The cells in your digestive tract can absorb only single amino acids or very small chains of two or three amino acids called peptides. So proteins from food are broken into their component amino acids by digestive enzymes — which are, of course, specialized proteins. Then other enzymes in your body cells build new proteins by reassembling the amino acids into specific compounds that your body needs to function. This process is called protein synthesis. During protein synthesis
1 Amino acids hook up with fats to form lipoproteins, the molecules that ferry cholesterol around and out of the body. Or amino acids may join up with carbohydrates to form the glycoproteins found in the mucus secreted by the digestive tract.
1 Proteins combine with phosphoric acid to produce phosphoproteins, such as casein, a protein in milk.
1 Nucleic acids combine with proteins to create nucleoproteins, which are essential components of the cell nucleus and of cytoplasm, the living material inside each cell.
The carbon, hydrogen, and oxygen that are left over after protein synthesis is complete are converted to glucose and used for energy (see Chapter 7). The nitrogen residue (ammonia) isn't used for energy. It's processed by the liver, which converts the ammonia to urea. Most of the urea produced in the liver is excreted through the kidneys in urine; very small amounts are sloughed off in skin, hair, and nails.
Every day, you turn over (reuse) more proteins than you get from the food you eat, so you need a continuous supply to maintain your protein status. If your diet does not contain sufficient amounts of proteins, you start digesting the proteins in your body, including the proteins in your muscles and — in extreme cases — your heart muscle.
Nucleoproteins are chemicals in the nucleus of every living cell. They're made of proteins linked to nucleic acids — complex compounds that contain phosphoric acid, a sugar molecule, and nitrogen-containing molecules made from amino acids.
Nucleic acids (molecules found in the chromosomes and other structures in the center of your cells) carry the genetic codes — genes that help determine what you look like, your general intelligence, and who you are. They contain one of two sugars, either riboseor deoxyribose. The nucleic acid containing ribose is called ribonucleic acid(RNA). The nucleic acid containing deoxyribose is called deoxyribonucleic acid (DNA).
DNA, a long molecule with two strands twisting about each other (the double helix), carries and transmits the genetic inheritance in your chromosomes. In other words, DNA supplies instructions that determine how your body cells are formed and how they behave. RNA, a single-strand molecule, is created in the cell nucleus according to the pattern determined by the DNA. Then RNA carries the DNA's instructions to the rest of the cell.
Knowing about DNA is important because it's the most distinctly "you" thing about your body. Chances that another person on Earth has exactly the same DNA as you are really small. That's why DNA analysis is used increasingly in identifying criminals or exonerating the innocent. Some people are even proposing that parents store a sample of their children's DNA so that they'll have a conclusive way of identifying a missing child, even years later.
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