the Cage of a green plant. A kernel, ribosomes and fiber synthesis

you are: the Cage of a green plant

For some last decades biologists managed to make to themselves full enough representation about structure, himizme and functions of the major cellular organell. The largest organella cages - a kernel (fig. 2.3 see). This spherical body in diameter 5-10 microns contains the most part of the genetic information of the cage coded in the form of long threads of a difficult chemical compound - a deoxyribonucleic acid (DNA). DNA are present at a cage in structure hromatina - the difficult substance consisting in basic from negatively charged DNA and positively charged fibers, belonging to a class gistonov. In not sharing cage hromatin forms a friable network. To the division beginning the true nature of this ' networks ' comes to light - hromatin is condensed and forms discrete, easily identified palochkovidnye the little bodies named chromosomes, which number for cages of each kind constantly. In any cage of a plant of peas there are, for example, 14 chromosomes - on 7 from each of parents. At the person the number of chromosomes is equal in a cage 46, on 23 from mother and from the father. Such cellular division at which the number of chromosomes doubles so each daughter cell receives a double set of chromosomes, carries the mitosis name (see more low).

The Full double set of chromosomes name diploidnym (2 n), and a set received from each of parents through sexual cages, - gaploidnym (n). All cages of the higher plant, behind an exception gaploidnyh sexual, at least diploidny. Gaploidnye sexual cages are mainly in mature pollen grains and in a germinal bag semjazachatka. In plant life cycle gaploidnyj the set turns out from diploidnogo as a result reduktsionnogo divisions, or mejoza (p. 34 see), proceeding in parent cages micro- and the megadispute, being accordingly in pylnikah and semjazachatke a flower. Arisen gaploidnye cages share such way and give rise man's and female gametofitam in which eventually and sexual cages, or gametes are formed, i.e. spermii and jajtsekletki. When - at sexual reproduction - female and man's gametes merge in a zygote, there is a restoration diploidnogo the number of chromosomes peculiar sporofitu. Tracing changes in number of chromosomes and in DNA maintenance, we see that in a floral plant the cycle in which diploidija it is replaced gaploidiej, and the subsequent merge gaploidnyh cages of a different genetic origin in new diploidnyj an organism is made generates new combinations of genetic signs.

THE MITOSIS

In a cage before visible display mitoticheskoj activity the quantity of chromosomal DNA doubles. The mitosis begins, when chromosomes have already doubled and it is visible that each of them consists from two number of lying threads, or two halves, so-called hromatid. The kernel and a nuclear membrane disappear, chromosomes are built in an equatorial plane of a cage, and the threads of a spindle connecting an equatorial plate with poles of a cage, take away to opposite poles sisterly hromatidy which turn now in two separate, but quite identical chromosomes. Are reconstructed typical.interfaznye kernels, in each of which the membrane, a kernel and a network hromatina is restored. Soon, after one kernel was divided into two, there comes a finishing stage of cellular division - formation of the cellular plate halving a cage in an equatorial plane (tsitokinez). Actually the mitosis lasts 1-2 ch, on synthesis of DNA and the other preparatory reactions necessary for following cellular division, leaves still nearby 6 ch (fig. 2.9).

MEJOZ

Mejoz differs from a mitosis that already doubled homologous chromosomes approach and settle down nearby, closely adjoining to each other on all length (sinapsis). During this period four pressed one to another hromatidy can be mixed up and exchange separate sites (krossingover); in any given point undergo krossingover two can only hromatidy. Steams of chromosomes are built in an equatorial plate, homologous chromosomes, like sisterly hromatidam at a mitosis, disperse to opposite poles and finally appear in two different cages. in each of these daughter cells occurs then the second division mejoza (as a matter of fact, a mitosis in a cage with gaploidnym number of chromosomes) at which to opposite poles disperse sisterly hromatidy. As a result of two mejoticheskih divisions four cages are formed, everyone with reduced twice (gaploidnym) number of chromosomes, and is possible (thanking krossingoveru), and with any new, distinct from parental types of chromosomes (fig. 2.9 see).

Until recently was considered that in a vegetative organism all somatic cages diploidny. Now, however, we know that in plants there are separate congestions of cages number of chromosomes, multiple diploidnomu: 4п, 6п, 8п etc. Such poliploidnye cages arise, obviously, as a result of the nuclear fissions which are not accompanied by a divergence of affiliated kernels on two separate cages. Ploidnost cages it is possible to regulate sometimes by means of those or other natural or synthetic substances. poliploidiju, for example, it is possible to cause kolhitsinom - alkaloidom from klubnelukovits (the truncated fleshy stalks) bezvremennika autumn.

Kolhitsin interferes with formation of threads of a spindle and by that prevents a divergence of two sets of chromosomes to opposite poles of a sharing cage. Sometimes arise aneuploidy. It results from casual loss or, on the contrary, acquisition of one or several chromosomes. Such copies (2n+1 or 2n-1) usually are to some extent abnormal, but they can be and viable.

DNA Molecules in chromosomes represent the linear polymers constructed of four types nukleotidov. The arrangement order nukleotidov in DNA chain defines the genetic information which is born by this chain. The heredity at a plant depends, thus, in essence on an arrangement nukleotidov in molecules of DNA which are in chromosomes of a kernel and in some other cellular organellah, namely in hloroplastah and mitohondrijah. One of the most fascinating heads of modern biochemistry is made by history of opening of the mechanism by means of which the information concluded in molecules of DNA, connection DNA - RNK (ribonucleic acid) is transcribed with formation related, and then information RNK is in turn broadcast also by such way defines the nature of again synthesised fibers. As this process has crucial importance for cellular structure and function, we here will describe it in detail enough though much in this area became already well-known even among nonspecialists.

On an extent bolshej parts of the life of a cage the kernel is separated from cytoplasm by the nuclear cover representing a double membrane with a numerous large time and long vyrostami, getting deeply in cytoplasm (fig. 2.3 see). Vyrosty often directly pass in endoplazmatichesky retikulum (Ayr) - strongly branched out membrannuju a network penetrating all cytoplasm. Ribosomes - spherical particles in diameter about 0,2 microns consisting in basic from high-molecular RNK and the squirrel, - can be attached to membranes endoplazmaticheskogo retikuluma, but can lie also in cytoplasm freely. The ribosomes attached to Ayr are a part rough endoplazmaticheskogo retikuluma; most likely, they participate in synthesis of that fiber which is allocated in a gleam Ayr, i.e. In the closed space between its membranes. As to the ribosomes which are in cytoplasm they sometimes groups join to information (matrix) RNK and then it is visible that they settle down along this long thread. Such congestions of ribosomes name polyribosomes: like ribosomes Ayr, they participate in fiber synthesis until keep contact with matrix RNK. Fibers - the large molecules constructed of amino acids located in a certain order, - are synthesised on a surface of ribosomes. The difficult mechanism of this synthesis includes a transcription, i.e. Transfer of the information containing in DNA in RNK, and then translation - transfer of the information coded in sequence; nukleotidnyh rests RNK in sequence aminokislotnyh the rests of synthesised fiber. As work of all biochemical factory of a cage is regulated by active albuminous molecules - enzymes, it is necessary to search for a key to cellular regulation that specificity of fibers finally is defined DNA.

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