The Genetic Code

The body makes more than 2 million different proteins, all from the same 20 amino acids and all encoded by genes made of just 4 nucleotides (A, T, C, G)—a striking illustration of how a great variety of complex structures can be made from a small variety of simpler components. The genetic code is a system that enables these 4 nucleotides to code for the amino acid sequences of all proteins.

It is not unusual for simple codes to represent complex information. Computers store and transmit complex information, including pictures and sounds, in a binary code with only the symbols 1 and 0. It is not surprising, then, that a mere 20 amino acids can be represented by a code of 4 nucleotides; all that is required is to combine these symbols in varied ways. It requires more than 2 nucleotides to code for each amino acid, because A, U, C, and G can combine in only 16 ways (AA, AU, AC, AG, UA, UU, etc.). The minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid, and indeed this is the case in DNA. A sequence of 3 DNA nucleotides that stands for 1 amino acid is called a base triplet. The "mirror image"

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 4 Genetics and Cellular Function 135

Interstitial cell of testis

Interstitial cell of testis

Pictures Genetic Triplet Code

Figure 4.5 Indirect Control of Testosterone Synthesis by DNA. There is no gene for testosterone, but DNA regulates its synthesis through the enzymes for which it does code. ( 1) DNA codes for mRNA. (2) In the cytoplasm, mRNA directs the synthesis of an enzyme. (3) When testosterone is needed, luteinizing hormone (LH) stimulates production of a second messenger within cells of the testis. (4) The second-messenger system activates the enzyme encoded by the mRNA. (5) The enzyme converts cholesterol to testosterone. (6) Testosterone is secreted from the cell and exerts various anatomical, physiological, and behavioral effects.

Figure 4.5 Indirect Control of Testosterone Synthesis by DNA. There is no gene for testosterone, but DNA regulates its synthesis through the enzymes for which it does code. ( 1) DNA codes for mRNA. (2) In the cytoplasm, mRNA directs the synthesis of an enzyme. (3) When testosterone is needed, luteinizing hormone (LH) stimulates production of a second messenger within cells of the testis. (4) The second-messenger system activates the enzyme encoded by the mRNA. (5) The enzyme converts cholesterol to testosterone. (6) Testosterone is secreted from the cell and exerts various anatomical, physiological, and behavioral effects.

Table 4.2 Examples of the Genetic Code sequence in mRNA is called a codon. The genetic code is expressed in terms of codons.

Table 4.2 shows a few representative triplets and codons along with the amino acids they represent. You can see from this listing that two or more codons can represent the same amino acid. The reason for this is easy to explain mathematically. Four symbols (N) taken three at a time (x) can be combined in Nx different ways; that is, there are 43 = 64 possible codons available to represent the 20 amino acids. Only 61 of these code for amino acids. The other 3—UAG, UGA, and UAA—are called stop codons; they signal "end of message," like the period at the end of a sentence. A stop codon enables the cell's protein-synthesizing machinery to sense that it has reached the end of the gene for a particular protein. The codon AUG plays two roles—it serves as a code for methionine and as a start codon. This dual function is explained shortly.

Table 4.2 Examples of the Genetic Code

Base Triplet of DNA

Codon of mRNA

Name of Amino Acid

Abbreviate Amino Acid

CCT

GGA

Glycine

Gly

CCA

GGU

Glycine

Gly

CCC

GGG

Glycine

Gly

CTC

GAG

Glutamic acid

Glu

CGC

GCG

Alanine

Ala

CGT

GCA

Alanine

Ala

TGG

ACC

Threonine

Thr

TGC

ACG

Threonine

Thr

GTA

CAU

Valine

Val

TAC

AUG

Methionine

Met

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

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Responses

  • zula
    What are the enzymes that codes for testosterone?
    7 years ago
  • KARRI
    What is the system that enables 4 nucleotides to code?
    7 years ago

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