Biology
KEY CONCEPTS:
- Bypassing Involves Ribosome Movement
Certain sequences trigger a bypass event, when a ribosome stops translation, slides along mRNA with peptidyl-tRNA remaining in the P site, and then resumes translation (see Figure 7.30). This is a rather rare phenomenon, with only ~3 authenticated examples...
- Suppressor Trnas Have Mutated Anticodons That Read New Codons
KEY TERMS:A suppressor is a second mutation that compensates for or alters the effects of a primary mutation. A nonsense suppressor is a gene coding for a mutant tRNA able to respond to one or more of the termination codons and insert an amino acid at...
- Using The Genetic Code
The sequence of a coding strand of DNA, read in the direction from 5 to 3, consists of nucleotide triplets (codons) corresponding to the amino acid sequence of a protein read from N-terminus to C-terminus. Sequencing of DNA and proteins makes it possible...
- Three Codons Terminate Protein Synthesis
KEY TERMS:The amber codon is the triplet UAG, one of the three termination codons that end protein synthesis. The ochre codon is the triplet UAA, one of the three termination codons that end protein synthesis. The opal codon is the triplet UGA, one of...
- Every Sequence Has Three Possible Reading Frames
KEY TERMS:A reading frame is one of the three possible ways of reading a nucleotide sequence. Each reading frame divides the sequence into a series of successive triplets. There are three possible reading frames in any sequence, depending on the starting...
Biology
Recoding changes codon meanings
KEY TERMS:
- Recoding events occur when the meaning of a codon or series of codons is changed from that predicted by the genetic code. It may involve altered interactions between aminoacyl-tRNA and mRNA that are influenced by the ribosome.
- Changes in codon meaning can be caused by mutant tRNAs or by tRNAs with special properties.
- The reading frame can be changed by frameshifting or bypassing, both of which depend on properties of the mRNA.
The reading frame of a messenger usually is invariant. Translation starts at an AUG codon and continues in triplets to a termination codon. Reading takes no notice of sense: insertion or deletion of a base causes a frameshift mutation, in which the reading frame is changed beyond the site of mutation. Ribosomes and tRNAs continue ineluctably in triplets, synthesizing an entirely different series of amino acids.
There are some exceptions to the usual pattern of translation that enable a reading frame with an interruption of some sort?such as a nonsense codon or frameshift?to be translated into a full-length protein. Recoding events are responsible for making exceptions to the usual rules, and can involve several types of events.
Changing the meaning of a single codon allows one amino acid to be substituted in place of another, or for an amino acid to be inserted at a termination codon. Figure 7.28 shows that these changes rely on the properties of an individual tRNA that responds to the codon:
- Suppression involves recognition of a codon by a (mutant) tRNA that usually would respond to a different codon (see 7.12 Suppressor tRNAs have mutated anticodons that read new codons).
- Redefinition of the meaning of a codon occurs when an aminoacyl-tRNA is modified (see 7.8 Novel amino acids can be inserted at certain stop codons).
Changing the reading frame occurs in two types of situation:
- Frameshifting typically involves changing the reading frame when aminoacyl-tRNA slips by one base (+1 forward or ?1 backward) (see 7.17 Frameshifting occurs at slippery sequences). The result shown in Figure 7.29 is that translation continues past a termination codon.
- Bypassing involves a movement of the ribosome to change the codon that is paired with the peptidyl-tRNA in the P site. The sequence between the two codons fails to be represented in protein. As shown in Figure 7.30, this allows translation to continue past any termination codons in the intervening region.
- Bypassing Involves Ribosome Movement
Certain sequences trigger a bypass event, when a ribosome stops translation, slides along mRNA with peptidyl-tRNA remaining in the P site, and then resumes translation (see Figure 7.30). This is a rather rare phenomenon, with only ~3 authenticated examples...
- Suppressor Trnas Have Mutated Anticodons That Read New Codons
KEY TERMS:A suppressor is a second mutation that compensates for or alters the effects of a primary mutation. A nonsense suppressor is a gene coding for a mutant tRNA able to respond to one or more of the termination codons and insert an amino acid at...
- Using The Genetic Code
The sequence of a coding strand of DNA, read in the direction from 5 to 3, consists of nucleotide triplets (codons) corresponding to the amino acid sequence of a protein read from N-terminus to C-terminus. Sequencing of DNA and proteins makes it possible...
- Three Codons Terminate Protein Synthesis
KEY TERMS:The amber codon is the triplet UAG, one of the three termination codons that end protein synthesis. The ochre codon is the triplet UAA, one of the three termination codons that end protein synthesis. The opal codon is the triplet UGA, one of...
- Every Sequence Has Three Possible Reading Frames
KEY TERMS:A reading frame is one of the three possible ways of reading a nucleotide sequence. Each reading frame divides the sequence into a series of successive triplets. There are three possible reading frames in any sequence, depending on the starting...