© A.C.E. (1992, pp. 24-29) Used by permission
Some of the most complex organic componds are proteins. Proteins are an essential ingredient in every cell in your body. In fact, if all the water were withdrawn from your body, proteins would make up roughly 50% of its remaining weight.
Proteins are often classified by their function. The major categories are listed in the following table.
Proteins are made of amino acids, which are organic compounds containing a carboxyl group and another functional group called an amine. The 20 known amino acids form the nearly 10,000 different kinds of proteins in the human body.
Amino acids contain three basic units.
Amino acids combine to form proteins. Because one protein may contain as many as 3,000 separate amino acids, proteins are often very large and very complex molecules.
Because there are 20 different amino acids, there are many possible combinations for protein molecules. Consider how many different arrangements of the 20 amino acids are possible, even in a small protein molecule with only 100 amino acids. It has been calculated that, even for this small protein molecule, there are more possible arrangements than there are atoms in the entire universe! The possibilities for a large protein are beyond comprehension.
Enzymes, which are some of the most important proteins in the body, are biochemical catalysts that speed up processes within the cell. Many cell processes normally proceed at a very slow rate. Without something to speed up these processes, the cell would cease to function at all.
Enzymes are large protein molecules that are twisted and bent into many unusual shapes. Each enzyme performs a specific task because of its particular shape. Enzymes have been described as puzzle pieces that fit only one particular place in a huge biochemical puzzle.
Nucleic acids. By now you have learned that proteins are a vital component of every part of the body-without proteins there could be no human body. With the unlimited possible arrangements of amino acids within proteins, how does the body know just how to synthesize the correct proteins? The answer lies in the amazing structure and function of nucleic acids, such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA is a very complex organic molecule found in the nucleus of each cell. DNA contains the coded pattern that forms all the proteins in that cell. Think of DNA as the commanding general of the cell. From within the safety of the cell's nucleus, General DNA controls the synthesis of proteins in the cell.
In order for DNA's orders to be carried out, the cell needs a "foot soldier"-someone to take the orders to the front line. This duty is performed by RNA. RNA, found in both the nucleus and cytoplasm of the cell, is the chemical messenger that carries out the coded plan contained in the DNA. The DNA is kept safe and remains unchanged as it directs the RNA to synthesize protein.
Both DNA and RNA molecules are composed of many structural units called nucleotides. A nucleotide consists of three parts: a nitrogen-containing base, a sugar molecule, and a phosphate group.
There are four possible bases that could be used in RNA. They are guanine, cytosine, adenine, and uracil. DNA also uses guanine, cytosine, and adenine but substitutes thymine for uracil.
DNA is probably the most complex molecule ever created. It contains two strands, each of which consists of a chain of many thousand nucleotides. These two strands are twisted around each other in an arrangement called a double helix.
A double helix resembles a very tall, flexible ladder twisted into a spiral. Although this example is somewhat oversimplified, it does give you an idea of the shape of a double helix.
The uprights of a DNA "ladder" are formed by the phosphates and deoxyribose units. Each rung of the "ladder" is formed of two bases joined by hydrogen bonds. These bonds actually hold the two strands of DNA together.
Because each of the bases has an active site similar to an enzyme, only certain bases will bond with certain other bases. Adenine (A) fits only thymine (T), and guanine (G) fits only the active site of cytosine (C).
When it becomes necessary for DNA to direct the production of a new protein, enzymes provide the necessary information so that the DNA knows which protein needs to be synthesized and which portion of the DNA contains the code for that protein. The double helix then untwists, and the DNA "unzips" the portion of the molecule with the code for that specific protein. As the base pairs separate and expose their active sites, free nucleotides floating in the nucleus are attracted to the exposed active sites on one side of the unzipped DNA. A guanine attached to the unzipped DNA attracts a new cytosine; a thymine attracts an adenine; a cytosine attracts a guanine; and an adenine attracts a uracil (U). This action, called transcription, creates a new RNA with the same coded sequence of nucleotides as the parent DNA (except that uracil bases substitute for thymine bases). The new RNA is then released to move into the cytoplasm and carry out the synthesis of new protein. After the RNA is released, the DNA zips back together and waits until the next transcription.
There are actually three different kinds of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Although scientists have not been able to determine the function of rRNA, both mRNA and tRNA work together to synthesize proteins.
Each mRNA contains the coded message from the transcription of the DNA. The mRNA contains exposed active sites that need to be filled. The tRNA carries nucleotides to the mRNA to fill those active sites.
Each tRNA consists of a triplet of nucleotides bonded to a specific amino acid. Let's consider three molecules of tRNA. The first tRNA molecule contains G, A, and C nucleotides (GAC) and carries one amino acid. The second tRNA molecule contains A, U, and G nucleotides (AUG) and carries a different amino acid. The third tRNA molecule contains C, C, and A nucleotides (CCA) and carries a third amino acid.
tRNA (GAC) = amino acid 1
tRNA (AUG) = amino acid 2
tRNA (CCA) = amino acid 3
The tRNA carrying amino acid 1 moves along the mRNA until it finds the complement of its nucleotide pattern. When it finds active sites in the pattern CUG, it attaches to those nucleotides. Amino acid 2 finds its complement (UAC) and attaches. As it attaches, amino acid 2 bonds to amino acid 1. When amino acid 3 finds its complement (GGU), it attaches and bonds amino acid 3 to amino acids 2 and 1. In this manner, whole proteins are built by the attachment of amino acids one behind the other in the manner of cars being linked together to form a freight train.
A.C.E. 1992, Science: Physical Science 1115, Rev.
Ed., Accelerated Christian Education Inc.