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UNDERSTANDING HEREDITY PROTEIN SYNTHESIS 1 Genes & Proteins • Genes - sequences of nucleotide bases • Genes code for proteins • Proteins - amino acids linked together by peptide bonds • 20 different amino acids exist 2 Genes & Proteins • Amino acid chains are called polypeptides • So: Protein = polypeptide Ribbon Structure of Mouse Cholera Antibody 3 Starting with DNA • DNA - found inside the nucleus • Proteins are made in the cytoplasm of cells by organelles called ribosomes • Ribosomes may be free in the cytoplasm or attached to the surface of rough ER 4 Starting with DNA • DNA ‘s code must be copied and taken to the cytoplasm • In the cytoplasm, this code must be read so amino acids can be assembled to make proteins • This process is called protein synthesis 5 RNA 6 Roles of RNA & DNA •DNA = COOKBOOK •RNA = KITCHEN CREW 7 Roles of RNA & DNA • RNA is a disposable copy of a portion of DNA • It is used to construct a protein • Comparison of RNA and DNA RNA DNA Sugar Ribose Deoxyribose Nitrogen Bases C G A U(racil) C G A T Structure Single Helix Double Helix 8 . Three Types of RNA • Messenger RNA - mRNA • Ribosomal RNA - rRNA • Transfer RNA - tRNA 9 Messenger RNA (recipe) • Long chain of nucleotides • Made in the nucleus • Transcribes DNA & leaves through nuclear pores • Carries information for a specific protein to ribosome 10 Ribosomal RNA (head chef) • rRNA is a single strand 100 to 3000 nucleotides long • Globular in shape • Made in the nucleolus of a cell • Reads mRNA and assembles proteins 11 Transfer RNA (kitchen staff) • Clover-leaf shape • Single stranded molecule with attachment site at one end for an amino acid • Found out in the cytoplasm • Brings amino acid to ribosome 12 13 The Genetic Code • Codon - a set of three RNA bases • Codon - designates a specific amino acid • An amino acid may have more than one codon and there are 64 codons 14 Transcription and Translation 15 Protein Synthesis • The production or synthesis of polypeptide chains (proteins) • Two phases: Transcription & Translation • mRNA must be processed before it leaves the nucleus of eukaryotic cells 16 Transcription • The process of mRNA copying the sequence of one strand of DNA • During transcription, RNA polymerase binds to DNA and separates the DNA strands • RNA Polymerase then uses one strand of DNA as a template to assemble nucleotides into RNA 17 • What would be the mRNA strand for the following DNA sequence? DNA 5’-GCGTATGAA-3’ mRNA 3’-CGCAUACUU-5’ 18 STEPS OF TRANSCRIPTON: 1. RNA polymerase binds to DNA just before code for protein begins 2. Nucleotide bases are added to make strand of mRNA 3. RNA polymerase detaches from DNA and mRNA 4. Enzymes proofread the mRNA sequence before it leaves nucleus 19 mRNA Transcript •mRNA leaves the nucleus through its pores and goes to the ribosomes 20 Translation • Translation = the process of decoding the mRNA into a polypeptide chain • Ribosomes read mRNA three nucleotide bases or 1 codon at a time 21 STEPS OF TRANSLATION: 1. mRNA transcript start codon AUG attaches to the ribosome 2. Amino acids are brought to the ribosome by tRNA 3. tRNA plugs into the mRNA with an anticodon 4. The amino acids are attached like beads on a string 5. tRNA is released to cytoplasm; mRNA is disassembled 6. Stop codon tells ribosome to release protein 22 Summary of Protein Synthesis Transcription DNA code Translation mRNA tRNA amino acid copy anticode assemble protein product 23 Mutations are Mistakes • Mistakes in gene code cause mistakes in the protein • These mistakes are called mutations • In protein synthesis mutations involving one base are called point mutations • Some mutations may do nothing to the final protein others may do serious damage 24