DNA (deoxyribonucleic acid) is found in most forms of life on this planet. DNA plays a critical role in the structure and function of nearly everything in a living organism and is especially important as a major factor in heredity. DNA is found on chromosomes in the nucleus of the cell.
Humans have 46 chromosomes (23 pairs)—22 pairs of autosomes and one pair of sex chromosomes.
DNA is made up of four nucleotide bases (adenine, cytosine, guanine, and thymine) and sugar and phosphate groups.
Replication is a prime function of DNA. DNA replicates itself during cell division (mitosis) and gamete production (meiosis).
Novel arrangements of genetic variation can occur during meiosis via recombination and crossing over.
Protein synthesis is a second function of DNA. DNA carries the core information for the construction of a polypeptide (chain of amino acids), which in turn becomes a protein.
The main information in the DNA is carried in the sequence of nucleotide bases. Segments of three bases are called triplets. Each triplet codes for an amino acid or gives a start-or-stop signal during translation.
The DNA code is transcribed onto mRNA for translation into a protein. The transcribed triplets are called codons on the RNA.
A segment of DNA that contains the code for a protein is called a gene. A gene is transcribed from the DNA to a strand of mRNA (messenger RNA), which then moves into the cytoplasm and, with the help of ribosomes and tRNA (transfer RNA), is translated into a polypeptide. The polypeptide then folds in on itself to become a protein.
Genes can have single effects, pleiotropic effects, polygenic effects, and effects that are a mix of all three.
Regulation is a third function of DNA. DNA regulates itself via self correction mechanisms and interactions with enzymes.
Different sequences of nucleotides at the same place on the DNA (a locus) are called alleles. Alleles are variant forms of a gene.
Gametes (sex cells) carry the genetic information from one generation to the next. Each gamete has one half of the full genetic complement. Therefore, each gamete has one allele per gene.
As much as 95% of the human DNA does not appear to have active genes on it. It is currently unclear what the function of this "noncoding" DNA is.
There is a complex nonlinear relationship between genes and behavior.
All of the alleles in a population make up its gene pool. This gene pool represents the total genetic variation available to that population and, thus, the pool of variation upon which evolution can act.
