DNA was found in cells only at the end of the 19th century, but Mendel and other genetics of that period knew nothing of DNA’s role in hereditary when they carried out their investigations. In the 1930s it was established experimentally that DNA was the main molecule of inheritance. Scientists were interested in chromosomes, which carried genes. In the course of their studying, it was found out that chromosomes consisted of two types of chemicals: DNA and protein. And in the middle of the 20th century, it became obvious that DNA functions as hereditary material.
Later the scientists made an effort to establish the structure of DNA molecules. They already knew a lot about DNA, but one thing they couldn’t understand was the arrangement of atoms that gave DNA its unique properties—the capacity to store genetic information, copy it, and pass it from generation to generation (Campbell, Reece & Simon, 2007). The task was to find out how the structure of DNA can be responsible for the transmission of inherited information.
DNA is a nucleic acid, which consists of long chains (polymers) of chemical units (monomers) called nucleotides (Campbell, Reece & Simon, 2007; Saenger, 1984). As a result of the combination of the nucleotide, a polynucleotide is formed. There are four different types of nucleotides (abbreviated A, C, T, and G) that make up DNA (Campbell, Reece & Simon, 2007). They can be arranged in DNA in different sequences, forming a great number of combinations.
The nucleotides join to one another by covalent bonds between the sugar of one nucleotide and the phosphate of the next (Campbell, Reece & Simon, 2007). As a result of such a connection sugar-phosphate backbone is formed. It should be noted that each nucleotide consists of three components: a nitrogenous base, a sugar, and a phosphate group. The phosphate group, with a phosphorus atom (P) at its center, is the source of the acid in nucleic acid.
The sugar has five carbon atoms: four in its ring and one extending above the ring. The ring also includes an oxygen atom. The sugar is called deoxyribose because it is missing an oxygen atom. The name DNA is deciphered as deoxyribonucleic acid. It is situated in the nucleus of eukaryotic cells. Each nitrogenous base has a ring of nitrogen and carbon atoms with various functional groups attached and is basic (Campbell, Reece & Simon, 2007).
The four DNA nucleotides differ only in their nitrogenous bases. Thymine (T) and cytosine (C) are single-ring structures, unlike adenine (A) and guanine (G), which are double-ringed structures.
The physical structure of DNA was determined by James D. Watson and Francis Crick while studying an X-ray crystallographic photograph of DNA. They discovered that DNA had the shape of the helix (spiral). They concluded that the diameter of the spiral was permanent. They also suggested that it consisted of two polynucleotide strands based on the thickness of the spiral – double spiral, so to say.
Watson and Crick managed to construct a double spiral not at once but only when they realized that the four kinds of nitrogenous bases might pair in a particular way. The principle is the following: a double-ringed base on one strand must always be paired with a single-ringed base on the opposite strand. Moreover, each base has chemical side groups that can best form hydrogen bonds with one appropriate partner. Adenine (A) best forms hydrogen bonds with thymine (T), and guanine (G) with cytosine (C). It may be said that A is “complementary” to T, and G to C. The sequence of nucleotides in the DNA strand can be different and vary in a wide range. Thus, the structure of the DNA molecule explains the reproduction and inheritance of all living eukaryotic organisms. In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize for their work.
During the process of reproduction, genetic information must somehow be transmitted from one organism to another. This can be done only by copying the information. It was suggested that DNA replicates by pattern mechanism and each DNA strain serves as a pattern to transmit information to another strand. If the sequence of bases in one strand of the double spiral is known, the sequence of bases in the other strand can be easily determined by using the base-pairing rules: A pairs with T (and T with A), and G pairs with C (and C with G). For example, if one polynucleotide has the sequence CTGA, then the complementary polynucleotide in that DNA molecule must have the sequence GACT.
However, the mechanism of DNA replication seems quite simple; it is very difficult. In this process several enzymes take part. Those enzymes, which form the covalent bonds between the nucleotides of the forming DNA strain, are called DNA polymerases. The process of DNA replication goes on very fast and at the same time very accurate. Along with taking part in DNA replication, DNA polymerases also participate in repairing damaged DNA. There are a lot of factors that can bring DNA to damage.
DNA replication begins at definite sites on a double spiral, called the origins of replication. This process occurs in two directions, forming replication “bubbles”. The DNA molecule has a lot of points where the replication can begin simultaneously, speeding the process. At the end of replication, all the bubbles unite, resulting in the formation of two double-stranded new DNA molecules.
The process that produces haploid daughter cells in diploid organisms is called meiosis. It consists of two divisions, called meiosis I and meiosis II. As a result of this process, each of the four neogenic daughter cells has a haploid set of chromosomes.
The first who scientifically analyzed patterns of inheritance was Gregor Mendel. In the course of his experiments with garden peas, Mendel worked out the basic laws of genetics. He discovered that the inheritance of a large number of genetic features complies with simple rules. According to him “the heritable factors (genes) retain their individual identities generation after generation, no matter how they are mixed up or temporarily masked” (Campbell, Reece & Simon, 2007).
Reference List
Campbell, N.A., Reece, J.B., & Simon, E.J. (2007). Essential biology with physiology. [2nd Ed.] San Francisco: Pearson/Benjamin Cummings.
Saenger, W. (1984). Principles of Nucleic Acid Structure. New York: Springer-Verlag.