Non-Disjunction Can Occur in Anaphase I and II
Definition
Non-Disjunction
The failure of homologous chromosomes or sister chromatids to separate properly during meiosis.
- Non-disjunction can occur in two critical stages of meiosis:
- Anaphase I: When homologous chromosomes fail to separate.
- Anaphase II: When sister chromatids fail to separate.
To remember the stages of meiosis, think of the acronym PMAT: Prophase, Metaphase, Anaphase, Telophase.
Down Syndrome is Caused by Non-Disjunction
- Down syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21 instead of the usual two.
- This extra chromosome arises from non-disjunction during meiosis in one of the parent’s gametes.
- When a gamete with an extra chromosome 21 fuses with a normal gamete during fertilization.
- This results in a zygote with 47 chromosomes instead of the typical 46.
- Imagine a cell in Anaphase I of meiosis.
- Instead of the homologous chromosomes separating evenly, both copies of chromosome 21 move to the same pole.
- This error results in one gamete with an extra chromosome 21.
- If this gamete participates in fertilization, the zygote will have three copies of chromosome 21, leading to Down syndrome.
Characteristics of Down Syndrome
- Physical Traits:
- Distinctive facial features (e.g., almond-shaped eyes, flat nasal bridge).
- Short stature and low muscle tone.
- Health Challenges:
- Heart defects.
- Hearing and vision problems.
- Cognitive Effects:
- Mild to moderate intellectual disability.
- Delayed developmental milestones.
Why Does Non-Disjunction Occur?
- The exact causes of non-disjunction are not fully understood, but several factors increase its likelihood:
- Maternal Age: The risk of non-disjunction rises significantly with maternal age, especially after age 35.
- Environmental Factors: Exposure to certain chemicals or radiation may increase the risk.
- Cellular Errors: Issues with the spindle fibers or the cohesin proteins that hold chromatids together can lead to non-disjunction.
- How does our understanding of non-disjunction challenge the idea of "perfection" in biological systems?
- Consider how errors in meiosis contribute to genetic diversity and evolution.
Implications of Non-Disjunction Beyond Down Syndrome
- Non-disjunction can affect other chromosomes, leading to conditions such as:
- Klinefelter Syndrome (XXY): An extra sex chromosome.
- Turner Syndrome (XO): A missing sex chromosome.
- Can you explain how non-disjunction during meiosis I differs from non-disjunction during meiosis II?
- What are the outcomes in each case?
