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DNA Structure Chapter 10 Nucleic Acids • Polymers made of nucleotides • Sugar-phosphate backbone (sides) • Nitrogenous bases face in (rungs) – Purines (2 rings) • G and A – Pyrimidines (1 ring) • C, T, and U Nucleic Acid Types DNA • Sugar is deoxyribose RNA • Sugar is ribose – Has –H • • • • • Bases are A,C, G, and T Double-stranded helix Only in nucleus Modified only by mutations 1 type – Has -OH • • • • Bases are A, C, G, and U Single-stranded Not confined to nucleus Lots of processing and modifications • 3 types RNA Types • Ribosomal RNA (rRNA) – Combines with proteins to form ribosomes – Synthesize polypeptides • Messenger RNA (mRNA) – Complimentary DNA sequence – Carries DNA message from the nucleus to ribosomes • Transfer RNA (tRNA) – Transfers amino acids to ribosomes – Build polypeptide chains James Watson and Francis Crick • Nobel prize for DNA double-helix model – Rope ladder with antiparallel sides • 5’ to 3’ ends • Pyrimidines and purines – A with T form 2 bonds – G with C form 3 bonds – Supported by Chargaff’s rules Semiconservative model • Each strand of original DNA serves as a template • Nucleotides match to template according to base pairing rules (complementary strand) • 1 ‘parent’ DNA strand produces 2 new ‘daughter’ strands • Occurs rapidly, both strands simultaneously – Humans with 6 billion pairs a few hours, with only about 1 error every 10 billion nucleotides DNA Replication • Helicase – Unzips and separates strands • DNA polymerase – link nucleotides to growing daughter strands – Can only bind to 3’ – New strands can only grow 5’ to 3’ • Leading strand - toward fork (continuous) • Lagging strand – away from fork (fragmented) • DNA ligase – Links fragments together • Roles in maintenance, proofreading, and repair • Video 1 Central Dogma of Biology transcribed translated • DNA RNA protein – Francis Crick – Genes instruct, but don’t build • Transcription (same language) in nucleus • Translation (new language) in cytoplasm – mRNA codes for polypeptides Transcription • In the nucleus • RNA polymerase binds to 1 strand with promoter – Many work at once – RNA nucleotides added • Bind to 3’ end only • Builds 5’ 3’ – Separates DNA strands • Unstable complex = immediate release • Terminator sequence releases RNA polymerase – Release pre-mRNA mRNA Processing • Before leaving nucleus • Initially has introns (filler) and exons (code) • Nucleotide sequences added to either end – 5’ cap and Poly A tail • Introns removed and exons rejoined • Creates mRNA Decoding Codons • Only 4 nucleotide bases to specify 20 amino acids • Genetic instructions are based on triplet code called codons – 42 = 16 (not enough); 43 = 64 (plenty) • Demonstrates redundancy, but not ambiguity • Nearly universal across species Translation • Within ribosomes • mRNA has codon message from DNA • Translated by tRNA – Anticodon and amino acid on opposite ends • Ribosomes facilitate addition of tRNA to mRNA Ribosomes • Coordinate mRNA and tRNA • Composed of proteins and ribosomal RNA (rRNA) • Actually make polypeptides • 2 subunits, large and small – Small locks mRNA – Large has 2 sites • P site holds growing polypeptide • A site holds new tRNA molecule (amino acid) Building Ribosomes • Small subunit binds mRNA at a start codon (AUG) • 1st tRNA enters the P site carrying the amino acid met – Anticodon is what? • Large subunit binds to create a ribosome – Met is in the P site – A site is empty Translating mRNA • 2nd tRNA molecule into A site • Polypeptide in P site breaks off and attaches to amino acid in A site • P site tRNA leaves • Ribosome translocates – Shifts 5’ to 3’ – A site tRNA to P site • Repeats A New Polypeptide • Stop codon sequence signifies the end of a polypeptide chain – Enters A site, doesn’t carry amino acid • Polypeptide released from P site tRNA • Ribosome splits • Polypeptide assumes level of structure (1° to 4°) Mutations • Changes to the genetic information of a cell • Ultimate source of diversity because ultimate source of new genes • Point mutation – Replace 1 nucleotide with another – Effect depends on codon • Base insertions and deletions – Changes reading frame – Most often deleterious effects • E.g. The cat ate the rat.