分子生物学英文版Chapter4

CHAPTER4 Translation

4.1 Outline of Translation 4.2 Genetic Code

4.3 tRNA and Anticodon

4.4 Ribosome

4.5 Protein Synthesis

4.6 Posttranslational Events

4.1 Outline of Translation

From mRNA to protein

4.2 Genetic Code

Protein: 20 different amino acids

mRNA: 4 different bases

A single base as a codon: 4 codons

Pairs of bases as codons: 16 codons

Triplets of bases as codens: 64 codons

The genetic code is the correspondence between base sequences in DNA (or RNA) and amino acids in protein.

A codon is a triplet of nucleotides that represents an amino acid or a start/termination signal of translation.

The genetic code is triplet

终止密码子：UAA(赭石, ochre); UAG(琥珀, amber),

UGA(蛋白石, opal)

起始密码子: AUG (有些为GUG)

A reading frame is one of the three possible ways of reading a nucleotide sequence. Each reading frame divides the sequence into a series of successive triplets.

Open reading frame

An open reading frame (ORF)is a sequence of DNA consisting of triplets that can be translated into amino acids starting with an initiation codon and ending with a termination codon.

There are three possible ways of translating any nucleotide sequence into protein, depending on the starting point. For example:

For the sequence ACGACGACGACGACGACG……

the three possible reading frames are:

(AUG) ACG ACG ACG ACG ACG ACG ACG….. (AUG) CGA CGA CGA CGA CGA CGA CGA….. (AUG) GAC GAC GAC GAC GAC GAC GAC…..

遗传密码的特点

1. 遗传密码的简并性

简并性(degeneracy):指一个氨基酸有一个以上的密码子为其编码。

2. 在mRNA模板上的密码子是连续的，在前一个密码子与后一

个密码子之间没有间隔。

4.同一氨基酸的不同遗传密码的使用频率不同。

5. 遗传密码的通用性和特殊性

遗传密码对病毒,细菌,动物和植物而言都适用，但是也存在特定条件下的一些例外。

支原体：UAG编码色氨酸；

嗜热四膜虫：UAA编码谷氨酰胺；

线粒体DNA与核DNA的密码子存在差异。

4.3 tRNA and Anticodon

tRNA结构

1接受臂

CCA末端

氨基酸结合位点

2TϕC臂/环

ϕ=pseudouridine

假尿嘧啶

3 多余臂(extra arm)

4反密码子臂/环

5 D臂/DHU环

DHU=dihydrouridine

二氢尿嘧啶

A tRNA recognizes more

than one codon by unusual

(non-G·C, non-A·U) pairing

with the third base of a

codon.

Wobble hypothesis

Codon-anticodon

recognition involves

wobbling:

Wobble hypothesis

Wobble hypothesis

Aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases are enzymes that charge tRNA with an amino acid to generate aminoacyl-tRNA in a two-stage reaction that uses energy from ATP.

Step 1：Amino acid + ATP→Aminoacyl-AMP +PPi

Step 2：Aminoacyl-AMP + tRNA→Aminoacyl-tRNA+ AMP Total：Amino acid +ATP +tRNA→Aminoacyl-tRNA+AMP +PPi