Chapter 11 Biology 222 Exam 2

4. *Define the following terms as they apply to the genetic code: a. Reading frame b. Overlapping code c. Nonoverlapping code d. Initiation codon e. Termination codon f. Sense codon g. Nonsense codon h. Universal code i. Nonuniversal codons

(a) The reading frame refers to how the nucleotides in a nucleic acid molecule are grouped into codons, with each codon containing three nucleotides. Any sequence of nucleotides has three potential reading frames that have completely different sets of codons. (b) In an overlapping code, a single nucleotide is included in more than one codon. The result for a sequence of nucleotides is that more than one type of polypeptide can be encoded within that sequence. (c) In a nonoverlapping code, a single nucleotide is part of only one codon, which results in the production of a single type of polypeptide from one polynucleotide sequence. (d) An initiation codon establishes the appropriate reading frame and specifies the first amino acid of the protein chain. Typically, the initiation codon is AUG; however, GUG and UUG also can serve as initiation codons. (e) The termination codon signals the termination, or end, of translation and the end of the protein molecule. The three types of termination codons—UAA, UAG, and UGA—are also referred to as stop codons or nonsense codons. These codons do not encode amino acids. (f) A sense codon is a group of three nucleotides that encode an amino acid. Sixty-one sense codons encode the 20 amino acids commonly found in proteins. (g) A nonsense codon, or termination codon, signals the end of translation. Nonsense codons do not encode amino acids. (h) In a universal code, each codon specifies the same amino acid in all organisms. The genetic code is nearly universal but not completely. Most of the exceptions are in mitochondrial genes. (i) Although most codons are universal (or nearly universal) in that they specify the same amino acids in almost all organisms, there are exceptions in which a codon has different meanings in different organisms.

Define: Nonsense codon

A nonsense codon, or termination codon, signals the end of translation. Nonsense codons do not encode amino acids.

11. *Explain how some antibiotics work by affecting the process of protein synthesis.

A number of antibiotics bind the ribosome and inhibit protein synthesis at different steps in translation. Some antibiotics, such as streptomycin, bind to the small subunit and inhibit the initiation of translation. Other antibiotics, such as chloramphenicol, bind to the large subunit and block the elongation of the peptide by preventing peptide-bond formation.

Define: Sense codon

A sense codon is a group of three nucleotides that encode an amino acid. Sixty-one sense codons encode the 20 amino acids commonly found in proteins.

Define: Nonuniversal codons

Although most codons are universal (or nearly universal) in that they specify the same amino acids in almost all organisms, there are exceptions in which a codon has different meanings in different organisms.

Define: Initiation codon

An initiation codon establishes the appropriate reading frame and specifies the first amino acid of the protein chain. Typically, the initiation codon is AUG; however, GUG and UUG also can serve as initiation codons.

6. *How are tRNAs linked to their corresponding amino acids?

For each of the 20 different amino acids commonly found in proteins, a corresponding aminoacyl-tRNA synthetase covalently links the amino acid to the correct tRNA molecule.

Define: Non overlapping Code

In a nonoverlapping code, a single nucleotide is part of only one codon, which results in the production of a single type of polypeptide from one polynucleotide sequence.

Define: Universal code

In a universal code, each codon specifies the same amino acid in all organisms. The genetic code is nearly universal but not completely. Most of the exceptions are in mitochondrial genes

Define: Overlapping code

In an overlapping code, a single nucleotide is included in more than one codon. The result for a sequence of nucleotides is that more than one type of polypeptide can be encoded within that sequence.

3. *What is the significance of the fact that many synonymous codons differ only in the third nucleotide position?

In synonymous codons that differ only at the third nucleotide position, the "wobble" and nonstandard base pairing with the anticodons will likely result in the correct amino acid being inserted in the protein.

Define: Reading Frame

The reading frame refers to how the nucleotides in a nucleic acid molecule are grouped into codons, with each codon containing three nucleotides. Any sequence of nucleotides has three potential reading frames that have completely different sets of codons.

Define: Termination codon

The termination codon signals the termination, or end, of translation and the end of the protein molecule. The three types of termination codons—UAA, UAG, and UGA—are also referred to as stop codons or nonsense codons. These codons do not encode amino acids.

14. Using the genetic code presented in Figure 11.5, indicate which amino acid is encoded by the following codons on the mRNA. a. 5′-CCC-3′ b. 5′-UUG-3′ c. 5′-CUG-3′ d. 5′-AGA-3′ e. 5′-UAA-3′

a. 5′-CCC-3′ Pro b. 5′-UUG-3′ Leu c. 5′-CUG-3′ Leu d. 5′-AGA-3′ Arg e. 5′-UAA-3′ Stop