It is very vital for cells to have the ability to memorize, retrieve, and translate the genetic instructions that are required to initiate and sustain the existence of a living entity. This genetic material is passed down from one generation of an organism to the next through the reproductive cells of the organism, as well as during the process of cell division, when one cell gives rise to its daughter cells. Every living cell contains a set of instructions that are a storage location for the information-containing components known as genes. Genes are what determine the characteristics of a species as a whole as well as the individuals that make up that species.

When genetics was first formed as an area of study at the beginning of the 20th century, it immediately piqued the curiosity of researchers who were interested in the chemical composition of genes. The information contained in a multicellular creature's genes is copied and passed down from parent cell to daughter cell millions of times over the course of its lifetime, and despite this, it maintains its fundamental integrity throughout the process. What kind of molecule is it that can replicate so accurately and practically endlessly while at the same time being able to exert such fine control that it directs the formation of multicellular organisms as well as the day-to-day activities of each cell? In what different kinds of instructions does the genetic information specialize itself? And how can the enormous amount of information that is required for the development and maintenance of an organism fit inside the constraints of a single cell?

The idea that chromosomes, which are threadlike structures in the nucleus of a eukaryotic cell that become visible by light microscopy as the cell begins to divide, are where the hereditary information is stored was first recognized as early as the late nineteenth century as a result of painstaking observations of cells and embryos. Chromosomes are structures that become visible by light microscopy as the cell begins to divide. After some time, when biochemical research became feasible, it was found out that chromosomes are primarily made up of protein and deoxyribonucleic acid (DNA), with both components being present in nearly equal proportions. This discovery was made possible by the fact that DNA could be isolated from cells. For a very long time, people thought that DNA was nothing more than a structural component. The other key development that occurred in the 1940s was the revelation that DNA is very certainly the carrier of genetic information. This was accomplished in the decade. Research on the inheritance of bacterial traits brought about a significant advancement in our understanding of cells.

However, in the beginning of the 1950s, it was still a complete mystery as to how DNA instructions could specify proteins and how this information could be copied so that it could be sent from cell to cell. This enigma persisted throughout the 1950s. The question was resolved almost instantly in 1953 after James Watson and Francis Crick deduced the mechanism behind how DNA replicates from their model of the structure of DNA. When the double-helical structure of DNA was discovered, an immediate solution was found to the problem of how the information contained in this molecule could be copied, also known as reproduced. In addition to this, it provided the first suggestions as to how the organization of a DNA molecule's subunits may be used to encode instructions for the production of proteins. Because the concept that DNA is the genetic material is so essential to modern biological theory, it is difficult to fathom the enormous intellectual hole that this game-changing finding filled. However, this void was filled by the revelation that DNA is the genetic material.

This chapter will begin with an explanation of how the DNA molecule is organized. Despite the fact that it has a very straightforward chemical composition, DNA is an excellent candidate for its role as the fundamental component of heredity. The subsequent step in this process involves analyzing how the numerous proteins that can be present in chromosomes work together to package and organize this DNA. Because the chromosomes need to be copied and suitably distributed between the two daughter cells after each round of cell division, the packing of the chromosomes needs to be done in an orderly manner. In addition, it must be able to allow access to the chromosomal DNA for both the specialized proteins that regulate the expression of its various genes and the enzymes that repair any damage that may have been done to the DNA.

In the past 20 years, there has been a revolutionary change in our ability to pinpoint the precise arrangement of the subunits that make up DNA molecules. As a consequence of this, we are currently in possession of the DNA sequences for thousands of distinct animal species, in addition to the 3.2 billion nucleotide pairs that are essential for the development of an adult human from a fertilized egg. The discussion of the remarkable insights into the process of evolution that have emerged from extensive investigations of these sequences is presented as the concluding section of this chapter.

In the 1940s, biologists were having a hard time comprehending how DNA could possibly function as the genetic material. The molecule appeared to be overly simple; it was a lengthy polymer that was composed of just four distinct types of nucleotide subunits that were chemically similar to one another. At the beginning of the 1950s, DNA was analyzed using X-ray diffraction, which is a technique for determining the three-dimensional atomic structure of a molecule. This technique was used to examine DNA. Initial results from x-ray diffraction analysis suggest that the double helix structure of DNA is formed by two strands of the polymer coiled together. The fact that DNA is made up of two strands, or strands, was one of the most important pieces of evidence that led to the development of the Watson-Crick model for the structure of DNA. This model was first proposed in 1953 and immediately made DNA's potential for replication and information storage apparent.

A molecule of DNA is constructed from two very long polynucleotide chains that each include four distinct types of nucleotide subunits. Each of these chains is referred to as a "DNA chain" or "DNA strand" depending on the context. The chains are arranged in opposition to one another, and their cohesion is maintained by the formation of hydrogen bonds between the nucleotide bases. During the formation of a nucleotide, a nitrogen-containing base and one or more phosphate groups are joined to a sugar consisting of five carbons. The nucleotides that make up DNA can have the bases adenine (A), cytosine (C), guanine (G), or thymine (T). The sugar in DNA is deoxyribose, which is linked to a single phosphate group (thus the name deoxyribonucleic acid). Covalent bonds are created between the nucleotides in a chain by sugars and phosphates, which alternately make up the "back bone" of sugar-phosphate-sugar-phosphate. Because only the base is distinct in each of the four types of nucleotide subunits, each polynucleotide chain of DNA is analogous to a sugar-phosphate necklace (the backbone), from which dangle the four different types of beads. This is due to the fact that DNA is made up of polynucleotides (the bases A, C, G, and T). It is common practice to use the letters A, C, G, and T to represent both the four bases and the four full nucleotides. This refers to the bases along with the sugar and phosphate groups that are attached to them.

The double helix, the three-dimensional structure of DNA, is composed of two polynucleotide chains, each of which has distinct chemical and structural properties. Because hydrogen bonds are formed between the bases on the various strands, the double helix has all of the bases on the inside, while the sugar-phosphate backbones are on the outside. This arrangement is due to the fact that the two chains are connected by the bases. Each time, a smaller base with only one ring, called a pyrimidine, is joined with a larger base with two rings. The bases A and T, as well as G and C, remain constant partners.

Because these base pairs are complementary to one another, the base pairs that make up the interior of the double helix can be arranged in the configuration that is most favorable from an energetic point of view. Due to the fact that each base pair in this arrangement has a width that is approximately the same, the distance that exists between the sugar-phosphate backbones of the DNA molecule is preserved. The two sugar-phosphate backbones wind around each other to form a right-handed double helix. In order to maximize the efficiency of the base-pair packing, one full turn occurs every ten base pairs.

Only in the case where the two strands that make up the double helix are antiparallel—that is, only in the case where the polarity of one strand is oriented in the opposite direction from that of the other strand—are the members of each base pair able to fit together within the structure.

Because DNA must fulfill certain structural and base-pairing requirements, each strand of a DNA molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand. This exact complementarity is what allows DNA to store genetic information.