Mutations and Protein Structure

Meaning of mutation:

• Mutation is a permanent, heritable change in the DNA sequence.
• Since DNA carries the code for protein synthesis, any change in DNA may alter the protein produced.
• Mutations are an important source of variation and may be harmless, harmful, or occasionally beneficial.

Why mutations occur:

• DNA replication is highly accurate, but small errors may still occur during copying.
• Some mutations arise naturally on their own, while others are caused by mutagens such as ultraviolet rays, X-rays, and chemicals.
• Cells have proofreading and repair systems, but if an error escapes repair, it becomes a permanent mutation.

Types of mutations:

1. Base substitution / point mutation: one base pair is replaced by another
2. Insertion: one or more nucleotides are added to the sequence.
3. Deletion: one or more nucleotides are removed from the sequence.
4. Frame-shift mutation: caused mainly by insertion or deletion, shifting the reading frame of codons from the point of mutation onwards.

• The sequence of bases in DNA determines the sequence of codons in mRNA, and codons determine the sequence of amino acids in a protein.
• If the DNA sequence changes, the amino acid sequence may also change, which can affect protein folding, stability, and function.
• Some mutations have little effect, while others can severely damage protein activity.

• Because the genetic code is degenerate, more than one codon may code for the same amino acid.
• Therefore, some base substitutions do not change the amino acid; these are called silent mutations.
• In contrast, frame-shift mutations usually have a greater effect because they alter many codons after the mutation site.

Important disease examples:

• Sickle-cell anaemia is caused by a single base change in the gene for the beta chain of haemoglobin.
• This changes the sixth amino acid from glutamic acid to valine, producing abnormal haemoglobin and sickle-shaped red blood cells.
• Thalassaemia is linked to mutations affecting globin chain formation; the study material connects it with a frameshift mutation in the beta chain.

Link between mutation and phenotype:

• A mutation in DNA may change a codon.
• The changed codon may insert a different amino acid into the growing polypeptide chain.
• This may change protein shape and function, leading to altered traits or disease.

• Actinomycin inhibits mRNA synthesis.
• Chloramphenicol inhibits peptidyl transferase.
• Erythromycin inhibits translocation.
• Neomycin interferes with initiation and causes misreading.
• Streptomycin causes misreading of the genetic code.
• Tetracycline blocks the binding of aminoacyl tRNA to the ribosome.

• Mutation is a heritable, permanent change in the DNA sequence that may alter gene function.
• DNA replication is highly accurate but not perfect; errors and mutagens (UV rays, X‑rays, and chemicals) can produce mutations.
• Point mutations change single base pairs, while insertions and deletions can cause frame‑shift mutations that disrupt many downstream codons.
• Because the genetic code is degenerate, some point mutations are silent, but others can significantly change protein structure and function.
• Sickle‑cell anaemia is a key example where a single base change in the beta chain of haemoglobin alters an amino acid and produces disease.