Protein Synthesis (Transcription and Translation)

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

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

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 instructions Overview

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)

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 coding region and this is the instructions for the protein At the end of the gene there is a termination sequence that signals the RNA polymerase to stop copying

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

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!)

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

The Codon Wheel

The Codon Box

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

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

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.

Protein Synthesis