DNA ReplicationLife Science: Molecular

- James Watson and Francis Crick

“It has not escaped our notice that the pairing we have postulated immediately suggests a possible

copying mechanism for the genetic material.”

DNA Replication

n Replication is the process of copying the DNA.

n Before a cell can divide, the DNA must be copied so that each cell receives its own set of instructions.

Chargaff’s Rules

n Erwin Chargaff discovered...- DNA varied from one species to another in

particular in the relative amounts of A,C,T,G. - In any DNA the amount of A was the same as the

amount of T, and the amount of C was the same as the amount of G.

n Two complete DNA strands can be created from a single DNA molecule by matching complementary bases.

Complementary Base Pairing

3'

5' 3'

3' 5'

Replication Enzymes

n Replication requires enzymesn Topoisomerase unwinds the helixn Helicase separates basesn DNA Polymerase copies each strandn Ligase joins the lagging strand

n The two strands are antiparallel, running in opposite directions.

n DNA is read and copied from the 5’ end to the 3’ end.

Complementary Base Pairing

DNA Replication is Semi-Conservative

In semi-conservative replication DNA one old strand is always conserved in the new molecule.

Separating the Strands

n Strong covalent bonds hold the phosphate-sugar backbone together.

n Bases are held together with relatively weak hydrogen bonds.

n Helicase separates the two strands at the bases, so replication can occur.

Sugar-phosphate backbone

Weak hydrogen bonds

Copying the Strands

n DNA polymerase assembles new strands by matching unpaired complementary bases.

n Adenine always pairs with Thymine Cytosine always pairs with Guanine

Okazaki Fragments

n Okazaki fragments are short fragments that occur on the lagging strand during DNA replication.

DNA Replication

Okazaki Fragments

Okazaki Fragments

Which strand is leading? Which is lagging?