大二下学习分子试题2008quiz2答案

1、 2008-05-20 Answers for quiz2- PAGE 5 -简答题： factor: a transcription initiation factor that usually binds to the -35 and -10 regions of DNA and recruit RNA polymerase to form the holoenzyme. This factor can be divided into four regions called region 1 through region 4. the regions that bind to the -3

2、5 and -10 region of the DNA are region 4 and 2, respectively. This binding is accomplished though the helix-turn-helix motif.Group II intron: Chemistry (2): splicing occurs as two sequential ester-transfer reactions. Firstly: the 2OH of the branch point A attacks the phosphoryl group of a conserved

3、55splice site, resulting in the free of 5 exon from the intron. Then, the free 3 end of 5 exon attacks a phosphoryl group at the ligation of the 5 and 3Mechanisms (4): the splicing is catalyzed by the intron RNA itself, which is also named as self-splicing. TRNA editingRNA编辑A form of RNA processing

4、in which nucleotide sequence of the primary transcript is altered by either changing, inserting or deleting residues at the specific points along the molecule.4.SsrA:a. a tmRNA that is partly tRNA and partly mRNAb. it rescues ribosomes stalled by prematurely terminated mRNAc. the rescue concerns EF-

5、Tu-GTPd. SsrA binding adds a ten-amino-acid tag that is recognized by cellularproteases5. Nonsense mediated mRNA decay (NMD):When an mRNA contains a premature stop codon (nonsense codon), the mRNA is rapidly degraded by the process of nonsense-mediated decay. Translation of an mRNA with a premature

6、stop codon does not displace one or more of the exon junction complexes. This results in the recruitment of the Upf1, Upf2, and Upf3 proteins to the ribosome. Once bound the ribosome, these proteins activate a decapping enzyme that removes the 5 cap of mRNA. The umcapped mRNA is then rapidly degrade

7、d by 5 to 3Multiple choices/single choice:1：3);2: 1), 2), 4);3. 1), 4);4. 1), 2).Short questions:1.How does transcription terminate in prokaryote? (15)Transcription in prokaryote can be terminated by Rho-independent (2) or Rho-dependent (2) mechanism. Rho-independent termination requires the termina

8、tors contain a short inverted repeat rich in G and C followed by a stretch of about eight A:T base pairs. After the terminator transcription, the mRNA can form a strong hairpin structure (3)followed by a stretch of weak A:U base pairs with its DNA template (3), and mRNA easily dissociates from the D

9、NA template. Rho-dependent termination requires no intrinsic terminator RNA structure but requires Rho factor (1) that is a ring-shaped protein with six identical subunits (2). It uses the energy derived from ATP hydrolysis to stop transcription (2).2.In splicing, two ways ensure the accuracy of spl

10、ice-site selection. First, in transcription, RNA Polymerase II carries proteins involved in splicing at C-terminal tail. It will recognize the correct 5 and 3 splicing site before any competing site have been transcribed. Second, SR proteins which bounds to exonic splicing enhancers (ESEs) also ensu

11、re more efficient binding of U2AF and U1 to specific splicing sites. (3)The process of splicing : (12)Step1,formationoftheE(early)complexbyrecognitionofthe5splicesite,3splicesiteandAbranchpointbyU1snRNP,U2AFandBBPrespectively.Step2,U2snRNPbindtothebranchsitetoreplaceBBPformsAcomplex.Thebase-pairingb

12、etweenU2snRNAwiththebranchsitemaketheconservedAresidueinthebranchsiteextrudedfromthepairedregion,andthusthisAisreadytocarrythenucleophileattack.Thetri-snRNPU4/U6/U5joinsandAcomplexisarrangedtoBcomplexinwhichthethreesplicesitesarebroughttogether.InthiscomplexU4/U6snRNPsareheldtogethertightlybyextensi

13、vebase-pairingbetweenU4andU6snRNAs.Step3,U1leavesthecomplex,andU6occupiesthe5splicesitebybase-pairing.U4leavesthecomplex,allowingtheRNAcomponentsofU2andU6tobasepairtoproducetheactivesite.ThebranchsiteAattacksthe5splicesite,formingthe3-wayjunctionandCcomplex.The5splicesitethenattacksthe3splicesite,fr

14、eeingtheintronlariatandformingthemRNAproduct.回答剪接过程其它一些方面也会适当加分。3.All the possible anticodons: CAG,UAG,IAG,GAG,AAG(默认方向为5-3或标明3-5(以上每种anticodon写对一个给2分，五个共十分，写对了其中某些但多写很多其他错误的酌情扣2分左右，按3-5maximal number:5，（2分）minimal number:2，（3分）此处写出哪两种并正确可以加2分。加分原则不超过满分，犯明显错误的不能应加分点得到满分。如果以上得分点均未答，但正确写出摆动理论加2分。4. 分五

15、点给分，每点3分。答到二者作用机理，而未涉及到如何保证其正确率的酌情给分。Aminoacyl tRNA synthetase:Aminoacyl tRNA synthetases are responsible for selecting the right amino acids and attach them to the appropriate tRNA.3On the one hand, Aminoacyl tRNA synthetases discriminates different amino acids according to their size, shape, chemi

16、cal groups, as well as based on the energetically favorable chemical bonds or interaction. (For instance, the amino acids cysteine and tryptophan are distinguished by their substantially different size, shape and chemical groups. A3On the other hand, some aminoacyl tRNA synthestases use an editing p

17、ocket to charge tRNA with high accuracy. The above two strategies achieve 10000-fold selectivity.Ribosome: At least three mechanisms contribute to the the selection against incorrect codon-anticondon pairings. 3One mechanism that contributes to the fidelity of condon recognition involves two adjacen

18、t adenine residues in the 16S rRNA component of the small subunit. These bases form a tight interaction with the minor groove of the each correct base pair formed between the anticodon and the first two bases of the codon. These interactions result in lower rate of dissociation from the ribosome in

19、correctly paired 3A second mechanism involves the GTPase activity of EF-Tu. The mismatch in base pairing leads to a dramatic reduction in EF-Tu GTPase activity which is responsible for release of3A third mechanism is a form of proofreading that occurs after EF-Tu is released. The initially charged t

20、RNA must rotate into the pepetidyl transferasecenter of the large subunit in the process of accommodation. Incorrectly paired tRNAs frequently dissociate from the ribosome during the rotation. OPART IVProcess: Transcription：from DNA to pre-mRNARNA Processing (Splicing, 5Translation :from RNA to polypeptideFactor Transcription:Initiation: Cis-acting element: Promoter (BRE, TATA box, Inr, DCE I, DCE II, DPE, DCE III), Regulatory sequences: promoter proximal elements, upstream activator sequences, enhancers, silencers, boundary elements, insulators. Trans-acting fact