DNA REPLICATION
SEMI- CONSERVATIVE REPLICATION -ensures genetic continuity between generations
of cells
1. Double helix is uncoiled by an enzyme
2. Hydrogen bonds between complementary base pairs are broken by
DNA Helicase - 2 template strands formed (unzipped)
3. New DNA nucleotides ( that are free within the nucleus) are attached
to their complementary bases on the two template strands.
4. DNA Polymerase catalyzes a condensation reaction between
adjacent nucleotides.
Why does DNA polymerase work in one direction?
➔ DNA strands are antiparallel
➔ Shape of the nucleotides is different
➔ Enzymes have active sites with specific shapes
➔ Only substances with complimentary shapes can bind with active sites on
DNA polymerase.
Work of Messelsohn and Stahl
➔ Grew E-COLI in a substance containing 15N (nitrogen isotope)
➔ After many generations bases were labeled with 15N
➔ Bacteria were then switched into 14N and left to grow
➔ Small samples extracted, DNa purified + density measured
➔ DNA separated into bands by spinning at high speed.
- Generation 0- single band of 15N
- Generation 1- single band, higher hybrid of 15N and 14N
- Generation 2- two bands, one low, one high, 15N and 14N
- Generation 3 +4- light band (14N) became stronger + hybrid
bond became weaker.
Supports semi-conservative model because generation 1 contained heavy and
light, if it is supported conservation, only heavy DNA would have been present.
SEMI- CONSERVATIVE REPLICATION -ensures genetic continuity between generations
of cells
1. Double helix is uncoiled by an enzyme
2. Hydrogen bonds between complementary base pairs are broken by
DNA Helicase - 2 template strands formed (unzipped)
3. New DNA nucleotides ( that are free within the nucleus) are attached
to their complementary bases on the two template strands.
4. DNA Polymerase catalyzes a condensation reaction between
adjacent nucleotides.
Why does DNA polymerase work in one direction?
➔ DNA strands are antiparallel
➔ Shape of the nucleotides is different
➔ Enzymes have active sites with specific shapes
➔ Only substances with complimentary shapes can bind with active sites on
DNA polymerase.
Work of Messelsohn and Stahl
➔ Grew E-COLI in a substance containing 15N (nitrogen isotope)
➔ After many generations bases were labeled with 15N
➔ Bacteria were then switched into 14N and left to grow
➔ Small samples extracted, DNa purified + density measured
➔ DNA separated into bands by spinning at high speed.
- Generation 0- single band of 15N
- Generation 1- single band, higher hybrid of 15N and 14N
- Generation 2- two bands, one low, one high, 15N and 14N
- Generation 3 +4- light band (14N) became stronger + hybrid
bond became weaker.
Supports semi-conservative model because generation 1 contained heavy and
light, if it is supported conservation, only heavy DNA would have been present.
