METABOLISM, THE SYNTHESIS OF NUCLEIC：代谢，核酸的合成

METABOLISM, THE SYNTHESIS OF NUCLEIC：代谢，核酸的合成 1 METABOLISM, THE SYNTHESIS OF NUCLEIC ACIDS AND PROTEINS I. Nucleic Acid Structure: A. Nucleic acids: Bipolymeres: Consist of: Pentose phosphate chains Nitrogenous bases: Two types of nitrogenous bases: Purines: Adenine Guanine Pyrimidines: Thymine (DNA only) Cytosine Uracil (RNA only) Form linkages with pentose sugars: Nucleoside: Base + 5 Carbon sugar Nucleotide: Base + 5 Carbon sugar + PO: 4 Phosphate group (PO): 4 Combines with two different five carbon sugars: Forms backbone of nucleic acid molecule: B. DNA Structure: Long molecules Composed of two polynucleotide chains Coiled together - double helix Backbone: Alternating deoxyribose and PO groups: 4 Held together by hydrogen bonding between bases: Strands are complementary: Adenine always pairs with Thymine: Two hydrogen bonds B362 2 Guanine always pairs with Cytosine: Three hydrogen bonds Differences between specific DNA molecules due to difference in base sequence C. RNA STRUCTURE: Long molecules: Single stranded molecule: Contains: Five carbon sugar: Ribose Pyrimidine: Uracil Can coil back on itself: Base pairing occurs: Forms hair-pin shaped structures Helical organization Three kinds: Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA) II. Organization of DNA in cells: A. Prokaryotic cells: Closed circle: Twisted into supercoiled DNA Associated with basic proteins: Not associated with histones Basic proteins: Organized bacterial DNA into coiled chromatin-like structure B. Eukaryotic cells: More highly organized: Linear: Associated with histones: Small basic proteins rich in: Lysine Arginine B362 3 Arranged in nucleosomes III. DNA Replication A. Replication pattern varies between prokaryotic and eukaryotic cells: B. General steps Two strands of double helix: Unwind Separate - helicase Free nucleotides move in Line up along the two parental strands via base paring Linked together by DNA polymerases: 750 -1000 bases/second IV RNA transcription: A. DNA not direct template for protein sequence: Transfers information to RNA RNA is transcribed from DNA B. All RNA molecules have same backbone structure: Ribose alternating with phosphate: C. Differences in RNA molecules: Due to the differences in the order of bases connected to the sugar phosphate backbone: Any possible sequence of: Adenine Uracil Cytosine Guanine D. Single DNA strand is template for RNA synthesis: Base sequence of RNA complementary to base sequence of DNA template: E. RNA bases form H bonds with bases of DNA molecule: Ribose sugar phosphate backbone zipped up by RNA polymerase RNA polymerase must bind to promoter before transcription can start Promoter: Region in DNA where RNA polymerase must bind before it can make the linkage between ribonucleotides and phosphate B362 4 F. Messenger RNA (m RNA): Directs amino acid sequence in proteins Template for protein synthesis Transcribed from a specific gene which codes for a specific protein: Specific gene contains: Series of codons: Sequence of three bases coding for an single amino acid: Genetic code After transcription: mRNA associates with a ribosome V. Protein Synthesis: A. Translation: Synthesis of protein using an RNA template Amino acid activation: Amino acid attached to transfer RNA: tRNA = Adaptor molecule: Modifies amino acids so that they can combine with a mRNA codon Small - 60 - 70 nucleotides long Contain anticodon: Three bases complementary to the codon in RNA At least one specific tRNA for each amino acid Amino Acetyl tRNA Synthetase: Links amino acids to tRNAs Specific enzyme for each amino acid Occurs only on surface of ribosomes Ribosome: Orients the AA-tRNA combination and mRNA so that genetic code can be read: Have specific surfaces that bind mRNA and tRNA B. Prokaryotic ribosome Composed of 2 separate subunits: Subunits separate when not actively synthesizing protein 30s unit contains: 21 different kinds of protein 16s RNA 50s unit contains: 32-34 kinds of protein B362 5 RNA: 23s RNA 5s RNA Combined units = 70s C. Eukaryotic ribosome Composed of 2 separate subunits: Subunits separate when not actively synthesizing protein: 40s unit contains: 32 different kinds of protein 18s RNA 60s unit contains: 40 kinds of protein RNA: 28s RNA 5s RNA Combined units = 80s D. Initiation of protein synthesis requires: Initiation complex: Consists of: 30s ribosome mRNA N-formylmethionyl-tRNA Must be formed before protein synthesis starts: Combines with 50s subunit to form functional ribosome Initiation of protein synthesis is complex: Insures that protein synthesis starts at the correct location: At the beginning of a gene At the correct codon At the correct base in the codon E. Elongation of the polypeptide Chain: 50s subunit has two special sites involves in the elongation of the polypeptide chain: Acceptor (A) site: Place where new tRNA-aa attach to 50s subunit Peptidyl (P) site: Place where growing polypeptide chain held by a tRNA B362 6 Peptidyl transferase: Part of the 50s ribosome Makes peptide bond between adjacent amino acids F. Protein synthesis continues until a termination (nonsense) codon is reached: Termination Codons: UAA UAG UGA Code for no amino acid Stop polypeptide elongation G. Protein synthesis requires more energy than any other synthetic process in the cell: Four ATP molecules per peptide bond B362