Question

What are silent mutations? From your knowledge of the genetic code, why do you think most silent mutations affect the third position in a codon? (Section 10.2)

Short answer

Silent mutations do not change the amino acid sequence. They often occur at the third codon position due to genetic code redundancy.

Step-by-step solution

Define Silent Mutations

Silent mutations are changes in the nucleotide sequence of DNA that do not result in a change in the amino acid sequence of the protein encoded by the gene.

Genetic Code Redundancy

The genetic code is redundant, meaning that most amino acids are encoded by more than one codon. This redundancy provides protection against mutations.

Codon Position and Silent Mutations

A codon consists of three nucleotides. Silent mutations often occur in the third position of a codon because changes in this position are less likely to alter the amino acid that is produced. This is due to the 'wobble' hypothesis, where variability in the third nucleotide has little impact on the protein.

Example

For instance, the codons GGU, GGC, GGA, and GGG all code for the amino acid glycine. A mutation in the third position of GGU to GGC does not change the amino acid, thus making it a silent mutation.

Key concepts

Genetic Code Redundancy

The genetic code has a fascinating feature known as redundancy. In simple terms, it means that most amino acids are encoded by more than one three-nucleotide codon sequence.
This redundancy acts like a safety net against mutations.
Even if a change occurs in the DNA sequence, the resulting protein might remain the same.

Here’s a fun fact: There are 64 possible codons, but only 20 amino acids. This overlap reduces the chances of a mutation affecting the protein's function.
For example, multiple codons can specify the same amino acid, such as GGU, GGC, GGA, and GGG all coding for glycine.

So, if a mutation changes one codon to another that still encodes the same amino acid, the protein remains unaffected. This clever design by nature helps minimize the risk of detrimental mutations during protein synthesis.

Codon Position

Let’s break down the importance of codon positions. A codon consists of three nucleotides, often referred to as positions one, two, and three.
Mutations can occur in any of these positions, but their effects vary widely.

The first and second positions in a codon are crucial. Changes here are likely to alter the amino acid and potentially the protein's function.
However, the third position is more forgiving. This is where the magic of silent mutations usually happens.

Consider the codon for glycine: GGU, GGC, GGA, and GGG. A mutation in the third position might change GGU to GGC, but glycine is still produced. The protein remains unchanged.
The third position is less critical due to the 'wobble' hypothesis, offering more flexibility and helping the organism tolerate mutations without adverse effects.

Wobble Hypothesis

Francis Crick introduced the 'wobble hypothesis' to explain why silent mutations frequently occur at the third codon position.
This hypothesis states that the pairing between the third base of the codon and the corresponding base of the anticodon is flexible, or 'wobbly.'

In a nutshell, the first two bases of the codon form strong pairs with the first two bases of the anticodon on the tRNA.
The third base, though, is less stringently paired, allowing more variability.
This flexibility is crucial for the cell's ability to use fewer tRNAs without compromising the precision of genetic translation.

For example, the tRNA with anticodon CCG can pair with both GGC and GGU codons, and both translate to glycine.
This 'wobble' helps the cell be both efficient and resilient against minor mutations, ensuring smooth protein synthesis.
So, thanks to this hypothesis, even when silent mutations occur, the organism's functional integrity is maintained.