Introduction and Key Definitions
Deoxyribonucleic acid (DNA) is the blueprint that contains the instructions for cells (and thus an organism) to function. This includes for reproduction, growth, maintenance, and all other functions that sustain life. This role of DNA is mediated through what is called the central dogma. In the basic sense, DNA codes for RNA that subsequently codes for proteins. An exception to this is the case of some RNA viruses called retroviruses. These produce copies of their RNA in the form of DNA using reverse transcriptase enzymes.
When thinking of the various terms that refer to genetic material, it is important to distinguish the primary definitions. A strand of DNA is made of building blocks called nucleotides (described later). Specific stretches of double-stranded DNA are genes. These code for specific characteristics of a cell, organ, or organism. Chromosomes, however, are packaged DNA found in the nucleus of a cell. Each chromosome consists of a single molecule of tightly coiled DNA complexed with proteins (histones). In addition to making DNA fit into a cell, chromosomes are crucial for ensuring the accurate copying and distribution of DNA during cell division.
DNA Structure
Purine and Pyrimidine Bases
There are four bases (nitrogen-based molecules) that form DNA.
Adenine
Guanine
Cytosine
Thymine
Adenine and Guanine are the purine bases of DNA. Purines consist of a six-membered, nitrogen-containing ring (a pyrimidine) bound to a five-membered, nitrogen-containing ring. Cytosine and thymine are the pyrimidine bases (the only ring present). Uracil of RNA is also a pyrimidine base. The formation of the nucleotide polymer results in a single strand.
Nucleotides and Nucleosides
The combination of a base with a sugar (via a glycosidic bond) is a nucleoside. The naming convention for this combination is as follows:
Adenosine
Guanosine
Thymidine
Cytidine
In DNA, the sugar is 2- deoxyribose (structure of ribose without an OH group). The addition of a phosphate group to this combination results in a nucleotide. The phosphate forms an ester linkage to the sugar (via the 5´carbon of the sugar). Nucleotides are the basic building blocks of nucleic acid. They join to form the polymer known as DNA. The formation of the nucleotide polymer is by covalent binding of the phosphate of one nucleotide to the sugar of the next nucleotide.
Examples of nucleotides are:
dATP = deoxyadenosine triphosphate
dCMP = deoxycytidine monophosphate
dGTP = deoxyguanosine triphosphate
dTDP = deoxythymidine diphosphate
DNA Helix
In living organisms, the primary form of DNA (when not in the process of replication) is in the double-stranded form. The two strands form a double helix. To form this helix, the bases of one strand join with the bases of the other. When one base joins with another, it forms a base pair. This base pairing is highly specific and occurs via hydrogen bonding. For instance, adenine and thymine form one type of base pair while cytosine and thymine form the other type.
The two DNA strands are antiparallel and twist together. The description of antiparallel means that the direction of one strand is from 5′ to 3′ while the other runs 3′ to 5′. Essentially, the double-stranded DNA structure is similar to a twisted ladder. In addition to the sequence, DNA function relies heavily on its structure. One example is the separation of the strands allowing one to serve as a template for construction of a new strand.