-1B: DNA replication Flashcards
when does DNA replication occur?
-before cell division (mitosis/meiosis) so each new cell contains exact replica of DNA
-S phase of cell cycle, synthesis
why is it called semi-conservative replication?
-new daughter DNA strand contains 1 strand of parental DNA + one strand of newly synthesised DNA
-so one strand is conserved
important factors in DNA replication?
-complementary base pairing, A-T, C-G, allows for identical DNA strands to be created + reduces copying error which could result in a mutation
-enzymes -> DNA helicase + polymerase
describe stages of semi conservative replication
-DNA helicase breaks hydrogen bonds between complementary base pairs between two strands within a double helix, causing double helix to unwind
-two parental strands acts as template, free floating DNA nucleotides in nucleus are attracted to exposed complementary bases on parental template strands
-nucleotides join together via condensation reactions to form phosphodiester bond + polynucleotide, catalysed by DNA polymerase
-two new daughter strands each contain 1 strand of parental DNA + 1 strand of newly synthesised DNA
what is meant by parental DNA?
original copy of DNA
what is meant by daughter DNA?
newly created/synthesised DNA
what names are given to each of the ends of DNA strands?
-3’ (prime)
-5’ (prime)
what is special about how the strands run in DNA?
-run antiparallel to eachother
the active site of DNA polymerase is only complementary to what end? what does this mean?
-3’ end of newly synthesised strand
-enzyme can only add new nucleotides to new strand at 3’ end
what does the strands running anti parallel further mean?
-new strand made in 5’ -> 3’ direction
-DNA polymerase moves down template strand 3’ -> 5’
-DNA polymerase on another strand moves in opposite direction to one moving 3’ -> 5’ on template strand
who discovered the structure of DNA?
Watson + Crick, with the help of Rosalind Franklin electron diffraction
who conducted an experiment for evidence that DNA replication was semi-conservative?
-Meselson + Stahl
-conducted an experiment -> hypothesised that DNA replication is either conservative/semi conservative
difference between conservative + semi conservative replication?
-conservative: original (parental) DNA stays intact/together, new DNA molecules contain two newly synthesised strands
-semi conservative: daughter DNA contains one parental strand of DNA, one newly synthesised strand of DNA -> so one strand = ‘conserved’
what did the experiment use?
-isotopes of nitrogen
-14N -> light, 15N -> heavy
describe in brief how the experiment worked:
-2 bacteria samples grown, one in broth containing 14N (light), other in 15N broth (heavy), as bacteria reproduced it took up nitrogen from broth to help make nucleotides for new DNA -> nitrogen gradually came apart of bacterias DNA
-samples were taken from each batch + spun in centrifuge, 14N -> settled higher as less dense, 15N settled lower as more dense
-15N sample taken out + placed in 14N broth + left for one round of DNA replication before being taken out again + spun in centrifuge
-if conservative: original heavy DNA -> still be together, would settle at the bottom + new light DNA settle at the top
-if semi-conservative: new molecule contain one strand of parental (heavy) DNA + one strand of newly synthesised (light) DNA, so DNA would settle out between where light nitrogen DNA + heavy nitrogen DNA settled out
-DNA settled out in middle -> showing DNA molecules contained a mixture of heavy + light nitrogen, bacterial DNA had replicated semi-conservatively in light nitrogen
