点突然変異とフレームシフト突然変異

Silent mutations alter a DNA base pair but do not change the amino acid sequence due to genetic code redundancy. For example, ACC and ACA both code for threonine. Missense mutations change a single amino acid in the protein, such as TCC to TGC, substituting serine with cysteine, potentially altering protein structure and function.

Point mutations involve a single base pair alteration, while frameshift mutations result from insertions or deletions not in multiples of three. Frameshift mutations shift the reading frame, completely altering downstream codons and typically producing dysfunctional proteins. Single-base frameshifts are especially disruptive, whereas larger insertions or deletions may modify amino acid sequences or cause gene loss.

Nonsense mutations convert an amino acid codon into a stop codon, causing premature termination of protein synthesis. This results in truncated, nonfunctional proteins that cannot perform their intended cellular roles. The severity depends on where the stop codon is introduced and whether the protein's critical domains are affected.

Reversion mutations restore the original DNA sequence, completely reversing the initial mutation. Suppressor mutations introduce a second change at a different genetic location that compensates for the original mutation without reversing it. Both mechanisms can restore protein function, though suppressor mutations work by offsetting the initial mutation's effects rather than eliminating it.

Large insertions or deletions can cause gene loss or severely disrupt protein structure and function. When essential genes are affected, the organism cannot produce critical proteins necessary for survival. This makes such mutations lethal, as the cell or organism cannot maintain basic life processes without functional essential gene products.

The genetic code is redundant, meaning multiple codons encode the same amino acid. This redundancy allows silent mutations to occur when a base change produces a different codon for the same amino acid. For instance, both ACC and ACA code for threonine, so a mutation between them leaves protein function unchanged, demonstrating how genetic code redundancy buffers against some mutations.

Point mutations in bacterial genes can alter protein structure, potentially reducing antibiotic binding or enabling drug inactivation. These mutations may confer survival advantages in antibiotic-rich environments, driving the development of antibiotic resistance. Understanding how mutations contribute to resistance is critical for managing bacterial infections and predicting resistance patterns.