• Unlike most prokaryotic DNA which is referred to as ‘naked’, eukaryotic nuclear DNA is associated with proteins called?histones?(to form?chromatin)
• Histones package the DNA into structures called nucleosomes
  • The?nucleosome?consists of a strand of DNA coiled around a core of eight histone proteins (octamer) to form a bead-like structure
  • DNA takes?two turns?around the histone core and is held in place by an additional histone protein
  • The DNA molecule continues to be wound around a series of nucleosomes to form what looks like a ‘string of beads’
• Nucleosomes help to?supercoil the DNA, resulting in a compact structure which saves space within the nucleus
  • Nucleosomes also help to?protect DNA?and?facilitate movement of chromosomes?during cell division
  • An analogy for supercoiling is?twisting an elastic band?repeatedly until it forms additional coils
• Nucleosomes can be?tagged with proteins?to promote or suppress transcription

NOS Making careful observations—Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix.

• In the 1950s,?Rosalind Franklin?and?Maurice Wilkins?used a technique called?X-ray diffraction?to study the structure of DNA
  • Franklin’s work was?instrumental to Crick and Watsons' model?as the diffraction patterns indicated that DNA had a?double-helical?structure
• X-ray diffraction involves directing?a beam of X-rays?onto the molecule being studied
  • X-rays have a?shorter wavelength?and?higher energy?than visible light
• The short wavelength allows X-rays to pass?through the molecule, interacting with any?electrons?within the atoms
  • The interaction?causes X-rays to scatter?(diffraction) at angles that indicate the arrangement of atoms
  • The scattering pattern can be?recorded on a film?(similar to having an X-ray of a bone), with dark marks appearing where the X-rays strike the film
  • Rotating the sample allows for the?three-dimensional molecular structure?to be studied
• Franklin was able to?refine her methods?and produce a clear diffraction pattern of DNA
• Using mathematical techniques and available knowledge about DNA, Franklin deduced that
  • DNA strands were?helices?- as represented by the X-shape
  • The?pitch?of the helix - as represented by the angle of the X-shape
  • The?distance?between nucleotides
  • Phosphates are located on the?outside?of the molecule
  • DNA was?double stranded

• The discovery of the structure of DNA was due to?experimental evidence?and inputs from a?range of independent researchers
  • Franklin’s X-ray diffraction patterns identified a?compact double helix
  • Erwin Chargaff showed that DNA was composed of an equal number of?purine and pyrimidine?bases which suggested base pairing
  • Crick and Watson used this evidence to build various?physical models of DNA
  • One model had the bases facing outwards but Franklin argued they should?face inwards?due to their hydrophobic nature
  • It was determined that if adenine paired with thymine and cytosine paired with guanine in an?antiparallel?orientation a?highly compact structure?would result
• When Crick and?Watson?proposed their model for the structure of DNA, they realised that the double stranded structure suggested?a mechanism for its replication?during the cell cycle
  • This was a?key question?that any model would have to address
• Crick and Watson?stated that as one chain of the double helix was the complement of the other,?either chain could act as a template?during replication
  • They postulated that?hydrogen bonds break,?allowing separation of the chains
  • Each separate chain then?acts as a template?for the formation of a new complementary chain on itself
• This theory was called?semi-conservative DNA replication?as half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
• An?opposing theory?suggested DNA replicated ‘conservatively’
  • The entire original DNA double helix would stay together and serve as a template for a new DNA molecule
• Crick and Watsons' theory of semi-conservative DNA replication was later?proven by Meselson and Stahl