Deoxyribonucleic acid (DNA) is the chemical inside the nucleus of a cell that holds the genetic instructions to create living organisms. DNA has three known functions:
- DNA replicates itself
- DNA codes for RNA, which in turn codes for proteins (the primary building blocks of the cell, the tissues, and the body)
- DNA regulates gene expression, allowing for
- Cell growth
- Cell differentiation
- Cell replication
- Programmed cell death
The structure of DNA is complementary. It is built from deoxyribose (a sugar), phosphate groups and four nucleotides or bases: adenine, cytosine, guanine, and thymine (mercifully abbreviated to A, C, G, and T). Adenine can only bind with thymine, and cytosine can only bind with guanine, producing the complementary structure. The 3-dimensonal structure of DNA is like a ladder that has been twisted around its vertical axis: the deoxyribose and phosphate form the 'rails' of the ladder, while pairs of A & T and C & G form the 'rungs'. The advantage of the complementary structure is simply that the DNA 'ladder' can split with each half binding to complementary nucleotides in order to make two perfect copies of the original DNA.
If all the DNA in a single human cell were unraveled and stretched out into a straight line, it would measure about 2 meters (6 feet). Given the 100 trillion or so cells in your body, if all the DNA in all your cells were stretched out in a straight line, it would reach to the sun and back a thousand times.
Genes are those sections of the DNA that code for ribonucleic acid, or RNA. The complementary binding of nucleotides to one another allows DNA to code precisely. RNA delivers DNA's genetic message to the cell cytoplasm, where proteins are made. RNA is structurally similar to DNA except:
- RNA is single stranded;
- RNA uses the nucleotide uracil (U) in the place of thymine;
- RNA's 3-nucleotide codons (think of them as "3-letter words") code directly for specific amino acids, allowing for the synthesis of proteins in ribosomes.
Heredity is dependent on the genes found within the entire genome. The average gene is about 3,000 nucleotides long, but this can vary considerably. Surprisingly, only about 3% of the human genome is actually used by and for human physiology.
