In order to replicate a cell must create a fully copy of its DNA for the new cell. The replication of DNA can be broken down into three key steps:
1 – The strands that make up the double helix of DNA are ‘unzipped.’ The hydrogen bonds between the bases break and leave two single strands of DNA. These strands act as template strands.
2 – Free mononucleotides are attracted to each of the template strands, and line up by complementary base pairing. The bases of each nucleotide pair in a specific way:
- Adenine pairs with Thymine
- Guanine pairs with Cytosine
3 – An enzyme called DNA polymerase joins up the mononucleotides and hydrogen bonds form between the bases. The two new DNA molecules twist to form the double helix structure of DNA and we are left with two daughter molecules of DNA – both identical to the original molecule.
NOTE: Other enzymes, such as DNA primase and DNA helicase, are involved in the process of DNA replication.
DNA replication is semi-conservative.
Semi-Conservative Replication
Semi-conservative replication means that each new DNA molecule contains one strand from the original DNA molecule and one new strand. Two scientists (Melson and Stahl) preformed an experiment to prove that DNA replication is semi-conservative.
Melson and Stahl’s Experiment
DNA contains nitrogen atoms. To prove DNA replication is semi conservative Nelson and Stahl grew two samples of bacteria with different nutrients. Two different isotopes of nitrogen were used in the nutrients to grow the bacteria.
The isotopes of nitrogen used were 14N and 15N.15N is heavier than 14N.
The bacteria used the nitrogen in the nutrients to produce mononucleotides as they reproduced. This meant that eventually one sample of bacteria had light (14N) nitrogen in their DNA and the other sample had heavy (15N) nitrogen in their DNA.
DNA was extracted from both samples and a centrifuge was used to spin it. The DNA containing heavy nitrogen settled lower in the centrifuge tube than the DNA containing light nitrogen.
They then gave the bacteria containing heavy nitrogen nutrients containing light nitrogen. They were then left, allowing the DNA to replicate once. Another sample of this DNA was then taken to spin in the centrifuge.
If DNA replication was conservative one of the molecules would have two of the original strands containing heavy nitrogen and the other would have two new strands containing light nitrogen. This means one of the strands would be lower than the other in the centrifuge tube.
Because DNA replication is semi-conservative each of the DNA molecules had an original strand containing heavy nitrogen and a new strand contains light nitrogen. Therefore, when spun in the centrifuge the DNA molecules both settled in the middle of where ‘light’ and ‘heavy’ DNA settle:
This proved that DNA replication is semi-conservative.
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