Molecular Genetics 1

• Molecular Genetics 1


  Forensic Scientific Principles
• Lecture Aims
• After this lecture you should be able to:

  1. Describe the key experiments that identified the molecule of inheritance

  2. Describe the key characteristics of the molecule of inheritance

  3. Describe the relationship between the structure of DNA and its function

  4. Describe the central dogma of biology
• Inheritance


  Gregor Johann Mendel studied patterns of inheritance in peas and in 1866 published the fundamental principles of heredity<|>He proposed the existence of "particulate unit factors" for each hereditary trait
• the molecular basis of inheritance and variation was still not known
• Walther Flemming


  • Discovered thread-like structures called chromatin (chromosomes) within the nucleus
• Theodor Boveri


  • Studied chromosomes replicating and dividing (in sea urchins)
  • Each cell has to have the correct number of chromosomes
  • Chromosomes hold the key to the inheritance of characteristics
• 1868 - Friedrich Miescher isolated nuclei and identified a molecule containing Carbon, Oxygen, Nitrogen, Hydrogen and Phosphorous
• Nuclein


  Nucleic Acid
• Chromosomes contain protein and Deoxyribose Nucleic Acid
• The molecule of inheritance should have the following characteristics:

  • It must be able to replicate accurately
  • It must contain in a stable form, the information about an organism's structure and function
  • It must be able to change in order to generate variation
• 1928 - Frederick Griffiths


  Showed the presence of a "transforming factor" that could pass on new characteristics
• The lethal phenotype could be destroyed by heat
• The lethal phenotype can be transferred to a living, previously non-lethal, bacterium
• 1944 - Avery, McCarty & McCleod


  Showed that the transforming factor was DNA
• 1952 - Hershey & Chase

  1. Working with bacteriophage

  2. Bacteriophages are composed of protein and DNA

  3. They reproduce by injecting their genetic material into the host cell to be copied
• Infected bacteria were only radioactive if the viral DNA was labelled
• P32 DNA, S35 PROTEIN
• Sketch from Francis Crick 1956 (unpublished)
• 1953 - James Watson & Francis Crick


  1. Proposed a structure for DNA, the double helix

  2. Awarded the Nobel prize for Physiology or Medicine with Maurice Wilkins in 1962
• Wilkins & Franklin


  • Maurice Wilkins & Rosalind Franklin studied DNA by X-ray diffraction and concluded that DNA was helical
  • Photograph 51
• Chargaff's Rule


  • DNA contains 4 nitrogenous bases
  • 50% of bases were purines and 50% were pyrimidines
  • The amount of adenine was equal to that of thymine and amount of cytosine was equal to that of guanine
• The Watson & Crick Model

  Two polynucleotide chains wound around each other in a right handed double helix<|>The sugar-phosphate backbone is on the outside of the helix<|>The bases point towards the central axis<|>The two chains run antiparallel
• Structure of DNA


  DNA is a polymer of deoxyribonucleotide molecules<|>Deoxyribose Sugar + Base + Phosphate = Nucleotide
• The DNA Bases


  Four types of nitrogenous bases, join to the primary carbon in the deoxyribose sugar:<|>Purines - adenine and guanine, nine-carbon, double ringed structures<|>Pyrimidines - thymine and cytosine, six-carbon, single ringed structures
• Structure of DNA


  Sugar-Phosphate backbone<|>5' Phosphate group<|>3' Hydroxyl group
• Nucleotides are linked together by a covalent phosphodiester bond


  Between the 5' phosphate group of one nucleotide and the 3' carbon of another
• Base Pairing


  Antiparallel strands held together due to bonding between the bases<|>The bases lie perpendicular to the axis, bound together by hydrogen bonds<|>Two H-bonds between Adenine = Thymine<|>Three H-bonds between Guanine = Cytosine
• The Double Helix (B-form)


  • Base pairs are 0.34 nm apart
  • A complete 360° turn takes 3.4 nm (10 nucleotides per turn)
  • Unequal spacing between sugar phosphate backbones result in a major groove and a minor groove
• Chromosomes


  Single DNA molecule (the smallest chromosome = 50 x 10^6 bp = 1.7cm long!)<|>DNA wraps around associated proteins known as histones (DNA + Protein = Chromatin)<|>The molecule wraps around itself to form dense chromosomes
• The Meselson-Stahl Experiment


  • If the conservative hypothesis was correct no hybrid forms would be detected
  • If the dispersive hypothesis was correct DNA of intermediate density would be detected
  • DNA Replication is semi-conservative
• DNA replication


  1. Occurs in the nucleus

  2. Carried out by DNA polymerase III

  3. One strand is used as a template to synthesis a new strand based on Chargaff's base-pairing rules (A-T, C-G)

  4. The new strand is synthesised in a 5 to 3 direction
• Some sections of DNA contain nucleotide sequences that determines the order of amino acids in a protein (genes)
• DNA sequence is converted, by transcription, to an intermediate molecule - RNA
• The Structure of RNA


  Single stranded<|>Ribose instead of deoxyribose (2' hydroxyl group)<|>Uracil instead of thymine
• Sugar-phosphate backbone, joined 5 - 3


  Bases attached at the primary carbon<|>Synthesised from DNA in a process called Transcription
• Transcription


  1. Occurs in the nucleus

  2. Hydrogen bonds break and DNA unwinds

  3. RNA polymerase reads 5'-3' joining ribonucleotides according to their complementary bases (C-G, G-C, T-A and A-U)

  4. Single stranded RNA molecule released
• Messenger RNA (mRNA)


  mRNA molecules are processed, ready for translation
• Ribosomal RNA (rRNA)


  Most abundant type of RNA<|>Combines with proteins to form ribosomes found in the cytoplasm and on the Rough ER<|>Ribosomes hold mRNA molecules in place for translation<|>Peptidyl transferase activity of rRNA catalyses the formation of peptide bonds
• Transfer RNA (tRNA)


  The smallest form of RNA (73-95 nucleotides)<|>Forms a clover-leaf structure<|>Carries specific amino acids (bound to the 3' end) to the ribosome<|>tRNA "reads" the genetic code