A cell is similar to a car in many aspects. A car is made up of different parts each with a specialized function. It has wheels, an engine, steering wheel, brakes and a body that form a functioning car. Likewise, humans (and all complex living things) consist of parts made up of specialized cells each with their own functions. While there are approximately 10,000,000,000,000 (1023) number of cells in each of us, there are only about 200 different cell types i.e. liver cells, nerve cells, skin cells, blood cells and so on. DNA carries instructions on which, and how many, cells to make to complete the creation and maintenance of a human body. Furthermore, every cell type has its own specialized functions and needs different types, numbers, and amounts of various proteins. The types, numbers and amounts of proteins in each cell are also controlled by DNA. Thus to understand the working of a cell (and therefore the human organism) it becomes necessary to understand how DNA directs the production of proteins.
DNA in a cell is made up of four different parts called nucleotides that appear in different sequences and patterns in a long string. Each nucleotide consists of a sugar (deoxyribose) bound on one side to a phosphate group and bound on the other side to a nitrogenous base.
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Proteins are the workhouse of a cell. They play a vital role in the maintenance, growth, division of cells and in communication between cells. The human genome, a complete set of human genes, comes in 23 separate pairs of chromosomes. Chromosomes are densely packed structures containing DNA and proteins. DNA in the chromosomes consist of genes (functional DNA), regulatory DNA (which regulates the function of genes) and other nucleotides (repeating units base pairs). Coiled DNA in the chromosome is bound with certain chromosomal proteins belonging to the “DNA-bound” family of proteins. The function of these proteins is to package the DNA and control its function.