Figure

The expression of genetic information in a cell occurs through the transcription of coded information from DNA to RNA in the nucleus, followed by the translation of the RNA information into protein synthesis in the cytoplasm. The proteins then perform the functions that determine the characteristics of the cell.

The genetic language is similar in principle to a written language, which consists of a set of symbols, such as A, B, C, D, that form an alphabet. The letters are arranged in specific sequences to form words, and the words are arranged in linear sequences to form sentences. The genetic language contains only four letters, corresponding to the bases A, G, C, and T. The genetic words are three-base sequences that specify particular amino acids—that is, each word in the genetic language is only three letters long. This is termed a triplet code. The sequence of three-letter code words (triplets) along a gene in a single strand of DNA specifies the sequence of amino acids in a polypeptide chain (Figure 5-2). Thus, a gene is equivalent to a sentence, and the genetic information in the human genome is equivalent to a book containing 50,000 to 100,000 sentences. Using a single letter (A, T, C, G) to specify each of the four bases in the DNA nucleotides, it will require about 550,000 pages, each equivalent to this text page to print the nucleotide sequence of the human genome.

The four bases in the DNA alphabet can be arranged in 64 different three-letter combinations to form 64 code words (4 X 4 X 4 = 64). Thus, this code actually provides more than enough words to code for the 20 different amino acids that are found in proteins. This means that a given amino acid is usually specified by more than one code word. For example, the four DNA triplets C-C-A, C-C-G, C-C-T, and C-C-C all specify the amino acid glycine. Only 61 of the 64 possible code words are used to specify amino acids. The code words that do not specify amino acids are known as "stop" signals. They perform the same function as does a period at the end of a sentence—they indicate that the end of a genetic message has been reached.

The genetic code is a universal language used by all living cells. For example, the code words for the amino acid tryptophan are the same in the DNA of a bacterium, an amoeba, a plant, and a human being. Although the same code words are used by all living cells, the messages they spell out—the sequences of code words that code for a specific protein—vary from gene to gene in each organism. The universal nature of the genetic code supports the concept that all forms of life on earth evolved from a common ancestor.

Portion of a gene in one strand of DNA

Amino acid sequence coded by gene

T A C

A A A

C C A

A - G - G'

C - C - A-

A C C

G T A

FIGURE 5-2

The sequence of three-letter code words in a gene determines the sequence of amino acids in a polypeptide chain. The names of the amino acids are abbreviated. Note that more than one three-letter code sequence can indicate the same amino acid; for example, the amino acid phenylalanine (Phe) is coded by two triplet codes, A-A-A and A-A-G.

FIGURE 5-2

The sequence of three-letter code words in a gene determines the sequence of amino acids in a polypeptide chain. The names of the amino acids are abbreviated. Note that more than one three-letter code sequence can indicate the same amino acid; for example, the amino acid phenylalanine (Phe) is coded by two triplet codes, A-A-A and A-A-G.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Before we turn to the specific mechanisms by which the DNA code is used in protein synthesis, an important clarification and qualification is required. As noted earlier, the information coded in genes is always first transcribed into RNA. As we shall see in the next section there are several classes of RNA—messenger RNA, ribosomal RNA, transfer RNA, and small nuclear RNAs. Only messenger RNA directly codes for the amino acid sequences of proteins even though the other RNA classes participate in the overall process of protein synthesis. For this reason, the customary definition of a gene as the sequence of DNA nucleotides that specifies the amino acid sequence of a protein is true only for those genes that are transcribed into messenger RNA. The vast majority of genes are of this type, but it should at least be noted that the genes that code for the other classes of RNA do not technically fit this definition.

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Essentials of Human Physiology

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