Structure of RNA

All RNA have unary (sole) individual polynucleotide chain. Their Mm is less than Mm of DNA. The content of RNA in cells is changed depending on age, function, tempreture. Mononucleotides of RNA contain ribose as pentose, A, G as purine bases and U, C as pyrimidine base.

There are 3 kinds of RNA: messenger RNA – mRNA, transfer RNA – tRNA, ribosomal RNA – rRNA.

MRNA is discovered by Jackob and Maneau in 1961. It makes 2-3% from all quantities of cellular RNA. It does not have (has not) rigid specific secondary structure. Its sole polynucleotide chain forms the different loops bands and changs constantly dependly from ionic concentration of solution. In unworking condition mRNA turn into globulla. During the function the polynucl. Chain of RNA is opened and stretched mRNA is synthesized on DNA in nucleus and this process is called transcription. MRNA is carrier of the genetic information from DNA to aminoacid’s sequence of protein. The place of every a/a in proteins is coded by sequence of mononucleotide in mRNA.

The idea of genetic code has arised by known physics Gamov – an author of theory of extending universe. Gamov supposed that cells have the dictionary translating the fourth letter alphabet text in the twenty word’s alphabetic of protein.

In 1961 M. Nirenberg and G. Mattei synthesized m.RNA, consisting from uridil only – polyuridil and synthesized on them protein, consisting of residues of phenylalanine only. The number of the phenylalanine residues was less than number of uridile residues in three times so they established that phenylalanine is coded by three uridiles. This message was made on 5-th biochemical congress in Moscow in 1961 and has caused sensation since at last the biochemists can possible to understand what is gene, from what it consists of. In decoding a genetic code the large role had played the research Crick, Nirenberg, Holly, Corana. Last three scientist have received the Nobel premium in 1968.

Now the genetic code is completely deciphered. Now is proved that the arrangement of aminoacids in polypeptide is defined by three mononucleotides. So the genetic code is triplet. Triplets of mRNA has received the name “codon”.

From four of nucleotide (A, G, C, U) by rules rearrangements it is possible to construct 64 codons of mRNA.

From this number 61 codons ciphers 20 aminoacids. Three codons do not cipher any aminoacids, they play a role of “stop codons” or terminal codons. There are 20 a/a and 61 codons from 20 a/a only tryptophan and methionine have one codon only and other – 2-3-6 codons. This property of a genetic code are named a degeneration. The researches have shown that first two letters codone define the specificity everyone codone and the third letter has smaller significance for specificity.

The following property of a code is its universality. Same a/a is coded by identical codones as an viruses, microbes, in vegetative (plants), as and the animal cells.

At last cod is unremitting. It does not interrupt by any stopping marks. The reading out a code from one point only – is uncrossed cod.

This characteristic is lawful for a cod at a level mRNA only.

The coding of the genetic information in DNA occurs more difficultly and complicatly and this process still is not found out up to the end.

However recently it is established that one sequence of mononucleotides can be ciphered the structure two or even of three proteins. Experiments with DNA of bacteriophage XI74 proved that there are other genes inside gene of bacteriophage. Such phenomenon economizes a genetic material a square of cell, occupied by DNA.

Other rules of code are exposed to doubt also ( triplet, degeneration, uncrossing).

Thus mRNA accepts direct participation in biosynthesis of proteins. Now the questions of synthesis of artificial mRNA are successfully decided. Such mRNA for hemoglobin are synthesized by Corana in USA and Kiselev in Russia.

Ribosomal RNA (rRNA)

The rRNA makes more than 80% of all cellular RNA. RRNA come into the composition of ribosomes. Ribosomes are nucleoprotein complexes, they consist of rRNA (65%) and proteins (35%). Everyone of rRNA joints with 30 protein’s molecules. Every ribosome consist of two subunits – big and small in ratio 2,5:1. Ribosomes make 25% of cell’s mass, 60% of their mass are hydrated and that is why different hydrophylic substances penentrate to them easily.

The polynucleotide chain of rRNA coils and joints with proteins making compact body.

The function of ribosomes is possible when they are intact. One break of polynucleotide chain of mRNA and ribosome lead to lose their biological activity.

When mRNA joints with one ribosome the matrix is formed. When mRNA joints with few ribosomes the polysome is formed.

MRNA plays specific role in matrix and polysome. The role of rRNA is nonspecific and ribosomes of frog can synthesized hemoglobin of rabbit if they joint with mRNA of hemoglobin of rabbit.

tRNA (transport or transfer RNA) contains in cytoplasm and makes approximately 10% of all cellular RNA. tRNA is investigated better than mRNA and rRNA. Their molecules are small, their molecular mass is 20000D and consist of 75±5 nucleotides.

Basic role of tRNA is transport and installation of aminoacids on the corresponding codone of mRNA. Separate tRNA correspond to everyone aminoacid, so tRNA-s are specific to aminoacids, their specificity is provided by enzyme – aminoacylsynthetase. Nowdays not only primary structure (sequence of nucleotides) of tRNA is investigated, but their secondary structure is known.

In 1965 Holly and Baev have established structure for valine tRNA which has appeared universal for all tRNA. A molecule of tRNA in denaturated condition has form of trifolium or a clover-leaf shape. In active condition is curtailed in globulus. There are 4 active centers in tRNA. 1- acceptor stalk – site with a final sequence of nucleotides ACC. This center joints different aminoacids. Kind of jointing of aminoacids depends of nucleotide’s sequence of second center, which is different for different aminoacids. This center is jointed with aminoacylsynthetase, which is specific for each aminoacids and only than aminoacylsynthetase joints determined aminoacids to determined aminoacid to acceptor’s stalk.

Third center in tRNA is site for connection with ribosome. Fourth center is named anticodon which consist of three nucleotides, bases of which gives the tRNA a remarkable property to recognize the codon presenting over the mRNA and coding the aminoacid because bases in anticodon are complementary to bases of codone.

So the role of tRNA is recognize the place of every aminoacid on the mRNA and therefor in the protein’s molecule.

tRNA “markes” of aminoacids giving them the specificity and establishes aminoacid on determined codon in mRNA. The characteristic feature a structure of tRNA is the presence of minor (modified) bases – methyl-, oxy-, thyobases. For example – pseudouracil, dehydrouracil.

These modified bases are capable to unclassical pairing. In this connection the quantity of tRNA is less almost in 2 times than the quantity of codones. For example modified base hypocsantine (purine) can form hydrogen bonds with C, A, G. The second peculiarity of modified bases is the feaility, fragility of hydrogen bonds, which they form. This promotes to easier clearing from a complex codon – anticodon. If all 3 bases of anticodons have formed strong Watson-Creak’s pair the connection between codon – anticodon would be so strong, that releasing of tRNA from a complex with mRNA would occur slowly and limited speed protein’s synthesis.

<== попередня сторінка	\|	наступна сторінка ==>
Primary st. of DNA is a spirally one polynucleotides chain (PNC), the disposition of nucleotides in which determine all hereditary properties of organism.	\|	Enzymes

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