Replication of DNAObjectives:

1. Summarize the roles of the different enzymes involved in replication of DNA.2. Explain how the leading and lagging strands are replicated differently

Semiconservative Replication

• When Watson and Crick proposed the model of the structure of DNA they also proposed a method of its replication, Semiconservative.

Aspects of Semiconservative Replication

1. Original strands separate, and serve as template strands.

2. These template strands copy themselves forming two molecules of DNA

3. Each DNA molecule has one strand of parental DNA and one strand of new DNA

3 stages of the process of Semiconservative replication

UnwindingBase pairing

Joining of the strands

Unwinding

The Double Helix of DNA must be unwound in order to allow base pairing (copying) of the DNA molecule.

An enzyme called Helicase unwinds the DNA molecule.

Helicase does this by breaking the weak hydrogen bonds between the two strand of DNA.

The Enzyme Helicase

http://www.youtube.com/watch?v=UWNhwceMjfk

Base Pairing The enzyme DNA polymerase attaches

appropriate nucleotides to the new DNA strands

The nucleotides are added to the 3’ so we say that the DNA synthesis moves in the 5’ to 3’ direction.

Okazaki Fragmentsand the leading and lagging strand

The two strands are made in a different manner.One strand is called the leading strand. This strand

is built continuously from the 5’ to 3’ ends.But, since the strands are antiparallel, the lagging

strand can not be built continuously.This strand is built into small segments, called

Okazaki fragments.The fragments are later connected by the enzyme

DNA ligase.

http://www.youtube.com/watch?v=teV62zrm2P0

Joining the Strands

• When the DNA molecule is finished replicating the enzymes release the strands and they reform two double helix’s, Each with a parental template strand, and with a new strand from replication.

Comparing DNA Replication betweenProkaryotes and Eukaryotes

• In Prokaryotes the circular DNA strand is opened at one origin of replication. In Eukaryotes, with up to one million bases pairs to copy there are multiple origins of replication.

• Both replicate in two directions, that is to say they are both semiconservative (one parent strand and one new strand).