DNA is the genetic blueprint for an organism. In humans, a DNA strand contains 3 billion base pairs. That long DNA strand is divided into more manageable sections. Those sections are called chromosomes. I'll use humans again. Humans have 46 chromosomes. A chromosome contains genes. A gene is a section of a chromosome, which is the same thing as saying that a gene is a section of DNA. A gene is then composed of a certain number of base pairs. The number of base pairs depends on the gene; however, the organization of a single base pair is the same. A base pair is composed of two nucleotide units that have been "attached" to each other with the nitrogen base. A nucleotide is composed of a deoxyribose, a phosphate, and 1 of 4 possible bases. Those bases are Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). A always pairs with T, and C always pairs with G. The order of the nitrogen bases functions like a code that determines how our body grows and functions.
It is possible to compare the structure and organization of DNA to a city. Each nitrogen base could be compared to an individual home on a street. A bunch of homes in a row would then be similar to a gene. Not all of the homes on a street are identical homes (different bases), and not all streets have the same number of homes or types of homes (different genes). The entire series of streets in a given neighborhood would then be a single chromosome. Cities have many different neighborhoods just like DNA has different chromosomes. All of the streets/neighborhoods together in a single city would then be similar to all of the chromosomes of a single cell's DNA.
https://www.livescience.com/37247-dna.html
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