- 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.