For most of the time, DNA exists in the nucleus as thin strands. When the cells containing nuclei are ready to divide, the DNA copies itself then coils and condenses to form chromosomes.
Each chromosome is made from a single molecule of DNA. Each section of this single molecule of DNA contains a code for the production of a particular protein called a gene.
Each human body cell contains 46 chromosomes. These can be arranged into 23 pairs. Each chromosome in a pair carries the same types of genes.
The 23rd pair is the sex chromosomes. In females, the two chromosomes are identical in shape. These are X chromosomes. Females are referred to as XX. In males, one of the chromosomes is different in shape. This is a Y chromosome. Males are referred to as XY.