Protein Synthesis (Transcription and Translation)

1. Protein Synthesis (Transcription and Translation)

2. RNA vs DNA structure RNA • sugar: ribose • Single stranded • A, C, G, and Uracil (U) DNA • Sugar: deoxyribose • Double stranded • A, C, G, and Thymine (T) DNA is not the only nucleic acid. It has a helper called RNA.

3. 3 Types of RNA DNA cannot leave the nucleus, so RNA actually does the work to build the proteins (protein synthesis) • messenger RNA (mRNA): carries the DNA instructions from the nucleus to the ribosome • ribosomal RNA (rRNA): creates the ribosome • transfer RNA (tRNA): transfers amino acids to the ribosome to build the protein

4. Protein Synthesis Steps • Transcription – copy the needed gene onto an mRNA • RNA Editing – check the mRNA for mistakes and remove introns (junk DNA) • Translation – use the final mRNA to build a protein at the ribosome; each mRNA codon represents an amino acid; amino acid chains make a protein; mRNA is the instructions, tRNA brings in the correct amino acid based on the instructionsOverview

5. Transcription • The enzyme RNA polymerase builds the mRNA strand by unzipping the DNA with helicase and base pairing at the correct gene (similar to replication except using RNA nucleotides)

6. Transcription • Each chromosome contains thousands of genes! • How does RNA polymerase “know” where to start and stop making the RNA copy of DNA? • Each gene begins with a promoter region that signals the start of the gene • The middle is the codingregion and this is the instructions for the protein • At the end of the gene there is a terminationsequence that signals the RNA polymerase to stop copying

7. RNA Editing • DNA contains unnecessary information • Introns: Junk DNA, must be removed from the pre mRNA • Exons: expressed DNA, this is the DNA used to make the protein • RNA editing after transcription removes the introns, splices back together the exons, and adds a protective cap and tail to the mRNA so that it can leave the nucleus

8. Translation • The final mRNA can then leave the nucleus and go out to a ribosome where it is “translated” into a chain of amino acids (which form proteins!)

9. How does the ribosome “read” the mRNA? • The mRNA is read 3 base pairs (A, U, C, or G) at a time in groups called codons • The 64 possible codons each represent one of the 20 different amino acids • Sometimes different codons will give the same amino acid • example: RNA sequence UCGCACGGU would be read UCG CAC GGU which would give Serine – Histidine – Glycine • “AUG” (methionine) is always the start • Three stop codons

10. The Codon Wheel

11. The Codon Box

12. Translation • Remember: the mRNA was made and edited in the nucleus before translation begins

13. Translation • To begin translation, the mRNA molecule attaches to a RIBOSOME in the cytoplasm (or on the rough ER) • The ribosome attaches at the first AUG (methionine) • tRNAs with the complementary codon (called an anti-codon) are in the cytoplasm carrying the amino acids. They enter the ribosome and base pair to the mRNA to ensure the amino acids are placed in the correct order. The tRNAs will leave their amino acid behind, but they can be reused later after picking up a new amino acid

14. Translation • This continues like a polypeptide/protein “assembly line” until the first stop codon is reached. Then the ribosome releases both the mRNA and the new protein.

15. Protein Synthesis