Lecture 08 - Translation Based on Chapter 6Gene Expression: Translation

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What is the chemical composition of a protein? What is the structure of a protein? What is the nature of the genetic code? What is the structure and function of transfer RNA (tRNA)? How is polypeptide synthesis initiated on the ribosome? How is a polypeptide elongated on the ribosome? How is a polypeptide terminated in translation of an mRNA? How are proteins sorted in the cell?

1a. Chemical Structure of Proteins

1a. Chemical Structure of Proteins

1c. Chemical Structure of Proteins

2. Molecular Structure of Protein

Motif Structure

ConservedDomains

3a. The Nature of the Genetic Code

3b. The Nature of the Genetic Code

4. Deciphering the Gene Code

5a. Characteristics of the Genetic Code

5b. Characteristics of the Genetic Code

6. Translation: The Process of Protein Synthesis

• Ribosomes translate the genetic message of mRNA into proteins.

• The mRNA is translated 5’ to 3’, producing a corresponding N-terminal-to-C-terminal polypeptide.

• Amino acids are bound to tRNAs used by the ribosome to build proteins.

7. Transfer RNA

8. Structure of tRNA

9. Recognition of the tRNA Anticodon by the mRNA Codon

• tRNA.Cys normally carries the amino acid cysteine. Ehrenstein, Weisblum, and Benzer attached cysteine to tRNA.Cys (making Cys-tRNA.Cys) and then chemically altered it to alanine (making Ala-tRNA.Cys).

• When used for in vitro synthesis of hemoglobin, the tRNA inserted alanine at sites where cysteine was expected.

• They concluded that the specificity of codon recognition lies in the tRNA molecule and not in the amino acid it carries.

10. Adding an Amino Acid to tRNA

8a. Ribosomes and Ribosomal RNA

8b. Ribosomes and Ribosomal RNA

12. Initiation of Translation

•Protein synthesis is similar in bacteria and eukaryotes. Some significant differences do occur, and they are noted below.•In both, translation is divided into three stages:

Initiation Elongation Termination

•Initiation of translation requires: An mRNA. A ribosome. A specific initiator tRNA. Initiation factors (IF). GTP.

13b. Initiation in Bacteria

14. Initiation in Eukaryotes

• Initiation of translation is similar in eukaryotes but more complex:• Initiator methionine not modified, is attached to a special

tRNA.• Ribosome binding involves the 5’ cap, not Shine–Dalgarno• Eukaryatic initiator factor (eIF-4F) is a multimer of proteins• Initiator complex scans the mRNA for a Kozak sequence.

• The eukaryotic mRNA’s 3’ poly(A) tail also interacts with the 5’ cap. Poly(A) binding protein (PABP) binds the poly(A) and binds a protein in eIF-4F on the cap, circularizing the mRNA and stimulating translation.

15. Elongation of the Polypeptide Chain

16. Binding of Aminoacyl-tRNA

17. Peptide Bond Formation

18. Translocation

19. Termination of Translation

20. Protein Sorting in the Cell