Structure of DNA

DNA consists of a double-helix formed from a sugar-phosphate backbone and four different nucleotides, the order of which determines the genome of an organism.

Deoxyribonucleic acid (DNA) exists in every living organism and cell. It determines the characteristics of all living organisms, and is incredibly important to a variety of fields. By changing or introducing strands of DNA into cells, researchers and scientists can alter cell behavior and functionality.

DNA itself consists of a spiral (called a double-helix) made up of nucleotides and a phosphate backbone to hold it together. Genetic information is encoded by the four DNA nucleotides – adenine, thymine, cytosine, and guanine. It is important not to confuse DNA with RNA(formed by transcribing DNA), which can contain uracil. The order of the nucelotides determines the proteins produced by a cell, and this order is of great interest to both scientific fields and industry.

Perhaps the most important feature of DNA is that its contents, or sequence, are passed on from adult to offspring.  This unique passing of information enables the next generation to inherit beneficial phenotypes or physical traits (strength, size, color) but it also means a corrupt genotype will also be passed to offspring (mutations, deletions, insertions).

Regarding eukaryotes, a classification including plants and animals in which all organelles are membrane-bound, DNA is maintained inside of a membrane-bound nucleus.  DNA consists of a long string of nucleotides connected together through the interaction between the sugar group of one nucleotide and the phosphate of the next nucleotide.  To aid with strand stability, two complimentary anti-parallel strands bind together via hydrogen bonds, following base pairing rules that adenine binds to thymine, and guanine binds to cytosine.

Each strand of DNA is extremely long and is compacted into tight coils by histones.  These structures maintain organization of DNA strands and help regulate which genes are actively transcribed.  Each paired strand of DNA (double helix) is coiled into structures called chromosomes.  All genetic information in humans is protected and maintained on 23 pairs of chromosomes.

Within the cell nucleus, DNA forms a tight double helix, with two anti-parallel strands of DNA running opposite of each other and twisting to form its helical shape. Inside cells, the DNA helix further winds into “supercoiled” DNA by forming a complex with proteins called chromatin.  This serves several purposes, including condensing DNA into a smaller volume.  During replication, the chromatin is highly condensed and appears in an X-shape – forming a chromosome.

Function of DNA

The length and sequence of DNA is nearly identical within a single species, but is unique for each individual, and leads to genetic diversity within a species.  Genetic diversity occurs during meiosis and results in genetic recombination in which the offspring inherits genetic information from either parent.  The resultant genetic material is unique to the organism and the new DNA sequence represents a code of the organisms’ biological information. Clones, however, do have identical genetic information, but differ in the expression of the information. This is of particular interest to epigenetics, the study of how external factors can affect genetic information.

DNA has two main roles, the first is the maintenance and recombination of genetic code.  In the nucleus of somatic cells, which make up an organism, the number of DNA chromosomes for that organism is kept constant.  However, in germline cells, which are the sexual reproduction cells, the number of DNA chromosomes is maintained at half the usual number since the offspring receives half of its genetic material from each parent.  During somatic cell division, the quantity of DNA doubles in the inter-phase, or S phase, and is the pause between G1 and G2 of mitosis.  Each of the two sets of chromosomes, which are genetically identical to the old chromosomes, contains one of the old and one of the newly synthesized DNA strands as the cell splits into two daughter cells.

The other major role of DNA is its involvement in the formation of ribonucleic acid (RNA).  Through a process called transcription, an enzyme called RNA polymerase reads the DNA sequence and makes a copy called RNA.  The RNA strand, unlike DNA, leaves the nucleus and is translated into a protein, which is the workhorse molecule in the cell (e.g. enzymes, polymerases, structural and scaffolding functions, cell signaling, immune response).

Utility of DNA in Medicine

DNA provides extensive benefits in the laboratory and to therapeutic medicines.  Following well-established DNA isolation protocols, DNA can be isolated from specific tissues of animals or areas of plants, and from bacteria and DNA-containing viruses.  Upon isolation, the concentration of DNA can be measured, since the nucleotide bases on the DNA strands absorb ultraviolet light at a wave length near 260 nm.  The isolated DNA enables researchers to sequence the genetic code of the sample to look for mutations, insertions, deletions or amplifications of certain genes that may be the cause of disease in an organism.

With a full understanding of the genetic makeup of the organisms DNA, scientists can attempt to genetically correct any alterations by intracellular delivery of DNA molecules.  DNA can be introduced to the target cells via a process called transfection utilizing electroporation, a chemical reagent or by using liposomes that can fuse with the cell membrane.  Introducing DNA into a cell is a complicated process, requiring knowledge of gene promoters, screening techniques and transfection methods.  Most often, DNA is introduced into cells through plasmids, which are circular pieces of double-stranded DNA containing inherent vector genes for replication, the desired replacement gene insert, and a gene for antibiotic resistance or fluorescence.  The transfected DNA insert is either transiently expressed in the target cells or it integrates into the genome of the target cells where it will be endogenously expressed.  The resultant expression of the transfected DNA enables researchers to study the efficacy of the replacement therapy.