68. DNA Mutations

A mutation is defined as a change in the DNA base sequence. These changes can occur spontaneously, particularly during DNA replication before cell division.

1. Causes of Mutations

While many mutations happen by chance, certain factors can increase the risk:

• Carcinogens: Harmful chemicals, such as those found in cigarette smoke.

• Radiation: High-energy waves like X-rays or gamma rays.

2. Types of Mutations

There are three primary ways the DNA sequence can be altered:

• Substitution: One base is replaced by a different base (e.g., a G changes to a T). This may change a single amino acid in the resulting protein.

• Insertion: An extra base is added into the sequence. This is often more damaging because it causes a frame shift, pushing all subsequent bases along and changing every following codon.

• Deletion: A base is removed from the sequence. Like insertions, this causes a frame shift, altering all subsequent triplets in the gene.

3. Consequences of Mutations

The effect of a mutation depends on where it occurs and how it alters the genetic code.

Protein Alteration

If a mutation changes the sequence of amino acids, the resulting protein may fold into a different shape.

• Enzymes: A change in the active site's shape may prevent the substrate from fitting, meaning the enzyme can no longer catalyze its reaction.

• Structural Proteins: The mutation could weaken the protein, making it less effective at providing strength to tissues.

Non-Coding DNA

Most mutations occur in non-coding DNA, which does not provide instructions for proteins.

• Some of this DNA controls gene expression (turning genes on or off).

• A mutation here might cause a protein to be produced when it shouldn't be, or prevent its production entirely (e.g., a nerve cell accidentally producing hemoglobin).

Neutral Mutations

Many mutations have no significant effect because they only change the protein slightly, allowing it to function normally, or they occur in regions of DNA with no known function.

Summary Table: Mutation Types