DNA structure

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  • DNA structure
    • Hershey and Chase
      • Viruses were grown in radioactive sulfur with radioactively labelled proteins
      • Viruses were grown in radioactive phosphorus with radioactively labelled DNA
      • The viruses were allowed to infect a bacterium
      • The bacteria was found to be infected by the phosphorus (DNA) but not the sulfur
      • This showed that DNA was genetic material not proteins
    • DNA structure
      • Rosalind Franklin used x-ray diffraction to investigate the structure of DNA
      • DNA was purified and then stretched into thin glass tubes
      • DNA was targeted by an x-ray beam which was diffracted and it's scattering pattern was recorded
      • Three findings confirmed
        • DNA is a double stranded molecule
        • Nitrogenous bases are closely packed inside and phosphates form a backbone
        • The DNA strands twist at regular intervals to form a double helix
      • There is equal number of purines and pyramidines
    • DNA replication
      • Helicase - unwinds the double helix at the replication fork by breaking h bonds between base pairs
      • Gyrase - reduces torsional strain from helicase by relaxing positive supercoils
      • SSB proteins bind to the strands to prevent them reanealing
      • DNA primase generates an RNA primer on each strand which provides an initiation point for DNA pol III
      • DNA Polymerase III  moves towards the replication fork on the leading strand and away from the replication fork on the lagging strand
      • DNA Polymerase I - replaces RNA primers with free nucleotides on the lagging strand
      • DNA Ligase - joins together Okazaki fragments (short fragments on lagging strand)
    • DNA Sequencing
      • ddNTPs (Dideoxynucleotides) prevent further elongation of a nucleotide chain and terminate replication
      • The Sanger method can be used to determine DNA sequencing
      • 4 PCR mixes are set up, fragments are separated using gelelectrophoresis and fragments can be detected by automated sequencing machines
    • Non-coding DNA
      • Satellite DNA - used for DNA profiling, structural component of centromere
      • Telomeres - protects against chromosomal deterioration
      • Introns - removed by RNA splicing before mRNA formation
      • Non-coding RNA genes - codes for RNA molecules that are not translated into protein
      • Gene regulatory sequences - promoters, enhancers, silences. Involved in transcription
    • Nucleosomes
      • In eukaryotic organisms DNA is packaged with histone proteins
      • Nucleosomes help DNA to supercoil for efficient storage
      • Nucleosomes are linked by an additional histone protein to form  chromatosomes
      • Chromatosomes coil to form a solenoid structure which becomes condensed fibre  (30nm)
      • The fibers form loops to make chromatin
      • Chromatin supercoils to form chromosomes


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