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RNA & Protein synthesis DNA vs. RNA • DNA can be compared to the “master plan” of a builder • It stays safely in the nucleus (office) • Double stranded • Contains thymine • Contains deoxyribose • RNA can be compared to the “disposable copies/blueprints” • It goes to the proteinbuilding ribosome site in the cytoplasm (job site) • RNA is made from the DNA • Single stranded • Contains uracil • Contains ribose How are genes expressed? • Genes are coded DNA instructions that control the production of proteins within the cell. • The 1st step in decoding the DNA is to copy part of the nucleotide sequence into RNA • RNA = ribonucleic acid • The RNA’s make proteins What is RNA? • RNA consists of a long chain of nucleotides (like DNA) – A sugar called ribose – A phosphate group – A nitrogenous base Adenine (A) Uracil (U) … not Thymine (T) Cytosine (C) Guanine (G) • RNA is single-stranded RNA Nucleotide Phosphate Group Nitrogenous Base Ribose Sugar RNA Structure Nitrogenous Base Ribose Sugar Types of RNA • RNA’s assemble amino acids into proteins • There are 3 types of RNA – Messenger RNA (mRNA) – Ribosomal RNA (rRNA) – Transfer RNA (tRNA) Messenger RNA • Copies instructions in genes • Serves as a “messenger” from the DNA to the cell Ribosomal RNA • Ribosomes make proteins • Ribosomes are made of proteins and contain their own RNA Transfer RNA • Transfer amino acids to ribosomes based on specified codes in mRNA Section 12-3 Go to Section: How is RNA made? • RNA is made by transcription: DNA to RNA • Transcription uses an enzyme RNA polymerase • During transcription, RNA polymerase binds to DNA and separates the DNA strands, RNA polymerase then uses one strand of DNA as a template (stencil) from which nucleotides are assembled into a strand of RNA – For example: DNA: RNA: ACTGTGGACCT UGACACCUGGA http://www.johnkyrk.com/DNAtranscription.html TRANSCRIPTION The Genetic Code • Remember: Proteins are chains of amino acids called polypeptides • The order of the amino acids determines the properties of the protein • The instructions for making different amino acids are in the mRNA = the genetic code • The genetic codes is read 3 letters at a time, so each “word” is 3 bases long = codon How are proteins made? • Proteins are made by translation: RNA to protein • Translation occurs in the cytoplasm • During translation, the cell uses information from mRNA to make proteins. • mRNA instructs amino acids on tRNA to join together in the ribosome containing rRNA http://www.johnkyrk.com/DNAtranslation.html What is tRNA again? • Transfer RNA brings amino acids to the ribosome. • One end of the tRNA has an amino acid, the other end has 3 bases called the anticodon, that are complementary to one of the mRNA codons Amino acid Anticodon Codons • Three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide – For example: RNA: Codons: UCGCACGGU UCG-CAC-GGU Amino acids: Serine-Histidine-Glycine There are 64 possible codon combinations, but there are only 20 amino acids! *More than one codon can code for an amino acid When does it begin or end? • There is 1 “start” codon that initiates protein synthesis if it is the 1st codon (otherwise it codes for an amino acid) – AUG • There are 3 “stop” codons that do not code for any amino acid and signify the end of a polypeptide. – UAA – UAG – UGA Let’s try to make a protein! • • • • • DNA: CCAGATAGGTTT mRNA: GGUCUAUCCAAA codons: GGU-CUA-UCC-AAA tRNA: CCA-GAU-AGG-UUU Amino acids for the mRNA codons on tRNA’s: Glycine-Leucine-Serine-Lysine