes with equal number, one containing only gene A and the other containing only gene A, assuming that all gametes are fertilized at the same tax rate. The conjugation of these gametes will be randomly combined, so four combinations will be produced; Namely Aa, ZAa and AA. When A stands for dominance and A stands for invisibility, this is the phenomenon of Mendel separation. In this way, we can understand the genetic law discovered by Mendel from the behavior of chromosomes during the formation of germ cells. As summarized by American cytologist W. Sutton (1877- 19 16) in his article Genetics and Chromosomes (1903), the combination of male and female chromosomes and their separation in meiosis constitute the basis of Mendel's law. That is to say, in the process of the formation and fertilization of male and female gametes, the behavior of chromosomes is parallel to that of Mendelian genetic factors (that is, genes). As long as the gene is assumed to be on the chromosome, it will explain the performance of segregation and the law of free combination.
Sutton's generalization was not recognized by most people at that time. People who disagree think that the relationship between genes and chromosomes only happens at the same time, and it seems a bit specious to compare Mendel's genes and chromosomes. American biologist Morgan Hu Harper (1866- 1945) holds this view. So he tried to solve this problem through experiments. 19 10 years, he chose Drosophila as the material for the genetic experiment of sex determination. One day, he happened to find a slight and obvious variation in a male fruit fly in a culture bottle, that is, it is different from the usual red-eyed fruit fly, but has a white eye character. Then Morgan kept the male fruit fly with its red-eyed sister to see what would happen. As a result, he found that all hybrids are red-eyed. If FI is inbreeding (refers to the hybridization between closely related individuals), then the number ratio of horses with red eyes and white eyes is 3: 1. This example is like a Mendel gene based on a dictionary. Interestingly, Mann's white-eyed fruit flies are all male individuals. Subsequent mating shows that white eyes almost always appear in male fruit flies, but occasionally white eyes burst in fruit flies. This makes Morgan think that the genes that determine red eye and white eye are probably always associated with the sex-determining chromosome components, that is, it is conceivable that the white eye gene is located on the X chromosome, but there is no allele on the Y chromosome. Morgan called this phenomenon of chromosome inheritance determined by sex sex sex-linked inheritance. The discovery of sex-linked inheritance linked a specific gene (such as the gene that determines the eye color of Drosophila) with a specific chromosome for the first time, thus proving that the chromosome is the gene itself through experiments.
Since then, Morgan has further studied the law of gene transmission on the same chromosome. He crossed the blackbody residual wings (bV) of male Drosophila melanogaster with the gray long wings (BV) of female Drosophila melanogaster, and all the Fl obtained were gray long wings. Then he crossed f 1 male Drosophila with recessive parents. According to the law of free combination of separation phenomenon, he should expect four types of offspring, namely bV, By, BV and Concord. But there are only two kinds of experimental results, gray long wings and black body residual wings. Morgan explained his experimental results like this. He said: if we assume that the two genes B and V are on the same chromosome and the two genes B and V are on opposite chromosomes, we can explain the above genetic phenomenon. That is to say, although genes on different chromosomes can be combined freely, genes on the same chromosome (such as B and V, B and V) cannot be combined freely because they are always together. Morgan called this genetic phenomenon gene linkage.
Are linked genes completely non-exchangeable? Experiments have proved that this is not the case. Male fruit flies are rare in complete linkage. In most cases, each gene linkage group is not always closely linked together, and some exchanges may occur between opposite genes. For example, in the above experiment, if female Drosophila uses FI to backcross with recessive parents instead of male Drosophila, then four types of offspring can be obtained, but the number of exchange types is far less than expected. Their ratio is: bv. (0.42)，By(0.08)，bv(0.08)，bvn.42)。 Among them, only 16% is between switching classes GA and bV. So Morgan called his discovery the law of gene linkage and exchange.
The linkage and exchange of genes is a common phenomenon in biology. It is proved by experiments that the exchange rate between two specific genes in the same linkage group is always a constant or a constant value, no matter how two pairs of traits are combined during hybridization. If the exchange rate of corpus luteum gene and supercilious eye gene is 1.2%, the exchange rate of supercilious eye gene and pterygoid vein dichotomy gene is 3.5%, and the exchange rate of corpus luteum gene and pterygoid vein dichotomy gene is 4.7%. Therefore, the exchange rate of corpus luteum gene and supercilious eye gene plus supercilious eye gene and wing gene is exactly equal to that of corpus luteum gene and wing gene. That is to say, as long as the exchange rate between three genes in the same linkage group is known, it can be inferred that the third value must be the sum or difference of the former. If we take a certain exchange value as the unit of length, assuming that two chromosomes may exchange between any of their gene loci, then the exchange value is proportional to the distance between genes. Then the gene distribution map we draw will be a neat straight line. It can be inferred that genes are arranged in a straight line in a certain order and distance on chromosomes.
Morgan and his colleagues combined hybridization research with cytology, and proved by convincing experiments that genes exist on cell chromosomes and are transmitted regularly, thus establishing chromosome inheritance theory (or cytogenetics). In his book Gene Theory published in 1926, he summarized the great achievements of genetics development in the first 30 years of the 20th century as follows: Gene theory holds that all kinds of traits in individuals come from paired elements (genes) in reproductive quality, and these genes combine with each other to form a certain number of linkage groups; It is considered that when germ cells mature, the two genes in each pair are separated according to Mendel's first law (separation phenomenon), so each germ cell contains only one group of genes. It is believed that genes of different linkage groups can be freely combined according to Mendel's second law (free combination law). It is believed that the genes of two relative linkage groups sometimes exchange in an orderly way. It is believed that the exchange rate proves the linear arrangement of the elements in each chain group and the relative positions of the elements.
2.3 DNA is the chemical entity of genes
Cytogenetics has proved that chromosomes are carriers of genes, but little is known about the chemical properties of genes. For example, what is a gene and how does it play a role in genetic transmission? These questions were unanswerable in Morgan's time. However, Morgan touched on this issue after all. In the conclusion part at the end of Gene Theory, when discussing whether genes belong to the level of organic molecules, he estimated them by calculating the size of genes, and thought that genes could not be regarded as chemical molecules. Genes may not even be a molecule, but a group of organic substances that are not chemically combined. However, he does not rule out the hypothesis that "the gene is stable because it represents an organic chemical entity".
Cytochemistry plays an important role in finding chemical entities of genes. Cytochemical studies show that chromosomes, as the basic components of cell structure, are mainly composed of protein and nucleic acid. So is the genetic material protein or nucleic acid? According to the traditional concept, protein, as the main component of living matter and the embodiment of all life phenomena, not only exists in the biological world and participates in all life processes, but also its chemical structure is diverse and plastic, which seems to be very suitable as a genetic material. However, the scientific experimental department denied this view and confirmed that nucleic acid is genetic material and protein is only its product.
It is a long historical process to realize that nucleic acid is genetic material (or chemical entity of gene). As early as 1928, the British bacteriologist F. Griffith (1881-1941) discovered an amazing phenomenon when doing experiments with pneumococcus. When he injected a large number of dead pathogenic S-type pneumococci (with capsule appearance and smooth colonies on the culture medium) with a small number of surviving non-pathogenic R-type pneumococci (without capsule appearance and rough colonies on the culture medium) into experimental animals, he was surprised to find that all these experimental animals died of illness and many S-type pneumococci were isolated in the body. People call this phenomenon that R-pneumococcus transforms into S-pneumococcus transformation phenomenon. Why did this change happen? At that time, it was speculated that some substances of S-type pneumococcus must be absorbed by R-type pneumococcus and transformed into S-type pneumococcus. But what kind of chemical is this? It was not clear at the time.
1944, American biochemist Avery and others made an in vitro experiment and found that deoxyribonucleic acid (DNA for short) in S-type pneumococcus played a role in the transformation phenomenon. They first ground S-type pneumococcus and extracted it with water, and found that the extract contained compounds such as protein, DNA, fat and sugar. Then, the extract was put into a culture medium (an artificially prepared mixture suitable for the nutritional needs of bacteria) for cultivating R-type pneumococcus. As a result, it was found that S type pneumococcus was produced in the culture medium. This is the same as the transformation phenomenon seen by Griffith, so it can be considered that there is indeed some factor in this extract that contributes to the transformation of traits. But this factor is protein, or DNA, or other substances. In order to find the answer, Avery and others studied these substances one by one. When they extracted the purified DNA from S-pneumococcus and put it on the culture medium of R-pneumococcus, they found S-pneumococcus there, but when the DNA was replaced by protein or other substances, this phenomenon did not happen. When they add some protease to the DNA extract, it does not affect the experimental results, but after adding DNA enzyme, the transformation phenomenon disappears. It can be seen that what plays a unique role in the transformation stage is nothing more than the role of DNA genetic material. 1952, Hershey and Chase made an authoritative experiment after Winfrey and others. They used 32P and "S" to mark the DNA of habitual bacteria (virus parasitic in the sperm) and the part of protein respectively, and then infected the sperm with the marked agricultural bacteria. It was found that when the bacteria were infected, the phage DNA entered the parasitic cell, but its protein shell remained outside, and the DNA that entered the parasitic cell could replicate the same phage. This experiment further confirmed that DNAffiff is a chemical entity that transmits substances or genes.
Since DNA is genetic material, what conditions does it have to play this role? This is about the chemical composition and structure of DNA. DNA is a kind of nucleic acid. Nucleic acid was first discovered in 1844- 1895 by F. Mieschr, a young Swiss chemist. In order to find out the chemical properties of the nucleus, he treated the pus cells with hydrochloric acid. Nuclei were separated by dilute alkali, and the components were analyzed after precipitation. It was found that the contents of nitrogen and phosphorus were particularly high. Because this substance is separated from the nucleus and acidic, people call it nucleic acid. Later, after many scientists' research, it was finally clear that nucleic acid is a polymer composed of nucleic acid as the basic unit. Then acid itself is a complex compound, consisting of pentose, alkali and phosphoric acid. According to the different types of pentoses in nucleic acids, nucleic acids can be divided into two categories, namely RNA and DNA. The pentose part of the former is ribose. The latter is deoxyribose. Except for the different sugar components, the types of bases contained in these two types of nucleic acids are not exactly the same. RNA contains glandular origin (represented by A), birdsong (represented by G), shortness of breath (represented by C) and urine sickness (represented by U). DNA contains A, G, C, T (sudden breath of thymus) without U. In fact, there is only one difference between DNA and RNA, that is, T is replaced by U in RNA. Nucleic acid is called adenylate (AMP) or deoxyadenylate (dAMP), ornithine (GMP) or deoxyguanylate (dGMP), cytidine (CMP) or deoxycytidine (dCMP), uric acid (U'MP) and deoxythymidine (dTMP) according to the different bases contained in 255. These nucleic acids become polymers by dehydration. In nucleic acid molecules, the arrangement of nucleic acids has a certain order, and this linear sequence of nucleic acids is the primary structure of nucleic acids. Although there are only four kinds of nucleic acids that make up DNA or RNA, the diversity of nucleic acid molecules can be formed because of the different arrangement order. Suppose that a nucleic acid molecule consists of 100 different nucleic acids. Then it is possible to provide so many different 4'ho arrangements.
Before understanding the three-dimensional structure (or spatial structure) of DNA, it is very difficult to explain its genetic function from its chemical properties. This problem needs to be solved urgently. 1953, Watson (J.D.Wason, 1928-) and Crick (F. HCCRI Blow, 19 16) applied the new technology of physical chemistry and the new achievements of biological research, and compared their creative work with those of their predecessors from a comprehensive point of view. In this structure, the main chain composed of phosphoric acid and deoxyribose is on the outside and the base is on the inside. The bases between the two chains are connected by hydrogen bonds, which have certain rules, that is, A matches T, C matches G, each pair of bases is in the same plane, and different base pairs are parallel to each other and perpendicular to the central axis. —5 is the pattern diagram of the double helix structure of DNA molecule. (1) It is a DNA model displayed in the form of bones. (b) is the filled space model of DNA.
Obviously, such a molecular model contains considerable biological significance. It provides a chemical basis for the reproduction and inheritance of organisms for the first time. As Watson and Crick said, "the principle of base-specific pairing in DNA double helix model immediately shows the possible replication mechanism of genetic material." It is also proposed that "if the actual sequence of the base on one side of the pairing bond is known, people can write down the exact sequence of the base on the other side." Therefore, it can be said that one chain is the complementary chain of another chain, and it is this feature that suggests why DNA molecules replicate themselves.
Watson's and Crick's predictions were soon confirmed by the work of messer Song (M. Messeson, 1930-) and others. 1963, American scientist john maynard keynes (C advised ms) also successfully photographed the DNA replication process of E.coli by combining electron microscope and autoradiography technology, thus directly proving the correctness of Watson and Crick's speculation on DNA replication.
2.4 Modern people's understanding of the concept of genes
It is an epoch-making event to identify DNA as the chemical entity of genes and determine its double helix structure and replication mechanism, which has profoundly changed the concept of genes in classical genetics. According to the understanding of classical genetics, genes are abstract and inseparable genetic units. After DNA is defined as the chemical entity of gene, gene is the real chemical molecule. The concept of gene is defined as a DNA fragment with genetic function, which carries genetic information units usually encoded by protein and RNA. In other words, genes are linear sequences with specific continuous nucleic acids. Take phage M% as an example. It is a single-stranded RNA molecule consisting of 3569 nucleic acids (RNA can also be used as genetic material in some organisms). There are three genes, which are responsible for the synthesis of protein A, coat protein and RNA replicase respectively, and they are called protein A coat protein gene and RNA replicase gene respectively. At present, it is clear that there is a leader sequence consisting of 129 nucleic acids at the beginning of M% RNA molecule, followed by a protein gene (containing 1 179 nucleic acids), coat protein gene (containing 390 nucleic acids) and RNA replicase gene (containing 1635 nucleic acids) in A. Finally, the terminal sequence is composed of 174 nucleic acids. The leader sequence, terminal sequence and nucleic acid in the two spacer regions are not expressed, that is, they cannot be transformed into protein.
According to the above modern gene concept, not only can we completely explain everything that classical genetics can explain. It can also explain some phenomena that are difficult to explain in classical genetics. For example, classical genetics can only explain the differences of different traits by "different genes", and now it can be explained by how the acidic sequence of DNA or RNA chain changes, resulting in different protein; Some mutations can be explained not only by gene changes, but also by DNA strand rearrangement and its influence. Classical genetics can't answer why genes can be replicated again and again. Now it can be explained by the self-replication function of DNA. In addition, from the perspective of modern genetics, DNA that cannot be exchanged for further division or mutation may only contain one nucleic acid pair, so it may be exchanged or mutated within a functional unit, and sometimes it may only involve a small segment of the functional unit, such as point mutation of hemoglobin. Therefore, genes as functional units, mutant units and recombinant units are not trinity. That is to say, as a functional unit, a gene refers to a specific continuous nucleotide sequence, and the mutation can be one or several nucleotide pairs, not necessarily the whole gene. As for exchange, it is possible to exchange or recombine genetic material between any two pairs of nucleic acids (referring to the number of chromosomes in germ cells) in a genome. Therefore, genes are not inseparable, but separable.
In addition, the experiment also proved that genes can move, not only in the traditional allele exchange, but also in different segments of the same chromosome and non-homologous segments between different chromosomes. As early as 1940s, mcclintock, an American geneticist, had noticed the phenomenon that genes could move when studying the high-frequency variation of corn kernel color. In the course of her research, she found that the color of corn seeds is very unstable, and sometimes there are some spots on the seeds. Why is this happening? She put forward a brand-new concept to explain that genes can be moved. She called this mobile gene a control factor or transposon (now often called a jumping gene). These jumping genes can be transferred from one site to another on different chromosomes of maize, sometimes like a new biological switch, turning genes on or off. For example, a jumping gene DS is inserted near the gene gy that produces purple on the maize chromosome, which turns off ffi at a certain rate, so that its seeds cannot produce purple and turn yellow. When DS jumped away from the vicinity of Xi, the inhibition of Xi was released, and then it returned to purple. DS is also cute for the role of another jumping gene AC. When AC is not far from DS, the effect of DS can be prevented, and the inhibition of DS on to can also be released. If DS jumps far away from AC, or after AC itself jumps away, DS is not affected by AC, and DS suppresses to. These jumping genes beat so fast that the color genes they control are turned on and off, and spots appear on corn grains. It can be seen that jumping gene is different from the traditional concept of gene. Although a certain trait is not expressed, it can cause a wide range of genetic effects. Although mcclintock's discovery was great. But it didn't attract people's attention at that time.
About 20 years later, the concept of jumping gene was generally accepted by Malaxnv in the United States, John Dan in Germany and Shapiro in the United Kingdom, and they found transposons similar to those mentioned by mcclintock in the study of microbial genetics. The concept of jumping gene makes people realize that functionally related genes do not necessarily exist in the form of close linkage, but can be scattered on different chromosomes or different parts of the same chromosome, thus greatly enriching and developing the modern gene concept.
In addition, nearly half a century's genetic research shows that in addition to nuclear genes, there are off-campus genes, that is, genes existing in cytoplasm. For example, some organelles in cytoplasm, such as plastids, mitochondria and chloroplasts, all contain their own DNA. The functions of these DNA are very similar to those of chromosome genes in the nucleus, so people call them extranuclear genes. Heredity controlled by extranuclear genes is different from that controlled by nuclear genes. People usually call it cytoplasmic inheritance. The difference between cytoplasmic inheritance and nuclear inheritance lies in its maternal inheritance. The so-called matrilineal inheritance refers to the genetic mode of crossing with parents with relative traits, and its FI always shows matrilineal traits regardless of orthogonal or backcross. This is because egg cells contain a lot of cytoplasm, while sperm contains very little cytoplasm. Especially in the process of fertilization, sperm mainly enter the nucleus of egg cells. Therefore, the cytoplasm of fertilized eggs mainly comes from egg cells. So cytoplasmic inheritance is always maternal inheritance. Secondly, the genetic behavior of cytoplasmic hybrid offspring does not conform to the three basic laws of classical genetics, that is, there is neither a certain separation ratio nor a relationship of free combination, linkage and exchange. This is because in the process of cell division, cytoplasm does not separate and combine regularly like nuclear chromosomes. When cells divide after gene replication in cytoplasm, they are randomly distributed to daughter cells, rather than evenly distributed. The discovery of cytoplasmic inheritance expanded the concept of nuclear inheritance. Experiments have proved that some traits of many organisms (such as whether paramecium is poisoned or not) are determined by nuclear genes and extranuclear genes, such as the extranuclear genes that release toxins from grass-borne insects, and the corresponding nuclear genes need to have the functions of replication, proliferation and transmission.
Geneticists have always been puzzled about the working principle of genes, but the progress of biochemistry has made people realize that the role of genes may be related to enzymes. Because all biochemical processes in organisms must involve enzymes, some biochemical reactions cannot be carried out under the catalysis of enzymes. If there is no amylase, starch is not easy to decompose in organisms. From this