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THE CENTRAL DOGMA 10-1 DNA - Polymer of nucleotides - Nucleotide – a 5 carbon sugar (deoxyribose), a phosphate group and a nitrogenous base (adenine, guanine, thymine, cytosine) - C + T = pyrimidines – 1 ring - A + G = purines – 2 rings DOUBLE HELIX - DNA = 2 nucleotide chains wrapped in a double spiral (twisted ladder) - Structure discovered by Watson and Crick and Wilkins = Nobel prize – Rosalind Franklin died before - Alternating sugar – phosphate – sugar – phosphate …covalently bonded – sides of the ladder (AKA backbone) - Bases are covalently bonded to the sugar; rungs of ladder are made up of bases hydrogen bonded - Complementary base pairing – A + T and C + G DNA REPLICATION - Process by which DNA is copied - 2 nucleotide chains separate -> replication fork - Helicase opens chain, breaking H bonds - Topoisomerase – allows swiveling by nicking strand - Primase ~ 10 nucleotides set down first - DNA polymerase binds and adds complementary nucleotides to parent strands – can only add 5’ -> 3’ - Replication is discontinuous on lagging strand – forming Okazaki fragments – fragments are linked together by ligase - Other strand is leading strand; replication is continuous - When replication is completed, 2 new exact copies of original DNA molecule are produced -> cell is now ready to divide - Replication is accurate - Change in nucleotide sequence = mutation - Repair enzymes fix mutations 10 -2 RNA - Polymer (macromolecule) made up of nucleotides - Sugar is ribose - Uracil replaces thymine - 3 types: o mRNA – copies protein recipe (message) from DNA in nucleus, carries it out to cytosol o tRNA – cloverleaf shaped molecule that transports (tRNAsports) amino acids to ribosome o rRNA – with protein, makes up ribosomes ( reads and rides down mRNA) TRANSCRIPTION - Describes process by which info from DNA is copied to RNA - RNA polymerase – recognizes promoter region (beginning) and binds to it; DNA separates – only one strand is transcribed – coding (sense) strand - Nucleotides complementary to coding strand are added to RNA until termination signal is reached - RNA moves through nuclear pores -> into cytosol where it directs synthesis of proteins 10-3 PROTEIN SYNTHESIS - Proteins are polymers of one or more polypeptides, each consisting of a specific sequence of amino acids - Sequence determines the 3-d shape of the finished protein - Function depends on structure THE GENETIC CODE - Genetic code used to translate mRNA into proteins - A codon (sequence of three mRNA nucleotides) codes for a specific amino acid (codon =AKA = triplet) - Each amino acid may be specified by more than one triplet - Code is non-overlapping – read in groups of three - Code is non-random – similar types of amino acids have similar codons - Code is nearly universal - Start codon = AUG; methionine - Stop codons DO NOT code for amino acids (UAA, UGA, UAG) TRANSLATION - mRNA leaves nucleus through nuclear pores, migrates to a ribosome, where proteins are synthesized - Free floating amino acids are transported to the ribosomes by tRNA - The loop on the tRNA opposite the amino acid attachment contains the anticodon (complementary to and paired with the corresponding mRNA codon - Ribosomes = rRNA and protein, found free floating in cytosol and attached to the rough ER - Ribosomes floating freely make proteins for use in the cell itself; those attached to the ER make proteins for export - Binding sites on ribosomes = P site holds the growing polypeptide chain and the A site is empty for incoming tRNA with its attached amino acid - AUG is start codon – ribosome subunits assemble, methionine is first amino acid in poypeptide - As each amino acid is added to the polypeptide chain, ribosome moves down one codon (three mRNA nucleotides) and next codon is translated - Eventually, stop codon is reached and ribosome subunits disassemble, and polypeptide is complete - Several ribosomes translate an mRNA sequence - Polypeptide chain is the protein primary structure; this folds and coils and assumes its completed protein structure