DNA replication

In a nutshell

DNA replication must occur before a cell divides and it does so through semi-conservative replication. This method uses various enzymes to create DNA molecules that have one original parent strand and one new strand. This summary explains semi-conservative replication and the evidence that Meselson and Stahl collected to prove this method.

DNA replication

Prior to nuclear division, DNA replication must occur to ensure that all new cells contain the correct genetic information. DNA replication occurs during the interphase stage of the cell cycle. It is described as semi-conservative as each DNA molecule contains one original parent strand and one new strand. The process is described below.

Biology; Biological molecules; KS5 Year 12; DNA replication — 1. Free nucleotides, 2. Original parent strands, 3. Newly synthesised strands

A.	This is the parent DNA that is going to be replicated.
B.	An enzyme called DNA helicase unwinds the DNA double helix from one end. It does this by breaking the hydrogen bonds between the complementary bases.
C.	Free nucleotides carrying complementary bases line up alongside the parent DNA strands. Hydrogen bonds form between the bases on the free nucleotides and the original parent DNA strands.
D.	An enzyme called DNA polymerase joins the free nucleotides together through condensation reactions to form the sugar-phosphate backbone. This is now the complementary strand.
E.	An enzyme called DNA ligase will close any gap in the strand that may have been missed.

DNA replication will make two DNA double helices that are identical to one another and identical to the original parent DNA.

Meselson and Stahl

There were two main theories of DNA replication. These were the conservative method and the semi-conservative method. These are compared below.

Conservative	Semi-conservative

One DNA molecule remains intact and is made from the original parent DNA (1.). The second DNA molecule is made up of new free nucleotides (2.).	Both DNA molecules that are produced will contain one parent strand (1.) and one new strand (2.).

Meselson and Stahl provided evidence that led to the semi-conservative method being the widely accepted method. They used E. coli bacteria and different nitrogen isotopes. ${^1}{^4}N$ is a lighter nitrogen isotope whereas ${^1}{^5}N$ is a heavier nitrogen isotope. Some bacteria were grown on a medium containing ${^1}{^4}N$ for many generations so all of their DNA contained ${^1}{^4}N$ and another group of bacteria were grown on a medium containing ${^1}{^5}N$ for many generations so all of their DNA contained ${^1}{^5}N$ .

The DNA was extracted and centrifuged in a special density gradient. Heavier DNA (containing only ${^1}{^5}N$ ) would sink to the bottom of the centrifuge tube, lighter DNA (containing only ${^1}{^4}N$ ) would collect nearer the top of the centrifuge tube and medium DNA (containing a bases with a mix of both nitrogen isotopes) would form a band in the middle of the tube.

Biology; Biological molecules; KS5 Year 12; DNA replication — 1. Bacteria grown in light nitrogen isotope, 2. Bacteria grown in heavy nitrogen isotope, 3. Bacteria transferred to light medium then DNA extracted after one generation, 4. DNA extracted after two generations, 5. DNA extracted after three generations.

The bacteria grown on the heavier ${^1}{^5}N$ medium were then transferred to the lighter ${^1}{^4}N$ . They were allowed to replicate once and then their DNA was extracted and centrifuged as before. The results showed a band at the middle of the tube which meant that the DNA molecule contained both ${^1}{^4}N$ and ${^1}{^5}N$ .

The bacteria were allowed to replicate again and another DNA sample was extracted and centrifuged. The results showed one light band and one medium band. This meant that one of the DNA molecules contained only new ${^1}{^4}N$ and the other DNA molecule contained a mixture of both isotopes. This proved the semi-conservative hypothesis.