ANATOMY OF OUR GENES: The Human Body
The human body is made of some 50 trillion to 100 trillion cells, which form the basic units of life and combine to form more complex tissues and organs. Inside each cell, genes make up a “blueprint” for protein production that determines how the cell will function. Genes also determine physical characteristics or traits. The complete set of some 20,000 to 25,000 genes is called the genome. Only a tiny fraction of the total genome sets the human body apart from those of other animals.
Most cells have a similar basic structure. An outer layer, called the cell membrane, contains fluid called cytoplasm. Within the cytoplasm are many different specialized “little organs” called organelles. The most important of these is the nucleus, which controls the cell and houses the genetic material in structures called chromosomes. Another type of organelle is mitochondrion. These “cellular power plants” have their own genome and do not recombine during reproduction.
Chromosomes carry hereditary, genetic information in long strings of DNA called genes. Humans have 22 numbered pairs of chromosomes and a single pair of sex chromosomes—XX in females and XY in males. Each chromosomal pair includes one inherited from the father and one from the mother. If unwound, the microscopic DNA strands in one cell’s nucleus would stretch to over six feet (two meters) in length.
DNA (deoxyribonucleic acid) is the set of genetic instructions for creating an organism. DNA molecules are shaped like a spiral staircase called a double helix. Each stair is composed of the DNA bases A, C, T, and G. Some segments of these bases contain sequences, like A-T-C-C-G-A-A-C-T-A-G, which constitute individual genes. Genes determine which proteins individual cells will manufacture, and thus what function particular cells will perform.
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