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Download Chapter 25: Molecular Basis of Inheritance
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PROTEIN SYNTHESIS 1 DNA and Genes 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 Genes & Proteins Proteins are made of amino acids linked together by peptide bonds 20 different amino acids exist 4 Amino Acid Structure 5 Polypeptides • Amino acid chains are called polypeptides 6 DNA Begins the Process • DNA is found inside the nucleus • Proteins, however, are made in the cytosol of cells by organelles called ribosomes • Ribosomes may be free in the cytosol or attached to the surface of rough ER 7 Starting with DNA • DNA ‘s code must be copied and taken to the cytosol • In the cytosol, this code must be read so amino acids can be assembled to make polypeptides (proteins) • This process is called PROTEIN SYNTHESIS 8 RNA Roles of RNA and DNA • DNA is the MASTER PLAN • RNA is the BLUEPRINT of the Master Plan 10 RNA Differs from DNA • RNA has a sugar ribose DNA has a sugar deoxyribose 11 • • Other Differences RNA contains the base uracil (U) DNA has thymine (T) RNA molecule is single-stranded DNA is double-stranded DNA 12 Structure of RNA 13 Three Types of RNA . • • • • • • • • Messenger RNA (mRNA) copies DNA’s code carries genetic info to ribosomes Ribosomal RNA (rRNA) along with protein, makes up ribosomes Transfer RNA (tRNA) transfers amino acids to ribosomes where proteins are synthesized 14 Messenger RNA • Long Straight chain of Nucleotides • Made in the Nucleus • Copies DNA • leaves through nuclear pores • Contains the Nitrogen Bases A, G, C, U • ( no T ) 15 Messenger RNA (mRNA) • Carries the information for a specific protein • Made of 500 - 1000 basepairs • Sequence of 3 bases called codon • AUG – methionine start codon • UAA, UAG, or UGA – – stop codons 16 Ribosomal RNA (rRNA) • rRNA is a single strand – 100 to 3000 nucleotides long • Globular in shape • Made inside nucleus of a cell • Associates with proteins – form ribosomes • Site of protein Synthesis 17 The Genetic Code • A codon designates an amino acid • An amino acid may have – more than one codon • There are 64 possible codons • Only 20 amino acids, – but Some codons tell the ribosome – to stop translating 18 The Genetic Code •Use the code by reading from •center to the outside •Example: •AUG = Methionine 19 Name the Amino Acids • • • • • GGG? UCA? CAU? GCA? AAA? 20 Remember the Complementary Bases On DNA: A-T C-G On RNA: A-U C-G 21 Transfer RNA (tRNA) • Clover-leaf shape • Single stranded molecule – attachment site at one end for an amino acid • Opposite end has 3 nucleotide bases called the anticodon amino acid attachment site UA C anticodon 22 Transfer RNA amino acid attachment site U A C anticodon 23 Codons and Anticodons • The 3 bases of an anticodon are complementary to 3 bases of a codon • Example: Codon ACU Anticodon UGA UGA ACU 24 Transcription and Translation Pathway to Making a Protein DNA mRNA tRNA (ribosomes) Protein 26 DNA RNA Protein Nuclear membrane DNA Transcription mRNA Eukaryotic Cell Ribosome Translation Protein 27 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 28 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 29 Template Strand 30 Question: What would be the complementary RNA strand for the following DNA sequence? DNA 5’-GCGTATG-3’ 31 Answer: • DNA 5’-GCGTATG-3’ • RNA 3’-CGCAUAC-5’ 32 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 33 RNA Polymerase 34 Result of Transcription CAP New Transcript Tail 35 mRNA Transcript •mRNA leaves the nucleus through its pores and goes to the ribosomes 36 Translation • Translation is the process of of decoding the mRNA into a polypeptide chain • Ribosomes read mRNA three bases or 1 codon at a time and construct the proteins 37 Transcription Translation 38 Ribosomes • Made of a large and small subunit • Composed of rRNA (40%) and proteins (60%) • Have two sites for tRNA attachment --- P and A 39 Step 1- Initiation • mRNA transcript start codon AUG attaches to the small ribosomal subunit • Small subunit attaches to large ribosomal subunit mRNA transcript 40 Ribosomes Large subunit P Site A Site mRNA Small subunit A U G C U A C U U C G 41 Step 2 - Elongation • As ribosome moves, two tRNA with their amino acids move into site A and P of the ribosome • Peptide bonds join the amino acids Initiation aa1 aa2 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 43 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 44 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 Ribosomes move over one codon 45 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 46 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 Ribosomes move over one codon 47 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 48 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 Ribosomes move over one codon 49 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 50 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 aa200 51 Messenger RNA (mRNA) start codon mRNA A U G G G C U C C A U C G G C G C A U A A codon 1 protein methionine codon 2 codon 3 glycine serine codon 4 isoleucine codon 5 codon 6 glycine alanine codon 7 stop codon Primary structure of a protein aa1 aa2 aa3 peptide bonds aa4 aa5 aa6 52 53 Transcription • Promoters are regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA • Called the TATA box • Specific base sequences act as signals to stop • Called the termination signal 54 mRNA Processing • After the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional • Introns, non-functional segments of DNA are snipped out of the chain 55 mRNA Editing • Exons, segments of DNA that code for proteins, are then rejoined by the enzyme ligase • A guanine triphosphate cap is added to the 5” end of the newly copied mRNA • A poly A tail is added to the 3’ end of the RNA • The newly processed mRNA can then leave the nucleus 56