DNA: THE GENETIC MATERIAL
Nucleic Acids • Fourth category of biological macromolecules (others: carbohydrates, lipids and proteins). We breezed over nucleic acids at the start of class with a promise to return. We are back! • Major functions: information storage, energy transfer and some limited enzymatic function by RNA. • Examples: DNA, RNA, ATP
• We studied inheritance of traits in our last unit, now we will further examine the structure and function of genes by studying DNA. • This material falls into the category of Molecular Genetics.
DNA: Deoxyribonucleic Acid • Discovered in 1869 by Friedrich Mieschler, a German chemist; he called it “nuclein” because it was associated with the nucleus. Function was then unknown. • DNA contains the genetic code (information) and is capable of replicating itself, a remarkable feat for a chemical. • Much of life is controlled by DNA. Much of modern biology involves understanding and manipulating DNA.
History: Where is Genetic Information Stored In the Cell? • Several early experiments concluded that the genetic information is found in the nucleus of the cell (cells with nuclei removed could not divide, etc.). • Prokaryotes have DNA in a nucleoid region. Genetic information = Hereditary information
What is the Genetic Material? • Several early experiments were performed to determine where genes are found. • Chromosomes are 60% protein and 40% DNA. Had to figure out which was the genetic material that contained genes. • Protein was initially favored, as it is complex and DNA was perceived as simple (protein is made of 20 amino acids, while DNA is made of only 4 nucleotides).
1952 Hershey and Chase Is protein or DNA the genetic material? • Experimented with bacteriophages (viruses that attack bacteria) and radioisotopes (next slide). • Viruses are obligate, intracellular parasites that consist of DNA or RNA surrounded by a protein coat. • Is a virus alive? Not according to the cell theory (viruses lack cells), but viruses can still replicate and evolve. -When a virus attacks, it binds to a bacterium (or other cell) and injects its genetic material into the cell, where it “hijacks” the metabolism of the cell and makes more viruses, often lysing (splitting) the host cell. Alfred Alfred Hershey Martha Martha Chase Chase
Which is injected, protein or DNA? • Radioactive isotopes were used to label two molecules of viruses. Read samples with a precise geiger counter. -Labeled protein virus coat with sulfur radioisotope ( 35 S ). -Labeled virus DNA with phosphorous radioisotope ( 32 P ). -Grew phages (viruses) in separate colonies, labeling one set with 35 S and one with 32 P. Then allowed the viruses to attack bacteria in two separate trials.
Protein virus coat labeled with 35 S DNA ( blue ) being injected into bacterium 35 S remains outside cells Two experiments: one in which protein (S) is labeled, and one in which DNA (P) is labeled.
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