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A GTP linked to the 5′ end of the mRNA by a 5′-5′ linkage forms the basic cap. By the way, 5′ caps are never worn backward.
A specific endonuclease recognizes an AAUAAA sequence and cleaves the precursor. Poly(A) polymerase adds a tail of adenylate residues to the 3′ end of the transcript.
The addition of the 5′ cap, the addition of the poly(A) tail, and the removal of the introns by splicing
The carboxyl-
(a) Because cordycepin lacks a 3′-OH group, it cannot participate in 3′ → 5′ bond formation. (b) Because the poly(A) tail is a long stretch of adenosine nucleotides, the likelihood that a molecule of cordycepin will become incorporated into the poly(A) tail is higher than it is for most RNA. (c) Yes, it must be converted into cordycepin 5′-triphosphate.
A spliceosome is the splicing machinery in the nucleus. It is composed of snRNPs (U1, U2, U4, U5, and U6) and various protein splicing factors.
The carboxyl-
One gene with several introns can be spliced to yield several different mRNAs, and these mRNAs will produce different proteins. In essence, one gene can encode more than one protein.
Because the gene contains eight sites at which alternative splicing is possible, there are 28 = 256 possible products.
Alternative splicing allows the formation of more than one protein from a gene.
The splicing mechanisms are similar in two respects. First, in the initial step, a ribose hydroxyl group attacks the 5′ splice site. The newly formed 3′-OH terminus of the upstream exon then attacks the 3′ splice site to form a phosphodiester linkage with the downstream exon. Second, both reactions are transesterifications in which the phosphate moieties at each splice site are retained in the products. The number of phosphodiester linkages stays constant.
Alternative splicing and RNA editing. Covalent modification of the proteins subsequent to synthesis further enhances the complexity.
Attach an oligo(dT) or oligo(U) sequence to an inert support to create an affinity column. When RNA is passed through the column, only poly(A)-containing RNA will be retained.
Histones are required in large amounts and in equal quantities only when DNA is being synthesized. The multiple arrays with an equal number of histone types per array will facilitate rapid production of a large number in equal proportions. The lack of posttranscriptional processing may speed up the synthesis of the histones themselves, and the lack of a poly(A) tail may facilitate the degradation of the histone mRNA, which need be present only during DNA synthesis.
Ser-
C39
A mutation that disrupted the normal AAUAAA recognition sequence for the endonuclease could account for this finding. In fact, a change from U to C in this sequence caused this defect in the thalassemic patient. Cleavage was at the AAUAAA 900 nucleotides downstream from this mutant AACAAA site.