1 Messenger RNA. Ex Biochem c7-mRNA 2 7.1 Introduction Figure 7.1

1

Messenger RNA

Ex Biochem c7-mRNA 2

7.1 Introduction

Figure 7.1


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


7.2 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 57.3 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


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


Sequence of anticodon solely responsible for specificity of aminoacyl-tRNA

Figure 7.5


7.4 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 97.5 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


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 117.6 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 137.7 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


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


may be polycistronic: having several coding regions that represent different genes.

5’ and 3’ untranslated region (UTR) Leader, trailer

Figure 7.15

bacterial mRNA


7.8 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 10000 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


Figure 7.16


The mRNA is exported from the nucleus to the cytoplasm only after all modifications have been completed.

Figure 7.17


7.9 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 207.10 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 10-20 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 217.15 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.



Figure 7.28


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)

Documents

1 Messenger RNA. Ex Biochem c7-mRNA 2 7.1 Introduction Figure 7.1