Chromosome are tiny threadlike structure composed of nucleic acids and proteins, found in all plant and animal cells. The chromosome contains the nucleic acid deoxyribonucleic acid (DNA), which contains units called genes. The genes determine the hereditary characteristics of the cell, animal, or plant. Human beings have 23 pairs of chromosomes. During fertilization the sperm and egg unite, making pairs of chromosomes, half of each pair coming from each parent.
The following picture shows the chromosomes after the S phase. They appear as "X" shape structures because they have already replicated themselves and are ready to divide.