Chromosomes are composed of deoxyribonucleic acid strands made of gene combinations. All DNA strands consist of nucleotides, composed of a nitrogenous base, a molecule of sugar and a molecule of phosphoric acid. Specific combinations of the nitrogenous bases are gene codes that present as hereditary characteristics.
The four base chemicals are adenine, cytosine, thymine and guanine. Uracil is another base chemical, but it is only present in RNA. Adenine and guanine both have two nitrogen rings and are called purines. Cytosine, thymine and uracil only have one nitrogen ring and are called pyrimidines.
These base chemicals bind together, adenine to thymine and cytosine to guanine, to create a strand of DNA about 6 feet long. The bound chemicals take the shape of a helix, which looks like a twisted ladder.
The DNA strand then wraps itself around protein molecules called histones. When wrapped up together, the DNA and protein are called a nucleosome. Chromatin is then formed by the tightly looped nucleosomes.
In turn, the chromatin then wraps around other protein molecules into a structure called a solenoid. The solenoid then wraps itself into the familiar "X" shape of a chromosome. Every human has 22 pairs of matching chromosomes along with a 23rd pair of sex chromosomes, XX for females and XY for males.