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PROTEIN SYNTHESIS copyright cmassengale 1 DNA • DNA contains genes, sequences of nucleotide bases • These Genes code for polypeptides (proteins) • Proteins are used to build cells and do much of the work inside cells copyright cmassengale 2 Amino Acid Structure copyright cmassengale 3 Polypeptides • Amino acid chains are called polypeptides copyright cmassengale 4 Starting with DNA • DNA ‘s code must be copied and taken to the cytosol • In the cytoplasm, this code must be read so amino acids can be assembled to make polypeptides (proteins) • This process is called PROTEIN SYNTHESIS copyright cmassengale 5 Pathway to Making a Protein DNA mRNA Read by ribosomes Protein copyright cmassengale 6 DNA RNA Protein Nuclear membrane DNA Transcription Eukaryotic Cell Pre-mRNA RNA Processing mRNA Ribosome Translation Protein copyright cmassengale 7 RNA copyright cmassengale 8 RNA Differs from DNA 1. RNA DNA 2. RNA DNA 3. RNA DNA has a sugar ribose has a sugar deoxyribose contains the base uracil (U) has thymine (T) molecule is single-stranded is double-stranded 9 Structure of RNA copyright cmassengale 10 . Three Types of RNA • Ribosomal RNA (rRNA), along with protein, makes up the ribosomes • Messenger RNA (mRNA) copies DNA’s code & carries the genetic information to the ribosomes • Transfer RNA (tRNA) transfers amino acids to the ribosomes where proteins are synthesized copyright cmassengale 11 Ribosomal RNA (rRNA) • rRNA is a single strand 100 to 3000 nucleotides long • Globular in shape • Made inside the nucleus of a cell and exported to cytoplasm • Associates with proteins to form ribosomes • Site of protein Synthesis copyright cmassengale 12 Messenger RNA • Long Straight chain of Nucleotides • Copies DNA and carries the information for a specific protein • Made up of 500 to 1000+ nucleotides long, organized into 3-base codons copyright cmassengale 13 Remember the Complementary Bases On DNA: A-T C-G On RNA: A-U C-G copyright cmassengale 14 Genetic Code DNA contains a triplet code Every three bases on DNA stands for ONE amino acid Each three-letter unit on mRNA is called a codon Most amino acids have more than one codon! 20 amino acids: 64 different triplets ALL organisms use the SAME code 15 The Genetic Code •Example: AUG codes for Methionine 16 copyright cmassengale 17 Transfer RNA (tRNA) • Clover-leaf shape • Single stranded molecule with attachment site at one end for an amino acid • Opposite end has three nucleotide bases called the anticodon copyright cmassengale 18 Transfer RNA amino acid attachment site U A C anticodon copyright cmassengale 19 Codons and Anticodons • The 3 bases of an anticodon are complementary to the 3 bases of a codon • Example: Codon ACU Anticodon UGA copyright cmassengale UGA ACU 20 Two Parts of Protein Synthesis Transcription makes an RNA molecule complementary to a portion of DNA Translation occurs when the sequence of bases of mRNA DIRECTS the sequence of amino acids in a polypeptide 21 Transcription Translation 22 Transcription • The process of copying the sequence of one strand of DNA, the template strand • mRNA copies the template strand • Requires the enzyme RNA Polymerase copyright cmassengale 23 Template Strand copyright cmassengale 24 Question: What would be the complementary RNA strand for the following DNA sequence? DNA 5’-GCGTATG-3’ copyright cmassengale 25 Answer: • DNA 5’-GCGTATG-3’ • RNA 3’-CGCAUAC-5’ copyright cmassengale 26 Transcription • 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 copyright cmassengale 27 RNA Polymerase copyright cmassengale 28 mRNA Transcript •mRNA leaves the nucleus through its pores and goes to the ribosomes copyright cmassengale 29 Translation • Translation is the process of decoding the mRNA into a polypeptide chain • Ribosomes read mRNA three bases or 1 codon at a time and construct the proteins copyright cmassengale 30 Ribosomes • Made of a large and small subunit • Composed of rRNA (40%) and proteins (60%) • Have two sites for tRNA attachment --- P and A copyright cmassengale 31 Translation • Three steps: 1. initiation: mRNA binds to ribosome at start codon (AUG) 2. elongation: amino acids linked 3. termination: stop codon (UAG, UAA, or UGA) signals ribosome to release mRNA 32 Step 1- Initiation • mRNA transcript attaches to the small ribosomal subunit • Small subunit attaches to large ribosomal subunit mRNA transcript copyright cmassengale 33 Ribosomes Large subunit P Site A Site mRNA Small subunit A U G C U A C U U C G copyright cmassengale 34 Initiation aa2 aa1 2-tRNA 1-tRNA anticodon hydrogen bonds U A C A U G codon G A U C U A C U U C G A mRNA copyright cmassengale 35 Step 2 - Elongation • As ribosome moves, tRNA with their amino acids move into the ribosome • Peptide bonds join the amino acids copyright cmassengale 36 Elongation peptide bond aa1 aa3 aa2 3-tRNA 1-tRNA anticodon hydrogen bonds U A C A U G codon 2-tRNA G A A G A U C U A C U U C G A mRNA copyright cmassengale 37 aa1 peptide bond aa3 aa2 1-tRNA 3-tRNA U A C (leaves) 2-tRNA A U G G A A G A U C U A C U U C G A mRNA copyright cmassengale Ribosomes move over one codon 38 aa1 peptide bonds aa4 aa2 aa3 4-tRNA 2-tRNA A U G 3-tRNA G C U G A U G A A C U A C U U C G A A C U mRNA copyright cmassengale 39 aa1 peptide bonds aa4 aa2 aa3 2-tRNA 4-tRNA G A U (leaves) 3-tRNA A U G G C U G A A C U A C U U C G A A C U mRNA copyright cmassengale Ribosomes move over one codon 40 aa1 peptide bonds aa5 aa2 aa3 aa4 5-tRNA U G A 3-tRNA 4-tRNA G A A G C U G C U A C U U C G A A C U mRNA copyright cmassengale 41 peptide bonds aa1 aa5 aa2 aa3 aa4 5-tRNA U G A 3-tRNA G A A 4-tRNA G C U G C U A C U U C G A A C U mRNA copyright cmassengale Ribosomes move over one codon 42 aa4 aa5 Termination aa199 aa3 primary structure aa2 of a protein aa200 aa1 200-tRNA A C U terminator or stop codon C A U G U U U A G mRNA copyright cmassengale 43 End Product –The Protein! • The end products of protein synthesis is a primary structure of a protein • A sequence of amino acid bonded together by peptide bonds aa2 aa1 aa3 aa4 aa5 aa199 copyright cmassengale aa200 44