Transcript Messenger RNA

Messenger RNA
1
Ex Biochem c7-mRNA
7.1 Introduction
Figure 7.1
2
Ex Biochem c7-mRNA
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
3
Ex Biochem c7-mRNA
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

Figure 7.2
Complementary to mRNA
4
7.3 Transfer RNA
Forms a Cloverleaf

5
tRNA serve as ‘adaptor’
between AA and nucleotide



Ex Biochem c7-mRNA
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

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
6
Ex Biochem c7-mRNA
Sequence of anticodon solely responsible
for specificity of aminoacyl-tRNA
Figure 7.5
7
Ex Biochem c7-mRNA
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
8
7.5 Messenger RNA Is
Translated by Ribosomes
Ex Biochem c7-mRNA

Ribosome characterized by rate of sedimentation



A ribosome consists of:



Higher S: higher rate of sedimentation, larger mass
70S for bacterial and 80S for eukaryotic ribosomes
a large subunit (50S or 60S for bacteria and eukaryotes)
a small subunit (30S or 40S)
S: sedimentation
Figure 7.8
9
Ex Biochem c7-mRNA
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 Cterminus as ribosome move along mRNA
A polypeptide chain in the process of
synthesis is called nascent protein
10
7.6 Many Ribosomes
Bind to One mRNA

11
An mRNA is simultaneously translated by
several ribosomes: polyribosome, polysome


Ex Biochem c7-mRNA
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
原核細胞(prokaryotic cells) vs
真核細胞 (eukaryotic cells)
Ex Biochem c7-mRNA




12
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
7.7 The Life Cycle of
Bacterial Messenger RNA
Ex Biochem c7-mRNA

13
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
Bacterial mRNA


Unstable, has a half-life of only
a few minutes.
Transcription and translation
similar rate



Figure 7.14
Transcription ~40 nucleotide/sec
Translation ~15 AA/sec
~2 min for 5 kb, 180 KD protein
14
Ex Biochem c7-mRNA
bacterial mRNA


may be polycistronic: having several coding regions
that represent different genes.
5’ and 3’ untranslated region (UTR)

Figure 7.15
Leader, trailer
15
Ex Biochem c7-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:



16
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
Figure 7.16
17
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
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
19
7.10 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)


Ex Biochem c7-mRNA
~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
20
7.15 Eukaryotic RNAs
Are Transported


Ex Biochem c7-mRNA
21
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
Figure 7.28
23
Ex Biochem c7-mRNA


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)
24