Medical Genetics · MU Plovdiv

3. Gene mutations. Mutagenic factors. Importance of mutagenesis.

Inside: 13 sections · 11 labelled pictures · 2 tables · 25 flashcards · 12 min

Topic 3 of 33 in the MU Plovdiv syllabus for Medical Genetics (Medicine, Year 4). The free AceMed note covers it in 13 sections, about 1,601 words (12 min of reading), with 25 flashcards.

In this note

  1. Introduction
  2. What are gene mutations?
  3. Classification of gene mutations
  4. Mechanisms of gene mutations
  5. Mutagenic factors (RICH N)
  6. Effects of gene mutations on protein structure and function
  7. Mutagenesis
  8. Germline versus somatic mutations
  9. Mosaicism
  10. Classification of mutations in the human genome
  11. Monogenic defects
  12. Phenotypic expression of mutations
  13. Importance of mutagenesis and clinical relevance

Introduction

Gene mutations are changes in the DNA sequence, typically of one or a few nucleotides, that arise without the cell's intentional control. This topic covers their classification, mechanisms, the mutagenic factors that provoke them, their effects on protein structure and function, mutagenesis itself, germline versus somatic mutations, mosaicism, the classification of mutations in the human genome, phenotypic expression and clinical relevance.

What are gene mutations?

  • Gene mutations are changes in the DNA sequence (one or a few nucleotides) that occur without the cell's intentional control.
  • They are the source of genetic variation and play a crucial role in evolution: they create new genetic variants on which natural selection can act.
  • Many mutations appear after the cell tries to repair chemical injuries to the DNA. The DNA may survive the repair with a changed sequence, but it cannot survive the original damage.

Classification of gene mutations

A. By cell type affected and inheritance ("So Germy!")

  • Somatic mutations occur in body cells and are not passed on to offspring (unless reproduction is asexual).
  • Germ line mutations occur in reproductive cells and can be inherited by the next generation.

B. By effect on fitness (HBN = Harmful, Beneficial, Neutral)

  • Harmful mutations reduce the chances of survival or reproduction (e.g. cancer).
  • Beneficial mutations improve fitness and offer a survival advantage (e.g. resistance).
  • Neutral mutations do not affect fitness but may still be passed on. They form the basis of the molecular clock because of their steady rate of occurrence.
  • The effect is context-dependent: the same mutation can be neutral in one environment and beneficial or harmful in another.

C. By structural change in DNA (SID)

  • Substitution: one nucleotide replaced by another.
  • Insertion: an extra nucleotide is added.
  • Deletion: a nucleotide is removed.

D. By cause

  • Spontaneous mutations occur naturally due to internal factors (e.g. replication error).
  • Induced mutations are caused by external agents called mutagens (e.g. radiation, chemicals).

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