Each letter in a DNA sequence is called a base , basepair , or nucleotide . Normally, DNA occurs as a double strand where each A is paired with a T and vice versa, and each C is paired with a G and vice versa. A genome is all of an organism's DNA sequence. Each nucleus in one of your cells contains its own copy of your genome, which is about 3 billion letters.
Replication is the process of duplicating the genome when a cell divides.
The genome of a warm-blooded animal is divided into isochores , which are regions of perhaps a few million basepairs in which the percent of C and G nucleotides is roughly constant. A gene is a region of the genome that contains the instructions for making one protein. The cell begins to make a protein by transcribing the gene into an RNA molecule that is essentially a copy of the gene. Part of the cell's method for identifying introns is the presence of GT and AG splice signals that always occur as the first and last dinucleotide of an intron. Each piece between consecutive introns is called an exon , and the RNA molecule with the introns removed is known as messenger RNA , or mRNA .