This indicates that the DNA was made up of both types of nitrogen isotopes (i.e. Again, they extracted their DNA and separated it using centrifugation.Īfter the first round of DNA replication, they saw just one band of DNA which was an intermediate weight between 14-N and 15-N. The scientists then took the bacteria that had been growing in N-15 (which now just contains heavy DNA) and grew them in the light isotope N-14. The DNA from the bacteria grown in N-15 separated at a higher density (a band lower down the test tube) compared to the DNA from the bacteria grown in N-14. As the bacteria grow and reproduce, they incorporate the nitrogen into their DNA.ĭNA was extracted from the bacteria and centrifuged to separate the DNA according to its weight. Two populations of bacteria were grown - one in a solution containing heavy nitrogen and the other in a solution containing light nitrogen. Here’s how they carried out the experiment in more detail: If DNA replicates conservatively, Meselson and Stahl knew that they’d see some DNA made of just heavy nitrogen with the rest made of only light nitrogen, but if it replicated semi-conservatively it would be a mixture of the two isotopes. They grew bacteria in the presence of the heavy nitrogen and any new DNA that the bacteria made would incorporate this isotope and so would weigh heavier.

To figure this out they used a heavy isotope of nitrogen (N-15) which has an extra neutron compared to the normal, lighter form of nitrogen (N-14). If DNA replicated conservatively, the original DNA strands would remain intact and the newly synthesised DNA would consist of two freshly-made strands. Before this, scientists were unsure whether DNA replication was conservative or semi-conservative. The evidence that DNA replication is semi-conservative comes from a pretty clever experiment carried out by Matthew Meselson and Franklin Stahl. DNA mutations can have detrimental effects to the organism, since an altered base sequence can change the sequence of amino acids in a protein, causing it to fold differently and possibly lose its function. A mistake results in a change to the DNA base sequence, which is known as a mutation. DNA polymerase has an accuracy rate of about 99%, which means that mistakes do occur every once in a while. If it detects a mismatch, it can ‘snip out’ the wrong nucleotide and replace it with the right one. To avoid this, DNA polymerase ‘proofreads’ the complementary strand as it moves along the DNA.

It is important that DNA polymerase accurately copies the template strand to avoid placing the wrong DNA nucleotide in the incorrect position. Two daughter DNA molecules are formed, each containing half of the original DNA molecule. One of the strands will act as a template for synthesis of the other strand.Ĭomplementary nucleotides will attach to the template strand by hydrogen bonding.ĭNA polymerase catalyses the formation of phosphodiester bonds between nucleotides, forming a complementary strand alongside the template parent strand. It takes place in the following stages:ĭNA helicase unwinds the double helix, breaking the hydrogen bonds between complementary base pairs to separate the strands. Since half of the DNA is preserved from the previous round of DNA replication, we describe the process as semi-conservative.

When DNA is replicated, the new DNA molecule is made up of one strand of the original DNA whereas the other strand is made of freshly made DNA. During cell division, cells need to make a complete copy of their genetic information.

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