DNA is the hereditary material that is passed from one generation to another. Although today it is very well known, DNA was once a very mysterious concept among biologists. If you could ask any expert in the biological field during the 1950's about what held our genetic information, you would most like to have protein as you answer. This is because proteins are highly complex and diverse, and they are in charge of very essential activities in the life of the cell. However, the old biologists weren't completely wrong! Proteins do play a special role in heredity. They have a supporting role in looking after the DNA.
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The very first experiments to demonstrate that molecules can transfer genetic information from one organism to another were done by Frederick Griffith, in 1928, while he was working with bacteria, which causes a variety of infections in humans and a deadly pneumonia in mice. During his experiments, Griffith studied two strains of the bacterium: one was a virulent strain that caused death in mice, and the other one a nonvirulent strain that allowed injected mice to survive. He them mix dead virulent strains with nonvirulents and observe something odd: some nonvirulent strains became virulent. Griffith came to the conclusion that some type of molecule in the debris carried the genetic information for virulence, however he did not identify the molecule.
This was cleared in 1994 by Oswald Avery, Colin Macleod, and Maclyn McCarty, when they proved that the molecule that transformed the nonvirulent cells into virulent cells was DNA. They realized that by destroying proteins and RNA during the preparation of the experiment done by Griffith, and observing the same results. Nevertheless, when they destroyed any trace of DNA, the transforming ability was lost.
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DNA serves as the genetic material due to its uniqueness among cellular molecules regarding the ability to replicate. The copying process, is known as replication, and it allows the genetic information from one DNA molecule to be copied into that of another DNA molecule, thus passing genetic information from cell to cell, and from offspring to offspring. Nonetheless, there are some error in this process, and it leads to mutations. Mutations can be good or bad, once it is essential for evolution.
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DNA directs the activities in the cell and guides the development of an organism, however, these effects of DNA are indirect. The most key molecule in cells and in the development of the organism is proteins. DNA specify the sequence of amino acid subunits of which forms protein and determines its properties and function.
DNA acts trough RNA, with the process of transcription (The first step of gene expression = formation of a product). We say transcription because DNA uses the same language as RNA. Then, translation comes along and RNA will be used to regulate the sequence of amino acids in a protein.
Gene expression does not happen all the time, in fact some genes are expressed sometimes and sometimes not. The process of transcription and translation occurs in the the cytoplasm of prokaryotes cells. However, in eukaryotic cells, they occur in different places, so there will allow additional levels of gene regulation that are not possible in prokaryotic cells/
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