English Simple Dissecting Microscope (circa 1700s)

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DNA Video No. 1
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Deoxyribonucleic acid (DNA) is a complex organic molecule found in all prokaryotic and eukaryotic cells and in many viruses. DNA carries the chemical information needed to direct protein synthesis and cell replication.

A molecule of DNA consists of two strands of compounds, called nucleotides, linked together to form a chain. Each nucleotide consists of three chemical units: a sugar molecule called deoxyribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine (A), guanine (G), thymine (T), and cytosine (C). The nucleotide chains are arranged into a ladder-like structure that has been twisted into the shape of a winding staircase, called a double helix.

DNA was first discovered in 1869, but its role in genetic inheritance was not ascertained until 1943. In 1953 American biochemist James D. Watson and British biophysicist Francis Crick published the first model describing the structure of the DNA molecule. Their model proved to be so important for the understanding of protein synthesis, DNA replication, and mutation that they were awarded the 1962 Nobel Prize for their work.

In the laboratory, scientists usually investigate the physical and chemical properties of DNA in very dilute buffered aqueous solutions. However, in vivo, DNA exists in domains where the localized concentrations are very high, often exceeding 400-600 milligrams per milliliter (mg/ml) or 70 percent weight/volume. As the aqueous concentration of DNA is slowly increased, through evaporation of the aqueous solvent or by dialysis, the macromolecular solution undergoes spontaneous phase transitions to form at least three distinct liquid crystalline phases, which are termed lyotropic phase transitions. This is due to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions.


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