3. 2 posttranscriptional processingin eukaryote（真核）                                                                             ——mRNA

——★ mRNA转录后加工类型 :

 · ① Capping（加帽）

 · ② Addition of poly A tail(添加 poly A 尾)

 · ③ Splicing（剪接）

 · ④ Methylation(甲基化)

 · ⑤ RNA editing （RNA 编辑） 

 

 ① Capping（加帽）

——★ 

 

——★ The addition of 5'cap

——★Function:

  ·1.Help to stabilize mRNA(稳定mRNA防止降解)

  ·2.Permit initiation of translation (specific, where translation 

    should begin)-协助起始翻译过程.

② Addition of poly A tail(添加 poly A 尾)

★Specific endonuclease cuts at modification site

★PolyA 聚合酶催化无模板添加A延伸.

★polyA :∽more than 250 nt .

                                               ★Histone: no poly A tail.

 ★Roberts and Sharps found interrupted gene(断裂基因) in 1977 separately.

③ Splicing（剪接）

 ★断裂基因(Split gene)

   A eukaryotic gene in which the coding sequence is divided into two or more exons that are interrupted by a number of noncoding intervening sequences (introns). Also known as interrupted gene.——去除非编码区再连接后，可翻译出由连续氨基酸组成的完整蛋白质。 

· 外显子(Exon)和内含子(Intron)

★Exon

·DNA sequence presentin final mature RNA product and codes for primary sequence of protein.

★Intron

·DNA sequence not present in final mature RNAproduct and is removed during splicing process.

    

      

· Splicing of mRNA  (mRNA剪接）

                                —— Introns are removed and exons are joined.

★snRNA（U1,2,4,5,6）+ protein =  snRNP ( small nuclear ribonucleoprotein, 小核核糖核蛋白)

snRNP+hnRNA=剪接体      

              （Splicesome） 

· Two steps of transesterification reactions（二次转酯反应）

· 鸡卵清蛋白基因及其转录和转录后修饰

· Alternative splicing

· Alternative splicing oftropomyosin (TM, 原肌球蛋白)

One gene  →→ multiple mRNAs  →→ multiple proteins

                                                                    （Isoforms）

· Faulty splicing can cause disease

★ β-thalassemia(β-地中海贫血) : There is a point mutation in exon-intron junction of β-hemoglobin chain, leading to abnormal splicing of intron（β链的异常剪接） and further diminished or absent synthesis of β-chain protein of hemoglobin.

④ Methylation(甲基化)

⑤ RNA editing （RNA 编辑） 

★RNA editing: a kind of process which changes sequence of RNA transcript by untemplated insertion, deletion or substitution. It can alter amino acid sequence of protein coded which differs from that predicted by DNA sequence.-----非DNA模板依赖的mRNA序列改变。

★Differential RNA processing(差异加工)。

·Structure of mature mRNA

·UTR: untranslated region（非翻译区）

内含子功能

★1. Can be further processed after splicing to generate noncoding RNA molecules.

★2. Alternative splicing (选择性剪接)is widely used to generate multiple proteins from a single gene. 

★3. Represent mobile genetic elements and may be regarded as examples of selfish DNA.

★Provide more possibilities for genome!