TOPIC 2B 4 how DNA works

What are the two mechanisms of replication ?

1. Conservative replication

2. Semiconservative replication

What is conservative replication ?

1. The original double helix is intact and

2. some way instructed the replication of a completely new double helix,

3. Made up of new materials.

What is semiconservative replication ?

1. The DNA is “unzipped” and new nucleotides aligned along each strand,

2. Forming a new double helix

3. With one strand which is original and another which is completely new and identical.

4. This was Watson and Crick’s hypothesis

What is the experimental evidence for semiconservative replication and how did the experiment took place ?

1. An experiment that Mathew Messlson and Franklin Stahl created proved semiconservative replication.

2. They grew several generations of Escherichia Coli ( E. Coli ) in a medium where the only source of nitrogen was radioactive isotope 15N from 15NH4Cl.

3. 15N is more denser than 14N.

4. The bacteria would use the heavy nitrogen to make materials, like DNA

5. After a few generations all the bacteria had DNA of 15N .

6. They then moved the bacteria to a medium containing normal 14 NH4Cl as the only nitrogen source, and measure the density of the cells produced.

Explain the prediction for the experiment proving semiconservative replication ?

1. If the cells reproduce via conservative replication,

2. Then half of the cells would contain DNA with only heavy nitrogen, containing the density of the original DNA,

3. and the other half would only contain the DNA of the new nitrogen, containing the density of the new strands.

4. If the cells reproduce via semiconservative replication,

5. Then after one replication cycle all of the DNA would have the same density

6. Half-way between that of 15N and 14N-containing DNA

What is DNA helicase used for ?

It unzips the two strands of the DNA.

What is DNA polymerase used for ?

It lines up the new nucleotides along the DNA template strand.

What is DNA ligase used for ?

It is used to catalyze the formation of phosphodiester bonds between the new nucleotides.