Lecture Notes: X Inactivation

Introduction

• Topic: X inactivation
• Context: Difference in X chromosomes between females (two) and males (one)

Purpose of X Inactivation

• Function: Prevents female somatic cells from producing twice as many gene products as a male cell
• Discovery: First discovered by geneticist Mary Lyon
  • Sometimes referred to as lyonization

Timing of X Inactivation

• Takes place in female mammalian somatic cells
• Occurs in early embryonic stem cell stage
• Female zygote undergoes several cell divisions until one X chromosome is randomly inactivated

Molecular Mechanism of X Inactivation

• Inactivated X chromosome is highly methylated
  • Makes it transcriptionally inactive
• Inactivated X chromosome is called Barr body
  • Tightly packed, condensed DNA (heterochromatin)

Summary

• Histone modifications lead to the structure of the Barr body
• All transcripts in a female cell originate from the activated X chromosome

Conclusion

• Understanding X inactivation is crucial for realizing how gene dosage is regulated in female mammals.