DNA replication

When does DNA need to replicate? Why?

DNA has to replicate every time a cell divides so that both cells have identical copies of the entire genome.

What is the method called? Why?

The method of replication is called semi conservative replication, half(semi) of each new double helix is a strand that has come from the double helix of the original DNA. Conserved from the parent cell.

What is first replication step?

The enzyme DNA helicase “unwinds” the double helix by breaking the hydrogen bonds between the bases of two DNA strands.

What is the second step of replication?

Each original strand then acts as a template to build new strand. Free DNA nucleotides are attracted to the exposed bases on template strands, attract to correct base through complementary base pairing.

What is the third step of replication?

Free nucleotides are joined to new strand with condensation reactions catalysed by enzyme DNA polymerase, forming hydrogen bonds between bases.

What is the fourth step of replication?

Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

What did Watson and Crick identify?

That DNA is adapted to allow replication to occur efficiently, because hydrogen bonds can be easily broken to allow both strands to be a template. Also complementary base pairing means replication of new strands should be accurate as the correct bases can only pair each other.

How does DNA polymerase work?

Each DNA strand has a directional structure due to ends of each strand being either sugar attached to the 5th carbon (5’) end or a hydroxyl group attached to 3’ end. Polymerase only complimentary to 3’ end, so only add nucleotides in 3’ to 5’ direction. Therefore, new strand built is 5’ to 3’ as strands are antiparallel. One strand is continuously built, other is built in other direction in sections as DNA unwinds. Polymerase on opposite template has to detach and re-attach so it’s slow.

What was Meselson and Stahl’s hypothesis?

Meselson and Stahl, used the fact that nucleotides contain nitrogen so they used light N(14) and heavy N(15), making DNA samples of nucleotides with both isotopes separating their weight by centrifuging.

What was the experiment to prove that DNA replication is semi-conservative?

They grew 2 cultures of bacteria in nutrient broth that contained light N and heavy N. Heavy N put into broth containing only light N, so when replicated parent of original DNA would contain heavy nucleotides but to make new they would need to use light too.

What summary was made from the experiment for semi-conservative replication?

If semi-conservative, DNA would contain mixture of heavy and light DNA, one strand of each in the double helix.