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The Central Dogma

The Central Dogma. Genetic information transfer: DNA→RNA→Protein Transcription: DNA to RNA Translation: RNA to protein. Translation: from mRNA to polypeptide. Where? Ribosomes in cytoplasm Genetic code: decipher information from mRNA to protein To carry amino acid: tRNA as adaptor

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The Central Dogma

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  1. The Central Dogma • Genetic information transfer: DNA→RNA→Protein • Transcription: DNA to RNA • Translation: RNA to protein

  2. Translation: from mRNA to polypeptide • Where? Ribosomes in cytoplasm • Genetic code: decipher information from mRNA to protein • To carry amino acid: tRNA as adaptor • The component of ribosome: rRNA and proteins • Carrier of genetic information: mRNA

  3. A8 The Genetic Code • Genetic code • Reading frames • Universality of the code

  4. Because nucleic acids and proteins are different in structure, like different languages, therefore information encoded in DNA should be translated into proteins. During this process, genetic codes are required.

  5. Genetic code • The genetic code describes how base sequences are converted into amino acid sequences. Successive three bases which is called codon (密码子) define an amino acid. • 连续的三个碱基组成一个密码子

  6. 3’ 5’ The Genetic Code

  7. Property • Degeneracy or redundancy: except Met（甲硫氨酸） and Trp（色氨酸）, amino acid is encoded by more than one codon. • 简并性：有多于一个的密码子编码同一种氨基酸的现象。 • Synonyms[‘sinənim]: codons that specify the same amino acid. Variations between synonyms tend to occur at the third position of the codon, known as the wobble position（摇摆性）. • 同义密码子：编码同一种氨基酸的密码子互称同义密码子。它们之间的不同一般在第三位的碱基上。所以密码子的专一性取决于前两位碱基。 • Synonyms help to avoid mutation.

  8. Stop codons: UAA, UAG and UGA, also called termination codon. Signals for protein synthesis to stop. • 终止密码子：UAA,UAG,UGA • Initiation codon: AUG. All protein synthesis start with Met. • 起始密码子：AUG • （AUG既编码甲硫氨酸，又是起始密码子）

  9. Table 1 The Genetic Code

  10. 3.Reading frames • Every DNA sequence can be read as three separate reading frames depending on which base is chosen as the start of the codon. √ 5’-AUG ACU AAG AGA UCC GG-3’ 5’-A UGA CUA AGA GAU CCG G-3’ 5’-AU GAC UAA GAG AUC CGG-3’ × ×

  11. A set of codons that runs continuously and is bounded at the start by an initiation codon and at the end by a termination codon is known as an open reading frame (ORF). • ORF finder in genome sequencing project • 开放阅读框（open reading frame）.

  12. 4. Universality of the code通用性 • Code is universally used in different organisms. • Some variations in the code exist, although very rare. • e.g. In mt DNA , UGAs code for Trp, AGA and AGG are stop codons. Instead of Ile, AUA codes for Met. In protozoa, UAA and UAG encode Glu.

  13. Questions • Explain: codon, degeneracy, ORF • The importance of synonyms.

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