14.3 Basics of DNA Replication

14.3 Basics of DNA Replication

• The figures show the structure of the chromosomes.

• The structure of the double helix gave a hint as to how DNA divides and makes copies of itself.
• The sequence of one DNA strand can be predicted from its complement.
• The double-helix model suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new strand is copied.
• It wasn't clear how the replication took place.
• Three models were suggested: conservative, semi-conservative, and dispersive.

• There are three suggested models of DNA replication.
• The original strands of DNA are shown in gray and the newly synthesized strands are shown in blue.

• The parental and newly formed daughter strands are the same as before.
• The semiconservative method suggests that each of the two parental strands acts as a template for new DNA to be synthesised; after replication, each double-stranded DNA includes one parental or "old" strand and one "new" strand.
• In the dispersive model, both copies of DNA have double-stranded segments of their parents' genes.

• They wanted to understand how DNA replicates.
• For several generations, they grew E coli in a medium containing a heavy isotope of nitrogen (15N), which gets incorporated into nitrogenous bases, and eventually into the DNA.

• The E coli were grown first in heavy nitrogen and then in 14N.
• The heavier the DNA grown in 15N, the lower the level of cesium chloride solution in the ultracentrifuge.
• After one round of cell division, the DNA is halfway between the 15N and 14N levels, indicating that it now contains fifty percent 14N.
• 14N is only found in an increasing amount of DNA.