DNA replication

Question 1

What are the three possible mechanisms of DNA replication?

Answer 1

Conservative
semi-Conservative
dispersive

Question 2

How does DNA replicate?

Answer 2

DNA helix unwinds and unzips

DNA helicase breaks the H-bonds between the
complementary base pairs and DNA unzips

DNA-binding proteins hold the DNA strands
separate

Free mononucleotides come and pair up with
complementary bases on the leading strand with
the help of DNA polymerase that catalyzes the
formation of a polynucleotide chain continuously
in the 5’→3’ direction

DNA polymerase catalyses the formation of the 2nd
polynucleotide on the lagging strand
discontinuously forming Okazaki fragments

Nucleotides are joined by phosphodiester bonds

Okazaki fragments are joined by DNA ligase

Question 3

Explain Meleson and Stahls experiment.

Answer 3

 Escherichia coli bacteria were grown in a heavy 15N isotope so
they have ‘heavy’ DNA as the nitrogen was incorporated into the
nucleotides.
 E. coli were then cultured in ‘normal’ 14N.
 After one generation (one cell division so one DNA replication),
the DNA had ‘medium’ mass – half 15N and half 14N.
 After a second generation, some DNA had light (14N) mass and
the other had ‘medium’ mass.
This proved that DNA replication is semi-conservative, since new
DNA molecules containing one parent strand and one new strand.
Semi-conservative replication – when DNA makes a new
copy/molecule and each copy contains one parent strand and one
newly synthesized strand

 Bacteria grown in medium containing heavy nitrogen
isotope 15N and in a medium containing light nitrogen
isotope 14N
 Removed a sample from 15N to look at and rest were put in
light nitrogen isotope 14N to grow more
 Bacterial samples from both mediums were compared
 After centrifugation DNA with heavy nitrogen ended up
near the bottom of the tube and DNA with light nitrogen
near the top of the tube due to their density
 After one generation growing in 14N DNA showed
intermediate density, appeared in the middle of tube
 After two generations growing in 14N DNA showed both
intermediate and light density but no heavy density