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Molecular Biology of the Gene (Chapter 10) Early Searches for Genetic Material ■ ■ ■ Mendel(1822-1884): modes of heredity in pea plants Morgan(1866-1945): genes located on chromosomes Avery(1877-1955): transformation agent was DNA The Continued Search for Genetic Material ■ Hershey and Chase (1952) √ bacteriophages (phages) DNA, not protein, is the hereditary material Test:sulfur(S) is in protein, phosphorus (P) is in DNA; only P was found in host cell √ √ DNA Structure ■ ■ ■ ■ Griffith(1879-1941): bacterial work; transformation: change in genotype and phenotype due to assimilation of external substance (DNA) by a cell Chargaff 1940s & 50s ratio of nucleotide bases (A=T; C=G) Watson & Crick 1950s (Wilkins, Franklin) The Double Helix √ nucleotides: nitrogenous base (thymine, adenine, cytosine, guanine); sugar deoxyribose; phosphate group DNA Bonding ■ ■ ■ ■ Purines: ‘A’ & ‘G’ Pyrimidines: ‘C’ & ‘T’ ‘A’ H+ bonds (2) with ‘T’ and ‘C’ H+ bonds (3) with ‘G’ Van der Waals attractions between the stacked pairs DNA Structure DNA Replication ■ Watson & Crick strands are complementary; nucleotides line up on template according to base pair rules (Watson) DNA Replication: a closer look ■ ■ ■ ■ Origin of replication (“bubbles”): beginning of replication Replication fork: ‘Y’-shaped region where new strands of DNA are elongating Helicase:catalyzes the untwisting of the DNA at the replication fork DNA polymerase:catalyzes the elongation of new DNA DNA Replication ■ Antiparallel nature: – sugar/phosphate backbone runs in opposite directions (Crick) – one strand runs 5’ to 3’, while the other runs 3’ to 5’ – DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only DNA Replication ■ ■ ■ Leading strand: synthesis toward the replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand) Lagging strand: synthesis away from the replication fork (Okazaki fragments); joined by DNA ligase (must wait for 3’ end to open; again in a 5’ to 3’ direction) Initiation: Primer (short RNAsequence~w/ primase enzyme), begins the replication process DNA Replication DNA replication in real time Protein Synthesis: overview ■ Transcription: synthesis of RNA under the direction of DNA (DNA →mRNA) ■ Translation: actual synthesis of a polypeptide under the direction of mRNA (mRNA → tRNA → Protein) The Triplet Code ■ ■ ■ The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ Codons (3 mRNA bases) Remember : ‘U’ (uracil) replaces ‘T’ in RNA Transcription vocabulary ■ ■ Helicase: unwinds the DNA strand (breaks the H+ bonds) RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code ■ ■ ■ Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins Terminator region: sequence that signals the end of transcription Transcription unit: stretch of DNA transcribed into an RNA molecule Transcription sequence of events ■ ■ ■ Initiation~ transcription factors mediate the binding of RNA polymerase to an initiation sequence (TATA box) Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end Termination~ RNA polymerase reaches terminator sequence Transcription: overview Translation vocabulary ■ ■ mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome tRNA has an amino acid attached to one side, an anticodon on the other Translation sequence of events ■ Initiation~ union of mRNA & tRNA ■ Elongation~ •codon recognition •peptide bond formation ■ Termination~ ‘stop’ codon reaches site Translation Mutations: genetic material changes in a cell ■ Point mutations – Changes in 1 or a few base pairs in a single gene ■ Base-pair substitutions: – silent mutations no effect on protein – missense ∆ to a different amino acid (different protein) – nonsense ∆ to a stop codon and a nonfunctional protein ■ Base-pair insertions or deletions: – additions or losses of nucleotide pairs in a gene – alters the ‘reading frame’ of triplets~frameshift mutation ■ Mutagens: – physical and chemical agents that change DNA