Genes and DNA | DNA Replication | Transcription | Translation
The DNA double helix transmits genetic information from generation
to generation and is the repository of information required to guide
an organism's development and interaction with the environment.
The role of DNA in storing and transferring hereditary information
depends on the innate properties of its four constituent bases.
There are two purine bases, adenine (A) and guanine (G), and two
pyrimidine bases, cytosine (C) and thymine (T). Within the DNA double
helix, A is complementary to T, and G is complementary to C. Each
block of DNA that codes for a single RNA or protein is called a gene, and
the entire set of genes in a cell, organelle, or virus forms its genome. Cells and
organelles may contain more than one copy of their genome. There
are 46 chromosomes in a typical human cell; when "unraveled," the
total DNA of a single cell is approximately 1 m in length.
The 46 human chromosomes consist of 22 pairs of autosomes
and 2 sex chromosomes, either XX for females or XY for males. Such
a large amount of genetic material is effectively packaged into
a cell nucleus, which is also the site of DNA replication and transcription. Only
a small percentage of chromosomal DNA in the human genome is responsible
for encoding the genes that act as a template for RNA strands; there
are approximately 20,000–25,000 genes total, of which about
10,000–15,000 genes are expressed in any individual cell.