DNA structure and replication

DNA structure and replication

1. The Watson and Crick model suggested semi-conservative replication

   • DNA structure suggested replication mechanism
   • knowledge of DNA structure
     • molecular modelling by Linus Pauling
     • X-ray diffraction patterns by Rosalind Franklin
       • DNA helix tightly packed
       • bases fit together; strands not too far apart
     • base composition studies by Erwin Chargaff
   • their 1st model - sugar-phosphate strands wrapped around one another with nitrogen bases facing outwards
     • Rosalind Franklin countered this
     • nitrogen bases more hydrophobic than sugar-phosphate backbone, so would face inwards
   • tightly packed if:
     • pyrimidine paired with purine
     • bases 'upside down' in relation to one another
   • adenine and thymine both structurally and electrically (charges) compatible
   • adenine and guanine paired; 3 hydrogen bonds = structural stability
   • model of complementary base pairing lead to theory

2. The role of nucleosomes in DNA packing

   • help supercoil DNA
   • Nucelosomes - structures of DNA packaged by histones
     • central core of 8 histone proteins (octane) with DNA coiled around them
       • 2 copies of 4 diff types of histones
     • connected by 'linker' DNA
       • bound to core by histone molecule, H1
         • bind to form 30nm fiber; assists packing
     • facilitates packing of large genomes of eukaryotes
   • association of histones contributes to supercoiling pattern
   • supercoiling allows great length of DNA to be packed in small space in nucleus

3. The leading strand and the lagging strand

   • leading
     • continuous replication
     • continuously following the fork
   • lagging
     • discontinuous replication
     • fragments moving away from the replication fork
     • Okazaki fragements - new fragments created on lagging strand as fork exposed more template strand
   • arranged in anti-parallel fashion

4. Proteins involved in replication

   • replication carried out by complex enzyme system
     • helicase unwinds DNA at the fork
     • topoisomerase releases strain that develops ahead of helicase
   • replication
     • formation and movement of replication fork
     • synthesis of leading and lagging strands
   • single stranded binding proteins keep strands apart long enough for template strand to be copied
   • RNA primer needed to start replication
     • created by DNA primase
     • necessary to initiate DNA polymerase activity
   • many primers on lagging strand; only one on leading strand
   • DNA polymerase
     • responsible for covalently linking deoxyribonucleotide monophosphate for the 3' end of the growing strand
     • diff. kinds with diff. functions
       • proof-reading
       • polymerization
       • removal or RNA primers once not needed
   • DNA ligase connects the gaps between the fragments

5. The direction of replication

   • DNA polymerase can only add nucleotides to the 3' end of the primer
   • begins at site called origin of replication
   • occurs in both directions away from origin
   • five carbons of deoxyribose sugar have a number
   • phosphate group added to 3' of the deoxyribose of the nucleotide at the end of the chain
   • replications occurs in a 5' to 3' direction

6. Non-coding regions of DNA have important functions

   • cellular machinery operates according to genetic code
   • coding sequeneces - DNA sequences used to produce polypeptides
   • used as guide to produce tRNA and rRNA
   • enhancers and silencers regulate gene expression
   • most genome non-coding in eukaryotes
   • repetitive sequences common
     • moderately
     • highly
       • satellite DNA
     • 5-60% (60% in humans)
     • one occurs on the ends of eukaryotic chromosomes; telomere
       • protects cells from losing genes that cannot replicate during interphase

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