DNA Structure (Genetic Material)

DNA STRUCTURE Genetic Material Hershey and Chase conducted experiments in 1952 to determine if DNA or proteins were the genetic material of a cell It was previously known that viruses insert their genetic material into cells and so radioactively labeled viruses were prepared • Viruses grown in 35S had radioactive proteins but did not transfer this radioactivity to bacterium (remained in supernatant) • Viruses grown in 32P had radioactive DNA and did transfer this radioactivity to infected bacterium (found in pellet) Experiment 2: Testing DNA with 32P Experiment 1: Testing Protein with 35S 300 300 Protein labelled Infection Centrifuge DNA labelled Radioactive supernatant Conclusion: Proteins not genetic material Infection Centrifuge Radioactive pellet Conclusion: DNA is the genetic material X-ray Diffraction Franklin and Wilkins used X-ray diffraction to elucidate DNA structure • X-rays will diffract when targeted at crystallised DNA molecules • The scattering pattern created can be used to determine structure Wilkins From the patterns generated, the following properties were deduced: • Composition: DNA is a double-stranded molecule • Orientation: The bases face inwards and the phosphates face out • Shape: DNA forms a double helix (10 bases per twist) Nucleosomes Wilkins Diffraction Franklin Non-Coding DNA In eukaryotes, DNA associates with eight histone proteins to form a nucleosome Nucleosomes help to supercoil the DNA • Makes DNA compact (better storage) • Prevents DNA damage (less exposed) • Assists in cell division (more mobility) • Involved in transcriptional regulation DNA H1 Some regions of DNA do not code for protein • • • • • Histones (octamer) Satellite DNA (tandem repeats) Telomeres (chromosome ends) Introns (non-coding sequences) Non-coding RNA genes Gene regulatory sequences Tandem repeats are used in DNA profiling Eukaryotic Organisation of DNA DNA is bound with histone proteins to form nucleosomes that are then linked together to form strings of chromatosomes These coil to form solenoids, which condense into 30 nm fibres, before being compressed and folded into chromatin DNA Nucleosome Chromatosome Solenoid 30 nm fibre Chromatin Chromosome