页数：37页文件大小：1.61 MB来源： 分子生物学基因Geneppt课件
本课件为《分子生物学》第5章信使RNA（Messenger RNA），依据 Gene VII 教材编排，沿Transfer RNA is等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 5 Messenger RNA
5.1 Introduction5.2 Transfer RNA is the adapter5.3 Messenger RNA is translated by ribosomes5.4 The life cycle of bacterial messenger RNA 5.5 Translation of eukaryotic mRNA 5.6 The 5¢end of eukaryotic mRNA is capped5.7 The 3¢terminus is polyadenylated5.8 Bacterial mRNA degradation involves multiple enzymes5.9 mRNA degradation involves multiple activities5.10 Sequence elements may destabilize mRNA
Figure 5.1 Transcription generates an RNA which is complementary to the DNA template strand and has the same sequence as the DNA coding strand. Translation reads each triplet of bases into one amino acid. Three turns of the DNA double helix contain 30 bp, which cide for 10 amino acids.
Figure 5.2 A tRNA has the dual properties of an adaptor that recognizes both the amino acid and codon. The 3adenosine is covalently linked to an amino acid. The anticodon base pairs with the codon on mRNA.
Figure 5.3 The tRNA cloverleaf has invariant and semi-invariant bases, and a conserved set of base pairing interactions.
Figure 5.4 Transfer RNA folds into a compact L-shaped tertiary structure with the amino acid at one end and the anticodon at the other end.
Figure 5.5 A space-filling model shows that tRNAPhe tertiary structure is compact. The two views of tRNA are rotated by 90o. Photograph kindly provided by S. H. Kim.
Figure 5.6 The meaning of tRNA is determined by its anticodon and not by its amino acid.
Figure 5.7 A ribosome consists of two subunits.
Figure 5.8 A poly ribosome consists of an mRNA being translated simultaneously by several ribosomes moving in the direction from 5-3. Each ribosome has two tRNA molecules, one carrying the nascent protein, thesexond carrying the next amino acid to be added.
Figure 5.9 A ribosome assembles from its subunits on mRNA, translates the nucleotide triplets into protein, and then dissociates from the mRNA.
Figure 5.10 Protein synthesis occurs on polysomes. Photograph kindly provided by Alex Rich.
Figure 5.11 Messenger RNA is translated by ribosomes that cycle through a pool.
Figure 5.12 Considering E. coli in terms of its macromolecular components.
Figure 5.13 Over view: mRNA is transcribed, translated, and degraded simultaneously in bacteria.
Figure 5.14 Transcription units can be visualized in bacteria. Photograph kindly provided by Oscar Miller.
Figure 5.15 Bacterial mRNA includes non-translated as well as translated regions. Each coding region has its own initiation and termination signals. A typical mRNA may have several coding regions.
Figure 5.16 Eukaryotic mRNA is modified by addition of a cap to the 5end and poly(A) to the 3end.
Figure 5.17 Over view: expression of mRNA in animal cells requires transcription, modification, processing, nucleocytoplasmic transport, and translation.
Figure 5.17 -2 Exogenous mRNAs can be translated by cell-free systems or by injecti on into Xenopus oocytes.
CAP (also known as CRP) is a positive regulator protein activated by cyclic AMP. It is needed for RNA polymerase to initiate transcription of certain (catabolite-sensitive) operons of E. coli.
Figure 5.18 The cap blocks the 5end of mRNA and may be methylated at several positions.
cDNA is a single-stranded DNA complementary to an RNA, synthesized from it by reverse transcription in vitro.
Figure 5.19 Poly(A)+ RNA can be separated from other RNAs by fracti on at ion on Sepharose-oligo(dT).
Degradosome is a complex of bacterial enzymes, including RNAases, a helicase, and enolase (a glycolytic enzyme), which may be involved in degrading mRNA.
Figure 5.20 Degradation of bacterial mRNA is a two stage process. Endonucleolytic cleavages proceed 5-3behind the ribosomes. The released fragments are degraded by exonucleases that move 3-5.
Exosome is a complex of several exonucleases involved in degrading RNA.
Figure 5.21 Degradation of yeast mRNA requires deadenyl at ion, decapping, and exonucleolysis.
Figure 5.22 An ARE in a 3nontranslated region initiates degradation of mRNA.
Figure 5.23 An IRE in a 3nontranslated region controls mRNA stability.
Surveillance systems check nucleic acids for validity. The term is used in several different contexts. One example is the system that degrades mRNAs that have nonsense mutations. Another is the set of systems that react to damage in the double helix. The common feature is that the system recognizes an invalid sequence or structure and triggers a response.
Figure 5.24 Nonsense mutations may cause mRNA to be degraded.
1.Genetic information carried by DNA is expressed in two stages: transcription of DNA into mRNA; and translation of the mRNA into protein.2.The adaptor that interprets the meaning of a codon is transfer RNA, which has a compact L-shaped tertiary structure3. The ribosome provides the apparatus that allows aminoacyl-tRN As to bind to their codons on mRNA.4.The translational apparatus is not specific for tissue or organism; an mRNA from one source can be translated by the ribosomes and tRNAs from another source.
5.A typical mRNA contains both a nontranslated 5 leader and 3 trailer as well as coding region(s).6.A growing E. coli bacterium has ~20,000 ribosomes and ~200,000 tRNAs, mostly in the form of aminoacyl-tRNA.7.Many ribosomes may trans late a single mRNA simultaneously, generating a poly ribosome (or poly some).8.Eukaryotic mRNA must be processed in the nucleus before it is transported to the cytoplasm for translation.9.Yeast mRNA is degraded by multiple pathways.
Eukaryotic mRNAs are usually stable for several hours.
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