In most types of cells, DNA is used to store genetic information. DNA takes the shape of a double helix. Each strand of DNA consists of four genetic bases (nucleotides), cytosine, guanine, adenine, and thymine, joined together by a sugar- phosphate backbone. Cytosine can only form complementary base pairs with guanine, and adenine can only form complementary base pairs with thymine. Hydrogen bonds form between complementary base pair, while phosphodiester bonds form the sugar-phosphate backbone.

dna structure, beginner biology

The steps to semi-conservative DNA replication are:

1) The enzyme DNA helicase breaks the hydrogen bonds between complementary base pairs causing the double helix to unwind.

2) The original strand acts as a template for the new strand, where free-floating DNA nucleotides are attracted to their complementary bases.

3) The enzyme DNA polymerase catalyses the condensation reaction forming the sugar-phosphate backbone between the free-floating DNA nucleotides. Hydrogen bonds form between the bases of the original and new strands.

dna replication

The scientists Meselson and Stahl were able to prove that DNA replicates semi-conservatively through one of their experiments. The steps to their experiment are:

1) They grew bacteria in heavy nitrogen and a separate culture of bacteria in light nitrogen.

2) Then they took a sample of DNA from both cultures and centrifuged them. The heavy nitrogen DNA settled further down the test tube than the light nitrogen DNA. 

3) Afterwards, they took some of the bacteria growing in the heavy nitrogen and allowed them to grow in the light nitrogen.

4) The scientists then extracted the DNA from the new bacteria and centrifuged it. They found the this new DNA, which was grown in both heavy and light nitrogen, settled in the middle of the test tube between the light and heavy nitrogen DNA. This shows that one strand of the new DNA was made of heavy nitrogen, while the other was made of light nitrogen, confirming DNA replicates semi-conservatively. 

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