CI, chain-initiating; CT, chain-terminating.

CI, chain-initiating; CT, chain-terminating.

messenger RNA. The specific code in the transfer RNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an anti-codon. This is located approximately in the middle of the transfer RNA molecule (at the bottom of the cloverleaf configuration shown in Figure 3-9). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon

Figure 3-9

A messenger RNA strand is moving through two ribosomes. As each "codon" passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribo-some. The transfer RNA molecule transports each specific amino acid to the newly forming protein.

bases of the messenger RNA. In this way, the respective amino acids are lined up one after another along the messenger RNA chain, thus establishing the appropriate sequence of amino acids in the newly forming protein molecule.

Ribosomal RNA

The third type of RNA in the cell is ribosomal RNA; it constitutes about 60 per cent of the ribosome. The remainder of the ribosome is protein, containing about 75 types of proteins that are both structural proteins and enzymes needed in the manufacture of protein molecules.

The ribosome is the physical structure in the cytoplasm on which protein molecules are actually synthesized. However, it always functions in association with the other two types of RNA as well: transfer RNA transports amino acids to the ribosome for incorporation into the developing protein molecule, whereas messenger RNA provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured.

Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed.

Formation of Ribosomes in the Nucleolus. The DNA genes for formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus, and each of these chromosomes contains many duplicates of these particular genes because of the large amounts of riboso-mal RNA required for cellular function.

As the ribosomal RNA forms, it collects in the nucleolus, a specialized structure lying adjacent to the chromosomes.When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with "ribosomal proteins" to form granular condensation products that are primordial subunits of ribo-somes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature, functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not contain mature ribosomes.

Formation of Proteins on the Ribosomes—The Process of "Translation"

When a molecule of messenger RNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA

bases called the "chain-initiating" codon. Then, as shown in Figure 3-9, while the messenger RNA travels through the ribosome, a protein molecule is formed— a process called translation. Thus, the ribosome reads the codons of the messenger RNA in much the same way that a tape is "read" as it passes through the playback head of a tape recorder. Then, when a "stop" (or "chain-terminating") codon slips past the ribosome, the end of a protein molecule is signaled and the protein molecule is freed into the cytoplasm.

Polyribosomes. A single messenger RNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in Figure 3-9 and in Figure 3-10. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, 3 to 10 ribosomes being attached to a single messenger RNA at the same time. These clusters are called polyribosomes.

It is especially important to note that a messenger RNA can cause the formation of a protein molecule in any ribosome; that is, there is no specificity of ribo-somes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place.

Many Ribosomes Attach to the Endoplasmic Reticulum. In

Chapter 2, it was noted that many ribosomes become attached to the endoplasmic reticulum. This occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum; this causes these molecules to penetrate the reticulum wall and enter the endoplasmic reticulum matrix. This gives a granular appearance to those portions of the reticulum where proteins are being formed and entering the matrix of the reticulum.

Figure 3-10 shows the functional relation of messenger RNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of messenger RNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix.

Yet it should be noted that except in glandular cells in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell.

Chemical Steps in Protein Synthesis. Some of the chemical events that occur in synthesis of a protein molecule are shown in Figure 3-11.This figure shows representative reactions for three separate amino acids, AA1, AA2, and AA20. The stages of the reactions are the following: (1) Each amino acid is activated by a chemical

Transfer RNA

Messenger RNA

Small Ribosome subunit

Amino acid

Transfer RNA

Messenger RNA

Small Ribosome subunit

Figure 3-10

Polypeptide gp subunit chain tL>

Polypeptide gp subunit chain

Figure 3-10

Physical structure of the ribosomes, as well as their functional relation to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. (Courtesy of Dr. Don W. Fawcett, Montana.)

Activated amino acid





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