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From Gene to Protein; how are genes/ DNA expressed? RNA is a very flexible polymer. This means it can take on multiple shapes depending on the bonds in it. There are 3 types of RNA (each one has a unique shape as each one has a unique function): 1) mRNA- messenger (linear, like a line) 2) rRNA- ribosomal (large and like a glob) 3) tRNA- transfer (like a t) It is a two-step process: • 1-Transcription occurs: synthesis of mRNA under the direction of DNA. Occurs in the nucleus. • 2-Translation occurs: actual decoding of the mRNA into a polypeptide. Occurs in the ribosomes in the cytoplasm or on ribosomes on the rough ER We need DNA’s code (A,T,G,C combination) so we know what amino acids to connect so we can build a proper protein in the cell • So, first we copy the DNA into mRNA (Transcription) • Then, decode those DNA instructions (now in the form of mRNA) to construct correct amino acids into a protein. (Translation) The DNA’s information (genetic instructions) to make specific amino acids that will form the polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ called Codons. Example: DNA: TAC mRNA: AUG The DNA sequence is broken down into groups of 3 nucleotides, each corresponding to a mRNA codon. • RNA polymerase: Copies DNA into RNA. RNA polymerase lays down a 5’ (phosphate) structure first. Since RNA is a single stranded molecule, we only need to copy one side of the DNA. The side we use is the 3’ side. (NO lagging strand!!!) • Tell a partner WHY you thing we use the 3’ side of the DNA, be ready to share. • The 3’ side of DNA is called the antisense strand. The 5’ (uncopied) side is called the sense strand. • Tell your partner why you think we give them those names! Be ready to share. Don’t tell them the answer to the second question until you watch the animation. • RNA polymerase looks for a start sequence on the DNA, it is called the initiation sequence, or promoter. The stop sequence, or terminator is the end of the transcribed strand. • Any one of the 3 types of RNA can be made during transcription. Click on picture for animation Before pressing play the second time, label all of the things. • We must edit our mRNA in a process called “RNA processing” • Introns: non-protein making regions of the pre-mRNA are removed and caps and tails are added • The edited, linear mRNA is now exported from the nucleus to the ribosome in the cytoplasm. • Why is this process similar to a architect copying expensive blue prints before sending them to the job site? What do the blue prints, the copy, and the architect each represent? • Think of an additional analogy that can be compared to this process. Be ready to share. Picture is hyperlink to video Now we need the other 2 types of RNA, the tRNA and rRNA. Let’s start with the tRNA. tRNA is shaped like a “t” and has three jobs: 1. carry an inactive amino acid into the factory. 2. Provide the opposite codon to activate the amino acids. (this is called the “anticodon” Ex: mRNA= AUG/ tRNA = UAC 3. Hold the amino acids as they bond together to form the growing protein. rRNA is the same as saying a”ribosome”. • rRNA or a ribosome is the factory where we can build the protein following DNA’s instructions (mRNA codons) • Just like a real factory, there are different things that occur within the rRNA. • A site: Holds the tRNA carrying the amino acid to be activated and the matching anticodon (matches the codon on mRNA) • P site: Holds the tRNA carrying the growing polypeptide chain • E site: Exit of tRNA once that tRNA is done We can say there is a “recipe” for making a protein… it is Codon + anticodon + inactive amino acid = human protein Also, please memorize the start codon and what amino acid it makes and the stop codons (all 3) Click on the picture to watch a video of the process. Look though all the pictures and write down any questions you have for lecture.