topic 1 - DNA replication Flashcards
cgp (topic 1B) 18 - 19
what does DNA replicate by
semi conservative replication
what does semi conservative replication mean
during DNA replication, the two strands of nucleotides separate - both strands then form the template for free nucleotides to bind to to create the two identical daughter strands
what does genetic continuity mean
the passing of DNA and the genetic information stored in DNA from one generation to the next
describe the first step of the semi conservative process
the enzyme DNA helicase breaks the hydrogen bonds between the two polynucleotide DNA strands - this makes the helix unwind to form two single strands
describe the second step of the semi conservative process
each original single strand acts as a template for a new strand - complementary base pairing means that free floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand - A with T, C with G
describe the third step of the semi conservative process
condensation reactions join the nucleotides of the new strands together- catalysed by the enzyme DNA polymerase - hydrogen bonds form between the bases on the original and new strands
describe the fourth step of the semi conservative process
each new DNA molecule contains one strand from the original DNA molecule and one new strand
how does DNA polymerase move
in the opposite way along antiparallel DNA strands
what are the names for each end of a DNA strand
3’ (three prime) and 5’ (five prime)
why can the enzyme only add nucleotides to the new strand at the 3’ end
since the active site of DNA polymerase is only complementary to the 3’ and of the newly forming DNA strand
what does 3’ and 5’ refer to
the number of carbon atoms in a deoxyribose sugar molecule that a phosphate group binds to
3’ 5’ page 18
who provided evidence for semi conservative replication
meselson and stahl
who came up with the theory of semi conservative DNA replication
watson and crick
before meselson and stahl’s experiment what were people unsure of
whether DNA replication was semi conservative or conservative
what would happen if the method was conservative
the original DNA strands would stay together and the new DNA molecules would contain two new strands
what does meselson and stahl’s experiment use
two isotopes of nitrogen (DNA contains nitrogen) - heavy nitrogen (15N) and light nitrogen (14N)
what is the first step to the semi conservative method
two samples of bacteria were grown - one in a nutrient broth containing light nitrogen, one in a broth with heavy nitrogen - as bacteria reproduced they took up nitrogen from the broth to hep make nucleotides for new DNA so the nitrogen gradually became part of the bacteria’s DNA
what is the second step to the semi conservative method
a sample of DNA was taken from each batch of bacteria, and spun in a centrifuge - the DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the DNA from the light nitrogen bacteria since it’s heavier
what is the third step to the semi conservative method
then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen - the bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge
what is the fourth step to the semi conservative method
if replication was conservative the original heavy DNA, which would still be together, would settle at the bottom and the new light DNA would settle at the top
what is the fifth step to the semi conservative method
if replication was semi conservative the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen - so the DNA would settle out between where the light nitrogen DNA settles out and where the heavy nitrogen DNA settled out
what is the sixth step to the semi conservative method
the DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen - the bacterial DNA had replicated semi conservatively in the light nitrogen
