Chromosome: A carrier of genetic information that is visible under an ordinary light microscope. Each human chromosome has two arms, the p (short) arm and the q (long) arm. These arms are separated from each other only by the centromere, which is the point at which the chromosome is attached to the spindle during cell division. The 3 billion base pairs in the human genome are organized into 24 chromosomes. All genes are arranged linearly along the chromosomes. Generally the nucleus of a human cell contains two sets of chromosomes'one set given by each parent. Each set has 23 single chromosomes: 22 autosomes and an X or a Y sex chromosome. (A normal female has a pair of X chromosomes; a male has an X and Y pair.) A chromosome contains roughly equal parts of protein and DNA. The chromosomal DNA contains an average of 150 million nucleotide building blocks, called bases. DNA molecules are among the largest molecules now known.