Mutations, Insertions, Deletions

1. Definition of Mutations

• A mutation is a change in the DNA base sequence.

• Changes occur in the sequence of letters (A, T, C, G) that make up DNA.

2. Causes of Mutations

• Spontaneous mutations occur naturally, especially during DNA duplication before cell division (mitosis).

• Factors that increase mutation risk:

  • Carcinogens (harmful chemicals, e.g., in cigarette smoke)

  • Radiation (e.g., X-rays, gamma rays)

• Note: These factors increase the likelihood of mutations but do not always cause them.

3. Mutations and Proteins

• DNA sequences in a gene determine the sequence of codons (triplets of bases), each coding for a specific amino acid.

• Amino acids combine in order to form a polypeptide chain, which folds into a functional protein.

• A mutation can:

  • Change a codon, altering the amino acid.

  • Change the protein’s shape or function.

  • Potentially disrupt enzyme activity by preventing substrate binding.

4. Effects of Mutations

• Many mutations have no significant effect because:

  • Minor changes in proteins often do not disrupt function.

  • Most mutations occur in non-coding DNA, which does not code for proteins.

• Some non-coding DNA plays a role in gene expression, controlling whether genes are turned on or off.

5. Types of Mutations

5.1 Substitution Mutations

• One base is replaced by another.

• Can alter the amino acid coded by the codon.

5.2 Insertion Mutations

• An extra base is inserted into the sequence.

• Causes a frameshift, altering all subsequent codons.

• Results in a completely different amino acid chain after the insertion.

5.3 Deletion Mutations

• A base is removed from the sequence.

• Also causes a frameshift, changing all subsequent codons.

• Alters the amino acid sequence and potentially the resulting protein function.

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