The genetics, its subject, aims, stages of development

Genetics is a science about principles of heredity and diversity of organisms and about methods to direct them. The term “genetics” was suggested by English scientists W. Batson in 1906 (from Greek ‘geneticos’ - related with birth).

Heredity - is an organism's property to transmit their traits and development features in line of following generations. Because of heredity, many species having been preserved unchanged during hundreds millions years (opossum, latimeria, gatteria). In sexual reproduction, a material basement of heredity is sperms and ovicells, in asexual reproduction - single somatic cells.

While studying heredity, we need to distinguish a term “herediting”. Herediting is principles of hereditary traits transmittion process from one organism generation to another while reproduction. During sexual reproduction, herediting is performed through the sex cells, during asexual through the somatic cell division. The analysis of herediting principles is an important method to study heredity patterns.

Embryonic cell don’t carry all traits of adult individual. It carries only material for those traits, which may give these features in a future. This material of future traits development is called genes. The gene is a unit of heredity, determining one elementary trait, whether it is related with protein structure or elementary organism reaction. The genotype is integrity of all organism genes. The phenotype is integrity of all organism traits. It mast be concerned that terms genotype and phenotype commonly are used in a narrow meaning. They may be related with such traits, which are interested for researcher at this moment.

The diversity-it is a variety of individual or group traits and properties. The diversity is a reflection of unstable preserving of individual hereditary information. It includes a gene changing and gene combinating and changes in gene expression throughout individual development. Thus, heredity and diversity are two fundamental properties of life matter. That dialectic union provides organisms evolu­tion on Earth.

The genetics studies heredity and diversity in four aspects. Firstly, it studies a problem of genetic information storage. It makes clear the material place of genetic information storage and the ways of genetic information coding.

Secondary, it studies a problem of genetic information transmitting and principles of that transmitting from cell to cell, from generation to generation.

Thirdly, it analyzes a problem of genetic information realization. It studies how genetic information may be realized in definite traits of developing organism, in correspondence with external environment impacts.

Fourthly, it considers the problem of genetic information changing. It discovers the types and reasons of changing and mechanisms of its appearance.

The history of genetics starts from 1900. This is a year of rediscovering Mendel’s Laws of heredity by G. De Fris, K. Correns, A. Chermack. The first stage of genetics development covers the period between 1900 and 1912. It was a period of Mendel Laws of heredity recognition. The second stage of genetics development covers the period between 1912 and 1925. It was a time of accepting Morgan’s chromosomal theory of inheritance.

The third stage of genetics development (1925-1940) was characterized by discovering of artificial mutagenesis and by studying of genetic processes of evolution. In the fourth stage of genetics development (1940-1953) some works about genetic control of physiological and biochemical traits appeared. And the firth stage of genetics development (from 1953 to nowadays) is characterized by studying of genetic events on molecular level.

Gene level of hereditary information organization. The molecular genetics is a part of genetics, which studies molecular bases of heredity. It was founded in 40fl'-50lh years of 20lh century using a newly appeared ideas and devices in physics and chemistry. Perhaps molecular genetics, we may distinguish such levels of hereditary information organization as gene, chromosome and genome in prokaryotes and eukaryotes.

Gene level of hereditary information organization is closely connected with success of chromosomal theory of inheritance. Even in a first quarter of 20^th century, scientists mentioned that the gene is a material part of heredity, lying in chromosome, which is able to self-reproduction and is a minimal unit of recombination, mutation and genetic function.

G. Mendel suggested gene pointing by Latin alphabet letters. Genes, which encode development of alternative traits, are called allelic genes. Allelic genes situate in homologues locuses of homologues chromosomes. Each gene may have two conditions: dominant and recessive. Dominant gene is that dictates the appearance of heterozygotes. One allele is said to be dominant if an individual who is heterozygous for that allele has a same appearance as an individual who is homozygous for it.

Recessive gene is whose phenotype effect is masked in heterozygotes by presence of a dominant gene. A dominant allele is pointed by capital letter of Latin alphabet (A), and recessive allele is pointed by small letters (a). Organisms having similar alleles of one gene for example both dominate (AA) or both recessive (aa) are called homozygotes. Organisms having different alleles of one gene for example one dominate and other recessive (Aa) are called heterozygotes (pic 6.1). If an organism has only one allele of gene (like in male X chromosome), it is called hemizygote.

Ріс. 6.1. The scheme of allelic genes localization in homologous chromosomes: 1 - dominant homozygote; 2 - recessive homozygote; 3 – heterozygote

We see that last decade molecular genetics success changes old statements about gene. Today we may say that the gene - is a region of genomic nucleic acid, which is characterized by specific nucleotide sequence and which presents function unit different from other genes. Now it is stated that gene has a smaller divisions. It was discovered by American genetics S. Benzer. He studied a fine structure of E.coli T4 bacteriophag genes. He founded a gene to be divided into many parts during crossing over. Later, the same gene structure for eukaryotes was showed. The minimal unit of mutation is muton; the minimal unit of recombination is recon. Minimal size of them is one nucleotide pair.

Until 70th years of 20th century, it was believed gene to consist of unseparated DNA region. However, in 1977 it was shown that some adenovirus genes exist in a form of fragments instead of unseparated DNA region. These fragments may be exons (having useful information) and introns (without it). Introns are removed during gene expression (process of realization genetic information). Then, exons are bounded together by their ends. Such removing of unuseful information was named gene splicing (it was described above in chapter 2.1).

It is performed with help of special enzymes — revertases. In the beginning, this event seemed to be ridiculous, but later it appeared to be wide spread, especially in birds and mammalians. For example, gene of human - globulin contains three exons and two introns; gene of stable region of heavy chain of human immunoglobulin contains 4 exons and 4 introns. So it is said, gene to have a mosaic structure. Mosaic gene nucleotide sequence firstly is copied to pro-mRNA molecule. It is a precursor of mature mRNA.

Then, pro-mRNA is subject to gradually processing and splicing. And only after that it is ready to further transcription. The explanation for introns being isn’t still cleared. It is possible that exons will be bonded by different ways during splicing to form new proteins. Also it may be that introns serve as a material for new genes development during evolution. It was shown that intron mutations might break splicing process, terminate protein biosynthesis and change protein structure.

The term “gene” was firstly used for pointing some hereditary intends to form some phenotypic traits. In 1944 J. Beadle and A. Tatum pushed forward the hypothesis: one gene - one enzyme. Their idea since has been modified. Now, we know that many proteins are composed of several kinds of polypeptide chains, each specified by separate gene. The modem restatement of Beadle and Tatum proposal would be that one gene specifies one polypeptide. The DNA molecule may perform several functions. It has a nucleotide sequences not only having hereditary information but also controlling gene expression and replication.