DNA is the molecule present in every cell of a living organism and is comprised of individual molecules known as nucleotides. The nucleotides are strung together to form very long strand of DNA that clump together and form chromosomes. Along these strings of nucleotides are “coding regions”, or regions of DNA which the cell uses to produce specific proteins. These proteins are then used to build every part of an organism. Many genes have multiple versions called alleles, which encode for different resulting proteins.
For example, it is generally accepted that a single gene encodes for the color of the iris. One particular allele that exists encodes a protein that results in brown pigmentation, while another particular allele encodes for a protein that results in no pigmentation (and the result is blue eyes). The presence of even a single brown allele will result in pigmentation, making brown eyes the dominant allele.
Each cell in the body of a human carries a complete set of DNA, while the germ cells (sperm, egg) carry only half of the DNA necessary for a viable human. During sexual reproduction, a complete set of DNA is passed onto the offspring from the combination of the germ cell DNA. Each of the parts from the parents come with one version of every gene. For instance, a father’s sperm cell may carry a brown allele while the mother’s egg carries a blue allele. The resulting child’s complete DNA would have two copies of the eye pigmentation gene: a brown allele and a blue allele, which result in brown eyes.
DNA makes a very suitable molecule for carrying the inheritable traits of organisms. Despite only four different nucleotides, a great deal of information can be stored in a very long string of nucleotides. This information can be very specific and provide the details for the cell to produce the necessary protein. This data carrying format allows the cell’s polymerase molecules to effectively copy the DNA with little to no errors. The errors, or mutations, that do occur can be corrected with cellular splicing and replacement mechanisms. Also, DNA is a very stable molecule. This allows it to be unraveled for replication and protein synthesis with little damage to the actual DNA strand. The negative charge along the strand, due mainly to the phosphate backbone, allows positively charged molecules to bind. This has allowed cells to evolve with special DNA binding molecules with positive charges. For instance, histones are positively charged proteins that bind DNA and help pack it into dense chromatin, the mass of DNA that forms chromosomes.
