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Intermediate Inheritance or Incomplete Dominance Not all patterns of inheritance obey the principles of Mendelian genetics. In fact, many traits occur due to a combined expression of alleles. Ex.: Crossing pure red (RR) and pure white (WW) Japanese four-’clocks results in flowers that are all pink (RW) To know if something is dominant, co dominant or incomplete dominance, you must look at the phenotype. The ‘Big Idea’: The structure of DNA, the molecule of heredity, enables the molecule to copy itself. Why was Mendel successful with the pea? • Used pure breeding, contrasting varieties • Studied characteristics one at a time for many generations • Used statistics in analyzing his results • Obtained large numbers of offspring • Chose pea plants which normally selffertilize **Mendel had no knowledge of genes or chromosomes But what are genes made of? • The chromosome theory of heredity states: – Genes are small particles located on the chromosomes • The location of genes within the chromosomes, however, was less certain. • Biologist knew that chromosomes contain both protein and DNA, but they didn’t know which one made up the genes and directed protein synthesis. Frederick Griffith • In 1928, Griffith worked with bacteria called Streptococcus pneumoniae. • There were 2 strains: – Strain S – looked smooth because of a mucus coat – caused pneumonia and death when injected into mice – Strain R – looked rough (no mucus coat) – had no visible effect when injected into mice – Heating strain S bacteria killed them and injections of the heat-killed S bacteria did not harm mice. • Injected a mix of heat-killed S bacteria and living R bacteria into mice • Hypothesized mice would not be affected by the mixture • However – mice died of pneumonia • What killed the mice? • Allowed bacteria from dead mice to reproduce • Offspring had the mucous coats • Reasoned somehow a transforming material passed from heat-killed S bacteria to living R changing R into S • Concluded the transforming material was genetic material Oswald Avery • 1944 - Repeated Griffith’s work to determine which molecule in the heat-killed bacterial was most important for transformation • Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next • What is the molecular structure of genes • How do they coordinate roles of transmitting information generation to generation • What is the role of other nucleic acid, RNA? Edwin Chargaff • 1952 - Showed that the percentages of guanine and cytosine in DNA are almost equal • The same is true of adenine and thymine • Known as Chargaff’s Rule Maurice Wilkins and Rosalind Franklin • 1952 work with X-ray diffraction led to the discover of DNA’s structure Hershey and Chase • • • • • • • • • In 1952 conducted series of experiments using virus that attack bacteria Used a virus with a DNA core surrounded by a protein coat Labeled protein with radioactive sulfur (35S) Virus allowed to infect bacteria Bacteria did not show marker Labeled DNA with 32P Once DNA inside bacteria cell, viral DNA took over and made new virus particles. Viral DNA offspring also showed presence of radioactive isotope of P. Clear that DNA, NOT protein, directed synthesis of new protein and transmitted virus features to succeeding generation. • Convinced scientists that DNA was the genetic material. James Watson and Francis Crick • 1953 - Based on their knowledge of DNA’s chemical nature and the X-ray diffraction, formulated hypothesis that the DNA molecule is made up of 2 chains twisted around each other in a helical structure with pairs of nitrogenous bases projecting toward each other • Adenine will bond to thymine • Guanine will bond to Cytosine • Genes are considered to be segments of these molecules with the sequence of bases coding for the amino acids in protein • Where is DNA found in the cell? • How is it organized? • Where are the genes that Mendel first described a century and a half ago? Prokaryote Cells • lack nuclei and membrane bound organelles • DNA located in cytoplasm • have a single circular DNA Eukaryotic Cells • 1000 times the amount of DNA as prokaryotes • DNA located in nucleus in form of a number of chromosomes • DNA molecules long • chromosomes contain both DNA and proteinchromatin • Chromatin = DNA tightly coiled around proteins called histones Masses of chromatin fibers Chromatin fiber is a tightly wound supercoil Each supercoil is made up of coils. A coil is a chain of beadlike structures called nucleosomes