一、Term
Splicing (剪接)
Capping(加帽)
Polyadenylation (加尾)
Intron(内含子)
Exon(外显子)
RNA splicing (剪接)
Spliceosome (剪接体)
NTS: nontranscribed spacer非转录间隔区
Small nucleolar RNAs (snoRNAs)核仁小RNA
Small nucleolar ribonucleoprotein (snoRNPs)核仁小分子核糖核蛋白
Polynucleotide phosphorylase (磷酸化酶) (PNPase)
Trans-splicing反式剪接
二、Splicing (剪接)
Genes for mRNAs have been found with anywhere from zero to 60 introns.
Transfer RNA genes have either zero or one.
1、可用来证明内含子：
2、剪接信号
First two bases of the intron: GU
Last two bases of the intron: AG
Several lines of evidence demonstrate that nuclear mRNA precursors are spliced via a lariat-shaped, or branched, intermediate. 多条证据表明，核内的信使核糖核酸前体通过套索状或分支的中间体进行剪接。
In addition to the consensus sequences at the 5’- and 3’- ends of nuclear introns, branch point consensus sequences also occur. 除了核内含子5‘端和3’端的共有序列外，还存在分支点共有序列
3、Alternative splicing
One precursor RNA can be spliced in different ways.
4、Self splicing RNAs
Some RNAs could splice themselves w/o aid from spliceosome or any other proteins
自剪接分为Group I introns 和Group II introns
Group I introns：不需要蛋白质帮助，可以在体外进行；没有分枝点A；形成的是线性内含子
Group II introns：在线粒体和叶绿体中找到；不需要蛋白质不需要能量；基本上磷酸二酯键的数量没有改变
5、tRNA splicing
Splicing of tRNA also have two steps:
(1) A tRNA endonuclease cuts out the intron.
(2) A RNA ligase stitches the two half-tRNAs together.
三、Capping
1、Capping: A methylated guanosine bound through a 5’-5’ triphosphate linkage to the 5’-end of an mRNA. Happen in nucleus.
2、Types of capping:
1) Cap 0: 7-methyl-guanosine (m7G), lack of 2'-O-methylation一个m7G
2) Cap 1: 7-methylated guanine & a 2'-O-methyl on this sugar of the first base一个m7G加一个甲基
3) Cap 2: 7-methylated guanine & 2'-O-methyls on the sugars of the first and second bases一个m7G加两个甲基
3、Cap 合成过程
4、Cap functions
Protection of the RNA from degradation保护mRNA不被降解
Enhancement of the mRNA’s translatability. Increasing efficiency of translation提高翻译效率
Facility of mRNA transport from nucleus to cytoplasm从细胞核运到细胞质
Proper splicing of the pre-mRNA保证mRNA前体的正确剪接
四、3’ Polyadenylation
Polyadenylation: A process of adding poly(A) to RNA
Polyadenylation occurs post-transcriptionally by poly(A) polymerase
About 250 nucleotides long
Consensus sequence: AAUAAA
Function of poly (A)
Protection of mRNA 保护mRNA不被降解
Enhancement of the mRNA’s translatability增强mRNA的翻译效率
五、Other RNA Processing: rRNA and tRNA
Processing：The trimming of excess regions from an RNA precursor
RNA processing和splicing很相似，都是将不必要的RNA去除，但是processing不会再把RNA连接起来
（一）rRNA processing
1) Eukaryotic rRNA Processing:
(snoRNAs and snoRNPs)
2) Prokaryotic rRNA Processing:
(RNase III and RNase E )
1、真核
rRNA gene被NTS序列一个个隔开了，在形成成熟的mRNA过程中会去除
Small nucleolar RNAs (snoRNAs)
Small nucleolar ribonucleoprotein (snoRNPs)
E1, E2 (snoRNPs) : 18S rRNA maturation
E3 (snoRNP) : Cleavage upstream of 5.8S rRNA
2、原核：RNase III和RNase E
（二）tRNA Processing
1、Cutting apart polycistronic (多顺反子) precursors
(RNAse III)
2、Forming mature 5’ –ends
(RNAse P)
RNAse P: contains two subunits. one of these subunits is made of RNA , not protein.
3、Forming mature 3’ –ends
六、Trans-splicing反式剪接
The splicing we talked previously can be called cis-splicing because it involves 2 or more exons that exists together in the same gene.
Trans-splicing, the exons are not part of the same gene at all and may not even be found on the same chromosome.是两个独立基因之间的剪接
七、RNA editing RNA编辑
In trypanosome在锥体虫中，Some Us are added or deleted in the mature mRNA during the maturation, this is called RNA editing
mRNA在成熟过程中会进行RNA编辑