1) Describe, using simple diagrams, the structure of DNA and its organisation into nucleosomes and chromatin
the phosphates bind to the 3rd of 5th carbon
part of the sugar phosphate backbone: e.g. :
PURINES - 2 circles - 'A' & 'G'.
PYRIMIDENES has 1 circle - 'C' 'U' 'T'
phosophodiester bond is a chemical
bond that joins successive sugar
molecures in a polynucleotide
Compation of DNA
nucleosome = combined tight loop of DNA + Protein
Linker Histones = link nucleosomes ...............................
Core Histones = what DNA is rapped around (8 types)
nucleosomes are stacked on top of eachother.
Chromitin = fibre of tight nucleosomes.
Chromosomes: each occupy a distinct
area to stop tangeling - and can be
seen during devision.
2 )Outline the mechanism of DNA synthesis (replication) and describe how some antibiotics intergere with this process
DNA replicates by Semi-Conservative replication.
ALWAYS occurs in the 5' (prime) to 3' direction.
Double helix opens with the help of initiator proteins
the Y shaped junction is
called the replication
fork. the 2 forks move
away from eachother in
opposite directions -
DNA polymerase synthesises new
DNA usinf one old strand as a
template. Replication occurs at the
'origins of replications' which can
be at multiple points of the DNA
When DNA is opened in front of the
fork there is an excess of twisting
(on other side of the fork) - makes
TOPOISOMERASE prevents this and
Antibiotics can interfere with
DNA replication. QUINOLONES
(an antibiotic) interupts
TOPOISOMERASE in bacteria
Fluoroquinolones (a type of quinolones) -> causes supercoiling
Trimethropim - Sulfamethoxazole = inhibits DNA synthesis as it targets
nucleotide (thymide synthesis) - its has a much higher affinity for
bacteria than humans
3) Explain the very low level of mistakes in the DNA replication process
DNA editing allows correction of mistakes.. WHen polymerase adds
incorrect nucleotide. The newly synthesided DNA strand unpairs from
the teamplate and moves to error correcting catalytic site to be
removed. DNA polymerase has 2 seperate sites for DNA synthesis and
4) Outline ethods od DNA repair, with exaples of clinical consequences of defective repair mechanisms
SINGLE STRAND DEFECTS
BASE PAIR EXCISION
this is when one base pair is wrong (e.g. U
instead of C) then it is removed by URACIL
DNA GLYCOSYLASE the bit of the sugar
phosphate backbone is removed and DNA
polymerase adds new nucleotide and DNA
ligase seals the 'nick'
NUCLEOTIDE EXCISION REPAIR
this recognises and corrects distortions in
DNA. DNA HELICASE identifies wrong
section and cuts out a gap - e.g. 12
nucleotide gap. the DNA polymerase and
DNA ligase replace section