Chromosomes 3

Introduction

• Good morning, everyone, and thank you for attending the lecture before spring break.

Quiz and Grading Reminders

• Quiz will be passed back on Sunday.

• Grading for combination quizzes is almost complete; they should be available in the next day.

• Emphasis on three-point mapping as there will be a related question worth 20 points in the next exam.

• Resources available on Canvas for those struggling with three-point mapping, including videos and step-by-step guides.

Upcoming Topics

• Today’s focus: chromosome rearrangements.

• Discussion on changes in chromosome parts, including deletions, insertions, and inversions.

• Importance of understanding what type of rearrangement is involved: balanced vs. unbalanced.

Chromosome Rearrangements Overview

Types of Rearrangements

• Balanced Rearrangements: genetic information is neither gained nor lost (e.g., inversions, translocations).

• Unbalanced Rearrangements: genetic information is gained or lost (e.g., deletions, duplications).

Key Takeaways

• Changes in chromosomes activity occur during meiosis, affecting gametes and potential offspring.

• Importance of centromere in maintaining genetic information; without it, information is lost.

Mechanisms of Chromosome Rearrangement

• Rearrangements often caused by random breakages in DNA due to mechanical stress rather than specific enzymes.

• Recovery of rearrangement depends on the presence of a centromere; without it, the chromosome segment cannot be captured by microtubules during meiosis.

Balanced Rearrangements

Inversions

• Definition: segments of a chromosome are flipped (e.g., breaking between genes b and c).

• Results in the same gene content but rearranged order.

• Inversions can be categorized as:

  • Paracentric: centromere is not involved.

  • Pericentric: centromere is included.

Implications of Inversions

• Formation of inversion loops during meiosis to allow pairing of chromosomes.

• May lead to lower recombination frequencies due to tighter chromosome pairing during tetrad formation.

• Gene fusions resulting from inversions could lead to oncogenes and cancer.

Unbalanced Rearrangements

Deletions

• Result in loss of genetic information, leading to potential non-viability of gametes.

Duplications

• During meiosis, segments can be duplicated, potentially disturbing gene balance but varying in severity based on the genes affected.

  • Can lead to deletion loops if sections of the chromosome are lost.

Examples of Unbalanced Rearrangements

• Krabbe syndrome: significant deletions can lead to severe birth defects.

• MECP2 duplication syndrome: caused by duplications relevant to brain development.

Translocations

Definition

• Segments are exchanged between non-homologous chromosomes.

• Reciprocal translocations: genes are traded back and forth between chromosomes.

Implications of Translocations

• Can result in reduced fertility due to the complex pairing patterns caused by chromosomal changes during meiosis.

• Two types of segregation during gamete formation can lead to either viable or non-viable gametes.

  • Alternate segregation leads to normal gametes, while adjacent segregation leads to non-viable gametes.

Conclusion

• Importance of understanding how chromosome rearrangements affect genetic information and offspring viability.

• Reminder to enjoy spring break and practice sun safety.