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the mRNA generated by transcription in primary nuclear organisms is polysuper, i.e. several structuresgene, using common promoters and co-terminating signals to transreceive an mRNA, So this mRNA molecule encodes several different < a href"" > protein . For example, the Z, Y and A genes on lactose manipulators, transcription-generated mRNAs can be translated to produce three enzymes, namely semi-lactose glycosidease, through enzymes and acetyl-based transferase There are no nuclear modules in the original nuclear organism, so transcription and translation are continuous, often transcription has not been completed, translation has begun, so the mRNA generated by transcription in the primary nuclear organism does not have a special post-transcription processing modification process.
< the mRNA produced by > p class"tt1" ernutation is a sy monotone, i.e. an mRNA molecule is encoded for only one protein molecule.1. At the 5th end of the cap
mature etonymRNA, the structure of 5' has an m7G-PPNN structure, the structure is called methyl bird glycoside hat. As shown in Figure 17-9. Bird glycoside is connected to the 5' end of the primary transcript by using the 5'-5' coking phosphoric acid bond. When the 7th carbon atom on the bird glycoside is methylated to form m7G-PPNN, the hat formed at this time is called "cap 0", if attached to m7G-PPNMN, this nuclear sugar "2" carbon also A Based, forming m7G-PPNm, called "cap 1", if 5' end N1 and N2 in the two kernels are methylated, become m7G-PPNmPNm2, called "cap 2". It can be seen from the complexity of the formation of the true nuclear biological hat structure that the higher the degree of biological evolution, the more complex the hat structure.
Figure 17-9 Post-transcriptional modification of the mRNA show the 7-methylguanosine cap and poly-A tail.
the importance of the-core biomRNA 5-end hat structure is that it is the necessary structure for mRNA to be used as the starting point for translation, providing a signal for the identification of mRNA by the rnatic glycogen, which may also increase the stability of mRNA and protect mRNA from 5 extumsnucleic acidenzymes.
2. At the 3' end plus tail
most of the posnel mRNA has a 3' polytail (A), and the poly (A) tail is approximately 200bp.
polyjust (A) butchers are not encoded by DNA but are tweed and added to the nuclei. Catalyzed by polyA polymerase, the enzyme is able to identify the free 3'-OH end of mRNA, and adds about 200 A residues.
In recent years, it has been known that there is an AATAA sequence at one end of the 3' of most poon genes, which is a signal of mRNA 3'-end plus polyA tail. Bynuclease in this signal downstream 10-15 base outside the cutting off of the phosphate phosphate phosphate dester bond, in polyA polymerase catalysis, in 3'-OH one by one the introduction of 100-200 A base. The question of the function of polyA tail, although extensively explored, is not entirely clear. It has been speculated that polyA may be related to the transfer of mRNA from the nuclei of cells to cytones, but a significant amount of mRNA without polyA butchers, such as histone mRNA, also enters the cytocyte through the nucleoblast. It is also thought that this structure has some effect on the translation efficiency of the urn mRNA, and can stabilize the mRNA structure and maintain a certain biological half-life.
3.mRNA pregenitor (hnRNA
"tt1" > the structural genes of primary nuclei are continuously encoded sequences, while the etonymical genes are often fractured genes, i.e. nucleotides is separated by multiple insertions, and the number of inclusions in an ebony biostruct structure gene is often related to the size of the gene, for example, insulin is a very small protein, its structural gene has only two containing children, and some large proteins, its structural genes can have dozens of inclusions. After a complex process, the internals are cut off and the encoded nucleotide fragments (Extron exons, also known as exons) are connected (Figures 17-10).
Figure 17-10 Primary polymerase 11transcript of a eukaryote gene show (a) introns aftering and plus of polyA tail. b) Excision of introns to form the mature mRNA is called splicing.
the structure ofthe structure of the urn has expressive activity of exons, but also contains non-expression activity of the inclusion of extratones, but the inclusion of subsethics is meaningless, more and more experiments show that there are many genes containing children involved in gene expression regulation, in transcription, exons and endotides are transped into hnRNA. In the nuclei of the cell hnRNA to cut off the inclusions under the action of nucleic acid incision enzyme, and then under the action of the connective enzyme, the exon parts are connected, and become mature mRNA, this is the shearing effect, there are a few genes hnRNA does not need to be sheared, such as α-interferon gene, Figure 17-11 to the egg white protein gene as an example, introduced a typical transcription and processing process.
Figure 17-11 Egg white protein gene transcription and processing process
< > Indicates that the inclusions are A, B, C, D... Indicates thatmRNA stitching, need to be in the stitching site for stitching identification of a conservative consistent order, through the analysis of more than 100 kinds of urn genes, found that the external and endogenetic stitching part of the base sequence has a certain law (see Table 17-4).
Table 17-4 contains the base order of the transcription product of the inner element at its splicing
table plays an important role in stitching recognition, such as changing the GT of the stitching site of the rabbit's β-globulin to AT, the stitching reaction is affected.
mRNA preseor spelling mechanism
Figure 17-12 The RNA splicing mechanism. RNA splicing is catalyzed by a spliceosome formed from the assembly of U1, U2, U5, and sn RNPs (shown as green circles) plus other components (not show). After assembly of the spliceosome, the reaction eventes in two speps:in step 1the branch-point A nucleotide in the intron sequence, which is located colse to the 3'splice site, attacks the 5'splice site and cleaves it; the cut 5'end of the intron sequence the becomes covalently linked to this A nucleotide, forming the branched nucleotide show in Figure 8-55.In step 2 the 3'-OH end of the first exon sequence, which was created in the first step, adds to the beginning of the second exon sequence, cleqving the RNA molecule at 3'splice site; These splicin
