DNA, or deoxyribonucleic acid, is a long-chain molecule that plays a central role in life on earth. The information encoded in strands of DNA controls the genetic makeup of organisms. The DNA molecule has a backbone of sugars and phosphate groups off of which hang simpler units called nucleotide bases. DNA contains only four bases, called A, T, C, and G. The sequence of the nucleotides along the backbone encodes genetic information. The four roles DNA plays are replication, encoding information, mutation/recombination, and gene expression.
DNA exists in a double-helical arrangement, in which each base along one strand binds to a complementary base on the other strand. T’s can only bind to A’s and C’s only to G’s. Bacteria and mitochondria may also have single-strand DNA arranged in a ring. When a cell divides, the chromosomes containing the DNA strands must replicate, or make copies, of themselves so that both daughter cells receive the full set of genetic material. During replication, the DNA double helix unwinds, allowing each strand to act as a template for a newly synthesized complementary strand that forms a new double helix. The enzyme DNA polymerase assists in the process.
The base sequences of A, T, C and G along a DNA strand are organized into units called genes. An adjacent trio of bases, called a codon, specifies a particular amino acid. Therefore, the sequence of bases in genes determines the sequence of amino acids in proteins, which are the biochemical units of a cell’s structure and function. RNA, a chemical similar to DNA, is an intermediary in protein synthesis. The cell first transcribes genes onto segments of RNA using the base-pairing logic that holds the double helix together. However, RNA substitutes the base of U for that of T. The cell later translates the RNA strands into proteins, three bases at a time.
Mutation and Recombination
DNA plays a role in the evolution of a species. Chromosomal DNA helices don’t usually interact with each other. However, through the process of genetic recombination, segments of different chromosomes swap places with each other, creating new sequences of genetic material. If changes occur to the DNA sequences of sex cells, the changes can be inherited by the next generation. The new sequences might produce new proteins, some of which are beneficial to the organism. In this way, the characteristics of the organism might evolve over time. Natural selection of beneficial traits can produce changes that make an organism more fit for survival and reproduction. DNA can also repair itself through recombination. A mutation happens when an “illegal” base pairing occurs — for example, when an A lines up opposite a G instead of opposite a T. An inheritable mutation must occur in the chromosomes of the sex cells. Mutations and recombinations can be beneficial but can also create genetic diseases and malformed offspring.
Each cell contains a full complement of genes, yet cells from different tissues and organs look and behave differently. The reason is that only some of the DNA of each cell is used to make proteins. DNA plays a role as a traffic cop for the types of proteins a cell will make. It does this through interactions with proteins in the cells that cause only certain genes to express themselves. This is how a single fertilized egg cell differentiates into the many types of cells, tissues and organs found in complex organisms. The DNA can respond to the need for a particular protein by exposing the appropriate genes for transcription while keeping other genes inactive.