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Molecular Genetics Searching for Genetic Material, I • Mendel: modes of heredity in pea plants • Morgan: genes located on chromosomes • Griffith: bacterial work; – transformation: change in genotype and phenotype due to assimilation (uptake) of external substance (DNA) by a cell • Avery: transformation agent was DNA (stores and transmits genetic information) Searching for Genetic Material, II • Hershey and Chase √ bacteriophages (phages) - viruses that attack bacteria √ Used radioactive markers to show DNA, not protein, is the hereditary material √ sulfur (S) is in protein, phosphorus (P) is in DNA; only P was found in host cell DNA Structure • Watson & Crick The Double Helix nucleotides: • nitrogenous base • sugar deoxyribose • phosphate group DNA Bonding • DNA Nitrogen Bases: – Purines: • Adenine • Guanine – Pyrimidines: • Cytosine • Thymine • Chargaff rules A-T C-G DNA Replication • Watson & Crick strands are complementary; nucleotides line up on template according to base pair rules (Watson) DNA Replication (Synthesis) • 2 strands of double helix unwind – each is template for complementary strand • Replication is initiated – RNA primer is synthesized • DNA strand grows in two directions 3’ 5’ Leading strand 3’ RNA primers 3’ 5’ 5’ 5’3’ 3’ 5’ Two Okazaki fragments Fig. 12-12b, p. 273 Bidirectional Replication, I • Starting at origin of replication – proceeding in both directions • Eukaryotic chromosome – may have multiple origins of replication – may replicate at many points at same time Bidirectional Replication, II • • • • Initiation: Primer (short RNA sequence~w/primase enzyme), begins the replication process 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) Bidirectional Replication, III • Always proceeds in 5′ → 3′ direction • Leading strand – synthesized continuously • Lagging strand – synthesized discontinuously – forms short Okazaki fragments • Key enzymes: – DNA polymerase adds nucleotide subunits – DNA primase synthesizes RNA primers – DNA ligase links Okazaki fragments 3’ 5’ Leading strand DNA helix RNA primer DNA polymerase 3’ 5’ 3’ 5’ 3’ 5’ Replication fork Lagging strand (first Okazaki fragment) Direction of replication Fig. 12-12a, p. 273 Protein Synthesis: overview • • • • One gene-one enzyme hypothesis (Beadle and Tatum) One gene-one polypeptide (protein) hypothesis Transcription: synthesis of Messenger RNA (mRNA) using DNA code Translation: actual synthesis of a polypeptide (protein) using mRNA code Protein Synthesis RNA • Three types of RNA – mRNA (messenger RNA) – ling strands of RNA that are complementary to DNA to one strand of DNA; travel from nucleus to ribosome – tRNA – (transfer RNA) – small segments of RNA that transport amino acids to the ribosome – rRNA – (ribosomal RNA) – component of ribosomes The Triplet Code • The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3nucleotide ‘words’ called Codons • RNA Code uses A, C, G, but ‘U’ (uracil) replaces ‘T’ Eukaryotic RNA Processing • Exons – DNA coding that remain in the final mRNA for protein synthesis; genes that are expressed • Introns – DNA coding that does not remain in mRNA Transcription • 2 strands of double helix unwind – One strand is template for making mRNA • Key enzyme: – RNA polymerase – regulates synthesis of mRNA Transcription Translation • mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome – Each tRNA anticodon (nucleotide triplet) is specific for one amino acid • Thus, amino acids are bonded together in the order translated from mRNA Translation Applying Genetics • Selective Breeding – allowing only desired traits to pass onto next generation • Hybridization – crossing dissimilar organisms to get the best of both, e.g., crop plants • Inbreeding – continued breeding of organisms with similar characteristics, e.g., dog breeds Applying Genetics • Increasing Variation • Producing new kinds of bacteria via radiation or chemicals to develop oil eating bacteria • Producing new kinds of plants, e.g., polyploidy plants such as daylilies Genetic Engineering Genetic engineering – altering DNA o DNA extraction – removing DNA from cells o Cutting and pasting DNA – using restriction enzymes to cut DNA into pieces that can be analyzed o Separating DNA – fragments separated via gel electrophoresis What can you do with it? o Read the sequence o Cut and paste o Make copies Genetic Engineering Vocabulary • Plasmid – bacterial DNA that can be used to replicate foreign DNA • Genetic marker – a gene that makes it possible to distinguish the bacteria that carry the plasmid and foreign DNA from those that don’t • Transformation – process that results in genetic alteration of a (bacterial) cell resulting from the uptake DNA from another (bacterial) cell • Recombinant DNA – Recombinant DNA is DNA that has been created artificially. DNA from two or more sources is incorporated into a single recombinant molecule. • Gel Electrophoresis – electric current is used to separate fragments of DNA; DNA fragments travel toward a negative charge Applications of Genetic Engineering Clone – identical cells, DNA Who is Dolly? Transgenic organisms – contain genes from other organisms Examples: • Transgenic microorganisms – to produce insulin and other compounds for health • Transgenic animals – mice that act physiologically like humans • Transgenic plants – plants with natural insecticides Mutations • Kinds of Mutations – Gene Mutations • Point mutations – change in one or a few nucleotides, i.e., substitutions, additions, deletions • Frameshift mutations – additions (insertions) or deletions can shift the reading of the strand resulting in more dramatic changes – Chromosomal mutations – change in number or structure of chromosomes • Deletions – loss of all or part of a chromosome • Duplications – extra copies of a chromosome • Inversion – reverse the direction of parts of a chromosome Mutations • Most mutations are neutral, but some are harmful and some are root of genetic variability. Examples: resistance to disease such as HIV • Mutation causes: Mutagens – chemicals, X rays, gamma rays Gene Regulation • Gene Regulation – turning genes on and off • Prokaryotes (bacterium) • Operon – group of genes that operate together – ex: lac genes to use lactose for food. – Contains an operator, regulatory gene, promotor and genes that code for proteins. Gene Regulation • Operon – Promotor – Where RNA polymerase first binds. It is the region where transcription can occur so that proteins can be produced that allow the transport and break down of lactose. But these proteins are not needed unless lactose is present. – Operator – Like a light switch that turns transcription on and off. In lac operon, O region where repressor proteins are present. These are present, transcription cannot occur. These proteins fall off O region when lactose is present. – Regulatory gene - Mechanism for turning transcription on and off. In lac operon, makes a repressor protein that binds to the operator in the promotor sequence and prevents transcription of the digestive enzyme genes. Eukaryote Gene Regulation • Eukaryotes – no operons, but regulation is more complex due to cell specialization o Transcription factors – proteins that control gene expression o Hox genes – control differentiation in embryonic development to determine the body plan of an organism o RNA Interference – small pieces of doublestranded RNA in the cytoplasm that bind with mRNA and prevent translation