Since James Watson and Francis Crick revealed the structure of DNA, it has been accepted as the molecule of heredity. Before their discovery, the scientific community retained some skepticism that DNA was up to the job, because it seemed too simple a molecule to perform the four necessary functions: replication, encoding, cell management and the ability to mutate. The unique structure of DNA allows it to fulfill all these functions.
Cells in a multicellular organism must duplicate many times, and even single cells have to duplicate when they divide. Whatever molecule carries genetic information must be capable of copying itself in an almost error-free manner. DNA is made of four bases, abbreviated A, C, G and T. DNA is a double-stranded molecule, where an A in one strand always matches with a T in another strand, and a C always matches with a G. If the two strands of DNA are separated and a new strand is built, it will use the pattern of the existing strand to construct an exact copy, because where there was an A across from the T in the original, there will be an A across from the T in the copy.
The work of each cell is done by proteins, so one of the roles of DNA is to build the right proteins for every cell. DNA fills this role by containing three-base sections -- called codons -- that direct the formation of proteins. In a long stretch of DNA, each codon contains the information that directs the assembly of one amino acid onto a protein. Different codons correspond to the assembly of another amino acid onto a protein, so a whole section of DNA with a given sequence of bases will build a specific protein.
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In multicellular organisms, a single fertilized cell, a zygote, divides and duplicates many times to make an entire living being. Each cell has exactly the same genetic material, but different cells develop in different fashions. That is, in a process called differentiation some cells build the right proteins to become liver cells, and others become skin cells, others stomach cells. In addition, cells must change the way they operate as conditions change. Your stomach cells, for instance, have to produce more digestive hormones and enzymes when food is present. DNA does this through signals that turn on and off the production of proteins involved in digestion. The same kind of thing happens as cells differentiate: Signals trigger the right levels of protein production to form the appropriate cell.
The Ability to Mutate
Evolution is the change in characteristics as generations of an organism are produced. Evolution happens on small scales within an organism -- such as changes in skin or hair color in humans -- and also on large scales -- such as the creation of the vast range of life on Earth from an early single-celled organism. That can only happen if the genetic molecule can change, can mutate. As DNA replicates to make egg and sperm cells, changes can creep in on several levels. One way is through single-point changes that add, subtract or change an existing sequence. Other changes happen when DNA molecules cross each other, switching the arrangement of genes on each of the two crossed strands of DNA.