The human genome has 3 billion base pairs (equivalent to 6 billion bases total).  The genome is divided into 22 regular (autosomal) chromosomes, two different kinds of sex chromosomes (X and Y), and a tiny amount of DNA that resides in mitochondria (see Lesson 1). The chromosomes vary in size and in the number of genes they encode.  The Y chromosome is the smallest, with about 50 million base pairs and 200 genes.  Chromosome 1 is the largest, with about 240 million base pairs and 3000 genes.  The total number of genes in the human genome is estimated at 20,000-25,000.