1 Messenger RNA

Ex Biochem c7-mRNA Introduction Figure 7.1

Ex Biochem c7-mRNA 3 RNAs Messenger RNA (mRNA) Transfer RNA (tRNA) Small Provide amino acids corresponding to each particular codon in mRNA Ribosomal RNA (rRNA) Component of ribosome Provide apparatus for polymerizing amino acids into polypeptide chain

Ex Biochem c7-mRNA mRNA Is Produced by Transcription and Is Translated Only one of the two strands of DNA is transcribed into RNA. Coding strand: sense Same sequence as mRNA (except T-U) Template strand: antisense Complementary to mRNA Figure 7.2

Ex Biochem c7-mRNA Transfer RNA Forms a Cloverleaf tRNA serve as ‘ adaptor ’ between AA and nucleotide Covalently bind to a specific AA Anticodon A tRNA has a sequence of 74 to 95 bases. It folds into a cloverleaf secondary structure with four constant arms. Longer tRNAs have an additional arm. Figure 7.4

Ex Biochem c7-mRNA 6 tRNA is charged to form aminoacyl-tRNA by forming an ester link between the: 2′ or 3′ OH group of the adenylic acid at the end of the acceptor arm COOH group of the amino acid Figure 7.3

Ex Biochem c7-mRNA 7 Sequence of anticodon solely responsible for specificity of aminoacyl-tRNA Figure 7.5

Ex Biochem c7-mRNA Acceptor Stem and Anticodon Are at Ends of the Tertiary Structure The cloverleaf forms an L-shaped tertiary structure with: the acceptor arm at one end the anticodon arm at the other end Figure 7.6 Figure 7.7

Ex Biochem c7-mRNA Messenger RNA Is Translated by Ribosomes Ribosome characterized by rate of sedimentation Higher S: higher rate of sedimentation, larger mass 70S for bacterial and 80S for eukaryotic ribosomes A ribosome consists of: a large subunit (50S or 60S for bacteria and eukaryotes) a small subunit (30S or 40S) S: sedimentation Figure 7.8

Ex Biochem c7-mRNA 10 Ribosomes The ribosome provides the environment in which aminoacyl-tRNAs add amino acids to the growing polypeptide chain. A ribosome moves along an mRNA from 5′ to 3′ Protein assembled by sequential addition of AA in direction from N-terminus to C- terminus as ribosome move along mRNA A polypeptide chain in the process of synthesis is called nascent protein

Ex Biochem c7-mRNA Many Ribosomes Bind to One mRNA An mRNA is simultaneously translated by several ribosomes: polyribosome, polysome 30S assicoated with mRNA, 50S carry newly synthesized polypeptide Each ribosome is at a different stage of progression along the mRNA Independent to one another Figure 7.9

Ex Biochem c7-mRNA 12 原核細胞 (prokaryotic cells) vs 真核細胞 (eukaryotic cells) 原核細胞與真核細胞最明顯的差異在於真 核細胞擁有細胞核與為薄膜 (membrane) 所 侷限的細胞器官，而原核細胞沒有 原核細胞與真核細胞一樣是以 DNA 為遺傳 基礎，也同樣的擁有核醣體 (ribosome) 、 細胞膜與類似的基本代謝過程等 一般而言真核細胞的 DNA 遠較原核細胞複 雜，且平均尺寸也較原核細胞大十倍以上。 原核細胞 : 主要為細菌

Ex Biochem c7-mRNA The Life Cycle of Bacterial Messenger RNA Transcription and translation occur simultaneously in bacteria as ribosomes begin translating an mRNA before its synthesis has been completed. Figure 7.14

Ex Biochem c7-mRNA 14 Bacterial mRNA Unstable, has a half-life of only a few minutes. Transcription and translation similar rate Transcription ~40 nucleotide/sec Translation ~15 AA/sec ~2 min for 5 kb, 180 KD protein Figure 7.14

Ex Biochem c7-mRNA 15 may be polycistronic: having several coding regions that represent different genes. 5 ’ and 3 ’ untranslated region (UTR) Leader, trailer Figure 7.15 bacterial mRNA

Ex Biochem c7-mRNA Eukaryotic mRNA Modified During or after Its Transcription eukaryotic mRNA transcript is modified in the nucleus during or shortly after transcription Transcription rate ~40 nucleotides/sec ~ 5 min for a gene of bp The modifications include the addition of: a methylated cap at the 5′ end a sequence of poly(A) at the 3′ end, transcription enzyme continue past end of gene Substantial increase in mRNA stability Half-life 4-24 hr in animal cells

Ex Biochem c7-mRNA 17 Figure 7.16

Ex Biochem c7-mRNA 18 The mRNA is exported from the nucleus to the cytoplasm only after all modifications have been completed. Figure 7.17

Ex Biochem c7-mRNA The 5′ End of Eukaryotic mRNA Is Capped A 5′ cap is formed by adding a G to the terminal base of the transcript via a 5′ – 5′ link. 1-3 methyl groups are added to the base or ribose of the new terminal guanosine. Figure 7.18

Ex Biochem c7-mRNA The 3′ Terminus Is Polyadenylated poly(A) ∼ 200 nucleotides long is added to a nuclear transcript after transcription. By poly(A) polymerase Not coded in DNA The poly(A) is bound by a specific protein, poly(A)-binding protein (PABP) ~70 KD, bind every bases of poly(A) tail poly(A) stabilize mRNA against degradation. Poly(A) essential for translation PABP bind to initiation complex

Ex Biochem c7-mRNA Eukaryotic RNAs Are Transported RNA is transported through a membrane as a ribonucleoprotein particle. All eukaryotic RNAs that function in the cytoplasm must be exported from the nucleus.

Ex Biochem c7-mRNA 22

Ex Biochem c7-mRNA 23 Figure 7.28

Ex Biochem c7-mRNA 24 genetically-modified organism (GMO) Increased protein synthesis Transcription level (mRNA, RT-PCR, real-time PCR, northern blot) Translation level Post-transcription level Post-translation level Stability of mRNA Sampling time point Probe (labeled)