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Chapter 26: Q32P (page 1180)

Why is the codon a triplet rather than a doublet or a quartet?

Short Answer

Expert verified

A three-letter code provides enough codes for all the 20 amino acids and also provides the necessary stop codon.

Step by step solution

01

Codons

Codon is a sequence of nucleotides (generally three) in DNA or RNA that corresponds to the specific amino acid.

Codon is a sequence of nucleotides (generally three) in DNA or RNA that corresponds to the specific amino acid.

02

Codon triplet

Generally, the genetic code is a triplet, not a doublet or quartet. There are total of four nucleotides. So the number of different possible codons using four nucleotides is (4)n , where n is the number of nucleotides in the code.

For two-letter code: (4)2= 16

For three-letter code: (4)3 = 64

For four-letter code:(4)4= 256

There are 20 amino acids. Therefore we can say that two-letter code would not provide enough codons.

A three-letter code provides enough codes for all the amino acids and also provides the necessary stop codon.

A four-letter code provides many more codes than would be needed.

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Most popular questions from this chapter

Why does DNA not unravel completely before replication begins?

The amino acid sequences of peptide fragments obtained from a normal protein were compared with those obtained from the same protein synthesized by a defective gene. They were found to differ in only one peptide fragment. Their amino acid sequences are shown here:

Normal: Gln-Tyr-Gly-Thr-Arg-Tyr-Val

Mutant: Gln-Ser-Glu-Pro-Gly-Thr

a. What is the defect in DNA?

b. It was later determined that the normal peptide fragment is an octapeptide with a C-terminal Val-Leu. What is the C-terminal amino acid of the mutant peptide?

Human immunodeficiency virus (HIV) is the retrovirus that causes AIDS. AZT was one of the first drugs designed to interfere with retroviral DNA synthesis. When cells take up AZT, they convert it to AZT-triphosphate. Explain how AZT interferes with DNA synthesis.

Using the D, A, and D/A designations in Problem 3, indicate how base pairing would be affected if the bases existed in the enol form

a. Using the single-letter abbreviations for the amino acids in Table 21.2, write the sequence of amino acids in a tetrapeptide represented by the first four different letters in your first name. Do not use any letter twice. (Because not all letters are assigned to amino acids, you might have to use one or two letters in your last name.)

b. Write one of the sequences of bases in mRNA that would result in the synthesis of that tetrapeptide.

c. Write the sequence of bases in the sense strand of DNA that would result in the formation of that fragment of mRNA.
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