The Flow of Genetic Information :

DNA RNA PROTEIN

Transcription Translation

Concept:

The ribonucleic acid (RNA) needed in protein synthesis :

◆ A. Messenger RNA(mRNA) 信使核糖核酸

Translation is RNA-directed polypeptide synthesis.

CytoplasmSite:

All members of this class function as messengers conveying the information in a gene to the protein-synthesizing machinery, where each serves as a template( 模板） on which a specific sequence of amino acids is polymerized （聚合） to form a specific protein molecule.

Codon( 密码子 ): A codon is a triplet( 三联体 )of three nucleotides in a messenger RNA.

Total: 64 specific codons

Stop codon( 终止密码 ):The codons UAA,UGA,and UAG are stop codons—codons that do not encode （编码） amino acids but instead cause translation to cease.

Start codon( 起始密码 ):The start codon is AUG,which codes for methionine( 甲硫氨酸 ).Translation starts at it in the mRNA.

Synonymous codon( 同义密码 ):The synonymous codons are multiple codons which decode the same amino acid.

Codon dictionary:

Properties: The genetic code （遗传密码） is degenerate( 简并 ), unambiguous, nonoverlapping, without punctuation, and universal ( 通用） .

Degeneracy( 简并性 ): Multiple codons can decode( 编码 ) t

he same amino acid.Unambiguous: Given a specific codon, only a single

amino acid is indicated.Nonoverlapping: The reading of the genetic code duri

ng the process of protein synthesis does not involve any overlap of codons.

mRNA:

GUAC A G A C U A AG AC UU

Triplet codon

三联体密码

Start codon

起始密码子

Stop codon

终止密码子Codon

氨基酸密码子

UAA UAG UGA

Reading frame( 阅读框 )

◆ B. Transfer RNA(tRNA) 转运核糖核酸

• The term tRNA refers to the kinds of RNA that bind covalently to amino acids and deliver them to the site of protein synthesis.

tRNA LeuAmino acid acceptor arm 氨基酸臂

Anticodon loop反密码子环

A G U Anticodon反密码子

Anticodon:• An anticodon is a “triplet” of nucleo

tides,in a tRNA molecule,that aligns( 排列 ) by complementary base pairing with an mRNA codon.

Codon and anticodon ， their interaction is one of the key steps in translation.

tRNA tertiary structure

3’End

Amino acid acceptor arm

Anticodon

Wobble （摆动） :

Although the first two nucleotides of all codons base pair normally with their anticodon,the pairing of the third nucleotide and its anticodon partner demonstrates a number of specific exceptions. This phenomenon, called wobble.

密码子的第三个碱基与反密码子的相应碱基配对时，会出现不遵从碱基互补配对原则的情况 , 称为遗传密码的摆动现象。

eg:Inosine （肌苷） can wobble pair. One tRNA can be used to read three codons if inosine is used.(Inosine can wobble pair with U,C or A)

C C I

Gly

G G U (GGC,GGA)5’ 3’mRNA

eg. all three codons for isoleucine （异亮氨酸）(AUA,AUC,and AUU) can base pair with a single tRNA.

In fact,only 31 different tRNA are actually required to read all 61 sense codons （有意义密码） .

◆ C. Ribosomal RNA (rRNA) 核糖体核糖核酸

The term rRNA refers to the kinds of RNA that combine with specific proteins to form ribosomes( 核糖体 ),the site of protein synthesis in all organisms.

Ribosome

mRNA

Large subunit核糖体大亚基

Small subunit核糖体小亚基

rRNA结构

Peptidyl-tRNA binding site肽酰基 -tRNA

结合位点

Aminoacyl-tRNA binding site氨酰基 -tRNA

结合位点P A

Ribosmal tertiary structure

A. The activation and transportation of amino acid ( 氨基酸的活化与转运 )

Aminoacyl-tRNA synthetase( 氨基酰 - tRNA 合成酶 )

The processing of protein synthesis ( 蛋白质的合成过程 )

＋Amino acid tRNA ATP＋

＋Aminoacyl-tRNA氨基酰 tRNA AMP PPi＋

Transported form

Aminoacyl-tRNA synthetase

Met

Metenzyme

氨基酸的活化

Met His

Ala

Ser

Asp

TyrThr

His

Ala

B. The course of translation in ribosome( 核糖体 ):

Properties:◆This process involves tRNA,rRNA,mRNA, several

eukaryotic( 真核的 ) initiation factors(eIF)( 起始因子 ), GTP, and ATP.

◆The ribosome( 核糖体 ) must correctly identify the precise starting position in the mRNA for decoding( 解码 ).

◆The ribosome must be separated into its smaller components( 组成成分 ).

◆The initiation complex( 复合物 ) forms on the small subunits( 小亚基 ).

1.Initiation( 起始 ):forming an initiation complex( 起始复合物 ).

Initiation complex( in eukaryoticcell ( 真核细胞 ) ) :

1A 3 2

Met

AUG

2

Met

Initiation complex

1A

3 2

1A

3 2

1A2

fMet

AUG

Met

AUG

1A3

Met

AUG

1A 3 2

Met

P A GTP →GDP+Pi

2..Elongation ( 肽链延长 )

（ 3 ） Translocation( 移位 )—is the shifting of ribosome( 核糖

体 ) relative to the mRNA and the new peptidyl- tRNA( 肽酰 - tRNA). 核糖体与 mRNA 和新的肽酰 - tRNA 之间的位置发生相对移动 .

（ 1 ） Binding( 进位 )—The charged aminoacyl-tRNA( 氨基酰 tRNA ) joints the A site,binding the appropriate codon( 密码子 ).

（ 2 ） Transpeptidation( 转肽 )—The formation of a peptide bond （肽键） to the next added amino acid residue. This reaction is catalyzed( 催化 ) by a peptidyltransferase( 转肽酶 ), a component of the large ribosomal subunit( 大亚基 ).

Elongation( 延长 ) 1

AUG

Met

Met

AP

Transpeptidation转肽

AUG

Binding( 进位 )

1 GTP1 GDP

1 GDP

1 GDP

Elongation 2

APMet

AUG

Translocation移位

AUG

AP

AUG

Met AP

AUG

MetMet

The elongation factor( 延长因子 ) 2(eEF-2) and GTP are responsible for the translocation( 移位 ).

Elongation3

APMet

AUG

Met

The elongation cycle continues until the stop codon( 终止密码 ) in the mRNA is detected.

3. Termination 终止

Thus, a water molecule, rather than an amino acid( 氨基酸 ), is added. This hydrolysis ( 水解 )releases the protein( 蛋白质 ) and the tRNA from the P site.

The nonsense( 无义 ) or stop codon of mRNA (UUA, UAG, UGA) appears in the A site.

Releasing factors( 释放因子 ) (eRF) are capable of recognizing that a termination( 终止 ) signal resides in the A site .

The releasing factor, in conjunction ( 联合 )with GTP and the peptidyl transferase( 转肽酶 ), promotes( 启动 ) the hydrolysis( 水解 ) of the bond( 键 ) between the peptide and the tRNA occupying the P site.

The mRNA is then released from the ribosome( 核糖体 ), which dissociates( 分离 ) into its component small and large subunits( 亚基 ), and another cycle can be repeated.

Chain termiation 1

AP

UAA

eRFGTP

Termination 2

AP

eRFGTP

UAA

eRFGDP

UAA

Preinitiation complex:

1A 3 2

Met

AUG

Initiation complex

1A

3 2

1A

3 2

1A2

fMet

AUG

Met

AUG

1A3

Met

AUG

1A 3 2

Met

P A GTP →GDP+Pi

Elongation 1

AUG

Met

Met

AP

Transpeptidation

AUG

Binding

1 GTP1 GDP

1 GDP

1 GDP

Elongation 2

APMet

AUG

Translocation

AUG

AP

AUG

Met AP

AUG

MetMet

Elongation3

APMet

AUG

Met

The elongation cycle continues until the stop codon in the mRNA is detected.

Chain termiation 1

AP

UAA

eRFGTP

Termination 2

AP

eRFGDP

UAA

eRFGDP

UAA

★The properties of translation( 翻译的特点 ) ：

1. Template( 模板 ) ： mRNA ，阅读方向： 5`→ 3`

2. 原料：氨基酸 3. 肽链延长方向： N 端 → C 端

4. Post translational processing( 翻译后加工 )

Many other peptides are synthesized as proproteins( 蛋白原 ) that require modifications( 修饰 ) before attaining biologic activity. Many of the posttranslational modifications involve the removal of amino terminal amino acid residues by specific aminopeptidases ( 氨肽酶 ).

Covalent( 共价的 ) modification by acetylation( 乙酰化 ), phosphorylation( 磷酸化 ), and glycosylation( 糖基化 )is common, for example.

Question:

• Basis of the sickle-cell Mutation:

Sickle-cell( 镰刀型细胞 ) hemoglobin ( 血红蛋白 )has a Val ( 缬氨酸 )residue at position 6 of the -globin ( 珠蛋白 )chain,instead of the Glu （谷氨酸） residue found in normal hemoglobin A.Can you predict what change took place in the DNA codon for glutamate( 谷氨酸 )to account for its replacement by valine?

MANY ANTIBIOTICS WORK BECAUSETHEY SELECTIVELY INHIBIT PROTEIN SYNTHESIS IN BACTERIA

eg:Tetracycline( 四环素 )prevents the binding of aminoacyl-tRNAs to the A site.

Chloromycetin( 氯霉素 )and the macrolide( 大环内酯物 )class of antibiotics work by binding to 23S rRNA, which is interesting in view of the newly appreciated role of rRNA in peptide bond formation .

Cycloheximide( 放线菌酮 )inhibits peptidyltransferase( 肽基转移酶 )in the large ribosomal subunit in eukaryotes, presumably by binding to an rRNA component.