10 Questions
What is the primary reason why the addition or deletion of three base pairs left the distal portion of the gene with the wild-type reading frame?
Each codon contains three nucleotides.
What is the result of chemically synthesized mRNA molecules containing repeating dinucleotide sequences?
The synthesis of copolymers with alternating amino acid sequences.
What is the characteristic of the genetic code that allows for consecutive reading of codons?
Comma-free.
Which amino acids are specified by only one codon?
Methionine and tryptophan.
What is the term for the occurrence of more than one codon per amino acid?
Degeneracy.
How do the multiple codons specifying a given amino acid usually differ?
By only one base, the third or 3’ base of the codon.
What would be the result of a frameshift mutation, where a single base pair is inserted or deleted?
The reading frame would be shifted, resulting in a different amino acid sequence.
What is the significance of trinucleotides in protein synthesis?
They are sufficient to stimulate specific binding of aminoacyl-tRNAs to ribosomes.
What is the characteristic of the genetic code that allows for multiple codons to specify the same amino acid?
Degeneracy.
Which of the following statements is true about the genetic code?
It is non-overlapping, with each nucleotide belonging to only one codon.
Study Notes
Three Nucleotides per Codon
- Twenty different amino acids are incorporated into polypeptides during translation, requiring at least 20 different codons with the four bases available in mRNA.
- Francis Crick and colleagues published evidence in 1961 supporting a triplet code, using genetic analysis of mutations induced by the chemical proflavin.
Properties of the Genetic Code
- The genetic code is ordered, with multiple codons for a given amino acid and codons for amino acids with similar chemical properties being closely related, usually differing by a single nucleotide.
- The genetic code contains start and stop codons, with specific codons used to initiate and terminate polypeptide chains.
- The genetic code is nonoverlapping, with each nucleotide in mRNA belonging to just one codon, except in rare cases where genes overlap.
- The genetic code is comma-free, with no commas or other forms of punctuation within the coding regions of mRNA molecules.
- The genetic code is degenerate, with all but two of the amino acids specified by more than one codon.
Degeneracy of the Genetic Code
- The occurrence of more than one codon per amino acid is called degeneracy.
- All amino acids except methionine and tryptophan are specified by more than one codon.
- Three amino acids—leucine, serine, and arginine—are each specified by six different codons.
- Isoleucine has three codons, and the other amino acids each have either two or four codons.
- The degeneracy in the genetic code is not at random, but is highly ordered, with multiple codons specifying a given amino acid differing by only one base, the third or 3’ base of the codon.
Codon Structure and Reading
- A single base-pair addition or deletion will alter the reading frame of the gene and mRNA for that portion of the gene distal to the mutation.
- The codon structure can be read in different ways, such as AAA, GGG, CCC, TTT, or AA, AGG, GCC, CTT, T.
- Codons can be grouped into two categories, plus (+) and minus (-), based on whether they result from additions or deletions.
Initiation and Termination Codons
- In both prokaryotes and eukaryotes, the codon AUG is used to initiate polypeptide chains.
- In rare instances, GUG is used as an initiation codon.
- Specific codons are used to terminate polypeptide chains.
Learn about the structure of codons and how they relate to amino acid translation in molecular biology. Discover how the four bases in mRNA form codons and the role of mutagenic agents in genetic analysis.
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