DNA Molecule

DNA (deoxyribonucleic acid) is a complex molecule that expresses, stores, and replicates all of the genetic information that is contained in eukaryotic cells. This genetic information is responsible for all the characteristics expressed in a particular species (e.g., color, size, and gender). In addition to physical characteristics, information pertaining to behavior and intelligence is also stored in the DNA molecule.

The DNA molecule is composed of two long strands (which are sugar-phosphate) and repeating units called nucleotides. Each of these nucleotides has three components: a5-carbon sugar (which is deoxyribose in DNA vs. ribose in RNA), a phosphate group, and a nitrogenous base. Four nitrogenous bases make up the DNA molecule: two purines, adenine and guanine, which are composed of two nitrogenous rings, and two pyrimidines, thymine and cytosine, which are composed of one nitrogenous ring.

By forming hydrogen bonds, adenine pairs with thymine, and guanine pairs with cytosine. These pairs are known as base pairs and are responsible for the structure of the DNA molecule. The two adjacent polynucleotides are wound into two spiral-shaped strands called a double helix.

These long stands of the DNA molecule are organized by various proteins into individual units called chromosomes.Chromosomes are located in the nucleus of all eukaryotic cells. There are 46 chromosomes in each human cell, except in the sex cells (i.e.,the eggs and the sperm), which have 23 chromosomes. On the chromosomes, there are regions of DNA, called genes, which are responsible for individual inheritable characteristics. A gene carries the actual biological information and can be as short as 999 base pairs to as long as several hundred thousand base pairs. It was estimated in1996 that 30,000 to 40,000 genes exist in the genetic make up of Homo sapiens. However, upon the completion of the Human Genome Project in 2003, it is now postulated that only 20,000 to25,000 genes exist.

The history of the discovery of the DNA molecule is an interesting one. It first began with the idea that physical traits of species are actually inherited in a predictable pattern from the parents to offspring. However, it was not until the middle of the 20th century that scientists began to actually identify the mechanism of inheritance and its molecular basis.

Gregor Mendel, a mathematician, known as “the father of genetics,” conducted many scientific experiments involving pea plants, which he began in 1857. The results of his experiments illustrated that pea plants distributed characteristics to their offspring in a mathematically predictable pattern. Mendel postulated that these characteristics (e.g., height of the plant) were inherited by the parent plant. However, at this time, the term character was used to define a heritable feature, such as the color of a flower. These characters vary among individuals. Each variant of a character is called a trait, such as purple or white. Even though he could not physically prove the biological foundation of this phenomenon, his work dramatically increased interest in the study of genetics.

In 1928, Frederick Griffith hypothesized that a molecule responsible for inheritance must exist. His work involved experiments using mice and the bacteria Streptococcus pneumonia. First, a virulent strain of S. pneumonia was injected into a mouse, and the mouse died. Second, a nonvirulent strain of S. pneumonia was injected into a mouse, and the mouse did not die. The next phase of his experiments involved heating up the virulent strain to denature and kill it; then, this killed strain was injected into a mouse, and the mouse lived. Finally, he injected a mouse with nonvirulent S. pneumonia that had not been heated up together with a virulent S.pneumonia that had been heated up, and the mouse died. Griffith postulated the killed virulent bacteria had passed on the virulent characteristics to the nonvirulent strand to make it virulent. He called the passing on of the inheritance molecule “transformation.”

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