Proteins
Proteins are a class of organic compounds which are present in and vital to every living cell. In the form of skin, hair, callus, cartilage, muscles, tendons and ligaments, proteins hold together, protect, and provide structure to the body of a multi-celled organism. In the form of enzymes, hormones, antibodies, and globulins, they catalyze, regulate, and protect the body chemistry. In the form of hemoglobin, myoglobin and various lipoproteins, they effect the transport of oxygen and other substances within an organism.
The most important thing 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.
In the synthesis of protein, three types of RNA function. The first type is called ribosomal RNA (rRNA). This form of RNA is used to manufacture ribosomes. 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 Xand 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.
Transcription is one of the first processes in the mechanism of protein synthesis. In transcription, a complementary strand of mRNA is synthesized according to the nitrogenous base code of DNA. The enzyme RNA polymerase binds to an area of one of the DNA molecules in the double helix. The enzyme moves along the DNA strand and “reads” the nucleotides one by one. Similar to the process of DNA replication, the new nucleic acid strand elongates in a 5′-3′ direction, as shown in Figure
First 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. | | | | | | | The process of transcription. The DNA double helix opens and the enzyme RNA polymerase synthesizes a molecule of mRNA according to the base sequence of the DNA template. | | |
The nucleotides of the DNA strands are read in groups of three. Each group is a codon. Thus, a codon may be CGA, or TTA, or GCT, or any other combination