During the 1950s and 1960s, it became apparent that DNA is essential in the synthesis of proteins. Proteins are used in enzymes and as structural materials in cells. Many specialized proteins function in cellular activities. For example, in humans, the hormone insulin and the muscle cell filaments are composed of protein. The hair, skin, and nails of humans are composed of proteins, as are all the hundreds of thousands of enzymes in the body.
The key to a protein molecule is how the amino acids are linked. The sequence of amino acids in a protein is a type of code that specifies the protein and distinguishes one protein from another. A genetic code in the DNA determines this amino acid code. The genetic code consists of the sequence of nitrogenous bases in the DNA. How the nitrogenous base code is translated to an amino acid sequence in a protein is the basis for protein synthesis.
For protein synthesis to occur, several essential materials must be present, such as a supply of the 20 amino acids, which comprise most proteins. Another essential element is a series of enzymes that will function in the process. DNA and another form of nucleic acid called ribonucleic acid (RNA) are essential.
RNA is the nucleic acid that carries instructions from the nuclear DNA into the cytoplasm, where protein is synthesized. RNA is similar to DNA, with two exceptions. First, the carbohydrate in RNA is ribose rather than deoxyribose, and second, RNA nucleotides contain the pyrimidine uracil rather than thymine.
Types of RNA
In the synthesis of protein, three types of RNA function. The first type is calledribosomal RNA (rRNA). This form of RNA is used to manufacture ribosomes.Ribosomes are ultramicroscopic particles of rRNA and protein. They are the places (the chemical “workbenches”) where amino acids are linked to one another to synthesize proteins. Ribosomes may exist along the membranes of the endoplasmic reticulum or in the cytoplasm of the cell.
A second important type of RNA is transfer RNA (tRNA). Transfer RNA exists in the cell cytoplasm and carries amino acids to the ribosomes for protein synthesis. When protein synthesis is taking place, enzymes link tRNA molecules to amino acids in a highly specific manner. For example, tRNA molecule X will link only to amino acid X; tRNA Y will link only to amino acid Y.
The third form of RNA is messenger RNA (mRNA). In the nucleus, messenger RNA receives the genetic code in the DNA and carries the code into the cytoplasm where protein synthesis takes place. Messenger RNA is synthesized in the nucleus at the DNA molecules. During the synthesis, the genetic information is transferred from the DNA molecule to the mRNA molecule. In this way, a genetic code can be used to synthesize a protein in a distant location. RNA polymerase, an enzyme, accomplishes mRNA, tRNA, and rRNA synthesis.
1 . The enzyme selects complementary bases from available nucleotides and positions them in an mRNA molecule according to the principle of complementary base pairing. The chain of mRNA lengthens until a “stop” message is received.