Chapter 11 Questions

Sense codon

A sense codon is a group of three nucleotides that code for an amino acid. There are 61 sense codons that code for the 20 amino acids commonly found in proteins

How are tRNAs linked to their corresponding amino acids?

Each of the 20 different amino acids that are commonly found in proteins has a corresponding aminoacyl-tRNA synthetase that covalently links the amino acid to the correct tRNA molecule

Nonoverlapping code

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

Universal code

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

The following amino acid sequence is found in a tripeptide: Met-Trp-His. Give all possible nucleotide sequences on the mRNA, on the template strand of DNA, and on the nontemplate strand of DNA that can encode this tripeptide

1. 5'-AUGUGGCAU-3' a. DNA template - 3'-TACACCGTA-5' b. DNA non template 5'-ATGUGGCAT-3' 2. 5'-AUGUGGCAC-3' a. DNA template- 3'-TACACCGTG-5' b. DNA non template 5'-ATGTGGCAC-3'

What role do the initiation factors play in protein synthesis?

Initiation factors are proteins that are required for the initiation of translation. In bacteria, there are three initiation factors (IF-1, IF-2, and IF-3). Each one has a different role. IF-1 promotes the disassociation of the large and small ribosomal subunits. IF-3 binds to the small ribosomal subunit and prevents it from associating with the large ribosomal subunit. IF-2 is responsible for binding GTP and delivering the fMet-tRNA^fMet to the initiator codon on the mRNA. In eukaryotes, there are more initiation factors, but many have similar roles. Some of the eukaryotic initiation factors are necessary for recognition of the 5′ cap on the mRNA. Others possess RNA helicase activity, which is necessary to resolve secondary structures.

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

Synonymous codons code for the same amino acid, or, in other words, have the same meaning. A nucleotide at the third position of a codon pairs with a nucleotide in the first position of the anticodon. Unlike the other nucleotide positions involved in the codon-anticodon pairing, this pairing is often weak, or "wobbles," and nonstandard pairings can occur. Because the "wobble," or nonstandard base-pairing with the anticodons, affects the third nucleotide position, the redundancy of codons ensures that the correct amino acid is inserted in the protein when nonstandard pairing occurs

What events bring about the termination of translation?

The process of termination begins when a ribosome encounters a termination codon. Because the termination codon would be located at the "A" site, no corresponding tRNA will enter the ribosome. This allows for the release factors (RF 1, RF2, and RF3) to bind the ribosome. RF1 recognizes and interacts with the stop codons UAA and UAG, while RF2 can interact with UAA and UGA. A RF3-GTP complex binds to the ribosome. Termination of protein synthesis is complete when the polypeptide chain icleaved from the tRNA located at the "P" site. During this process, the GTP is hydroly s zed to GDP

Reading frame

The reading frame refers to how the nucleotides in a nucleic acid molecule are grouped into codons containing three nucleotides. Each sequence of nucleotides has three possible sets of codons, or reading frames

Termination codon

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

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

a. Amino Pro b. Amino Leu c. Amino Leu d. Amino Arg e. Amino no amino acid, stop codon

What are isoaccepting tRNAs?

tRNA molecules that have different anticodon sequences but accept the same amino acids.

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 can also serve as initiation codons although rarely

Arrange the following components of translation in the approximate order in which they would appear or be used in protein synthesis:

First: fMet-tRNA^fmet initiation factor 3 30S initiation complex 70S initiation complex elongation factor Tu elongation factor G release factor 1 Last:

Overlapping code

If an overlapping code is present, then 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

Which of the following amino acid changes could result from a mutation that changed a single base? For each change that could result from the alteration of a single base, determine which position of the codon (first, second, or third nucleotide) in the mRNA must be altered for the change to result.

Leu-> Gln Of the six codons that encode for Leu, only two could be mutated by the alteration of a single base to produce the codons for Gln: CUA (Leu)—Change the second position to A to produce CAA (Gln). CUG (Leu)—Change the second position to A to produce CAG (Gln). Phe ->Ser Both Phe codons (UUU and UUC) could be mutated at the second position to produce Ser codons: UUU (Phe)—Change the second position to C to produce UCU (Ser). UUC (Phe)—Change the second position to C to produce UCC (Ser). Phe ->Ile Both Phe codons (UUU and UUC) could be mutated at the first position to produce Ile codons: UUU (Phe)—Change the first position to A to produce AUU (Ile). UUC (Phe)—Change the first position to A to produce AUC (Ile). Pro-> Ala All four codons for Pro can be mutated at the first position to produce Ala codons: CCU (Pro)—Change the first position to G to produce GCU (Ala). CCC (Pro)—Change the first position to G to produce GCC (Ala). CCA (Pro)—Change the first position to G to produce GCA (Ala). CCG (Pro)—Change the first position to G to produce GCG (Ala). Asn-> Lys Both codons for Asn can be mutated at a single position to produce Lys codons: AAU (Asn)—Change the third position to A to produce AAA (Lys). AAU (Asn)—Change the third position to G to produce AAG (Lys). AAC (Asn)—Change the third position to A to produce AAA (Lys). AAC (Asn)—Change the third position to G to produce AAG (Lys). f. Ile-> Asn Only two of the three Ile codons can be mutated at a single position to produce Asn codons: AUU (Ile)—Change the second position to A to produce AAU (Asn). AUC (Ile)—Change the second position to A to produce AAC (Asn).

Nonuniversal codons

Most codons are universal (or nearly universal) in that they specify the same amino acids in almost all organisms. However, there are exceptions where a codon has different meanings in different organisms. Most of the known exceptions are the termination codons, which in some organisms do code for amino acids. Occasionally, a sense codon is substituted for another sense codon

Nonsense codon

Nonsense codons or termination codons signal the end of translation. These codons do not code for amino acids

How is the reading frame of a nucleotide sequence set?

The initiation codon on the mRNA sets the reading frame.

What did this one gene, one enzyme hypothesis state?

This states that the function of a gene is to dictate the production of a specific enzyme