Abnormal Meiosis | Non-disjunction and the formation of Down syndrome

Summary

In this video, Miss Angie explains the process of abnormal meiosis, focusing on the concept of non-disjunction. She provides a recap of normal meiosis and discusses how non-disjunction leads to issues like monosomy and trisomy, where cells have either missing or extra chromosomes. Miss Angie also clarifies the phases of meiosis where non-disjunction occurs, emphasizing anaphase I and II. She highlights the real-world implications, such as Down syndrome, caused by an extra chromosome 21. Finally, she shares exam tips on how to explain these concepts clearly for full marks, and encourages viewers to subscribe and access her study resources.

Takeaways

• 😀 Normal meiosis involves halving the chromosome number in two stages to create four non-identical cells.
• 😀 Non-disjunction occurs when chromosomes fail to separate properly, often during anaphase of meiosis.
• 😀 Non-disjunction can happen in meiosis I, where homologous chromosomes fail to separate, or in meiosis II, where chromatids fail to separate.
• 😀 The result of non-disjunction can be trisomy (extra chromosome) or monosomy (missing a chromosome).
• 😀 Trisomy occurs when a gamete with an extra chromosome fuses with a normal gamete, resulting in three chromosomes of a certain type.
• 😀 Monosomy occurs when a gamete missing a chromosome fuses with a normal gamete, resulting in only one chromosome of a certain type.
• 😀 Non-disjunction can lead to conditions such as Down syndrome, caused by an extra chromosome 21 (trisomy 21).
• 😀 To explain non-disjunction in exams, always mention that the spindle fibers fail to separate chromosomes during anaphase.
• 😀 An important key to explaining Down syndrome in exams is noting that non-disjunction results in a gamete with 24 chromosomes, which leads to a zygote with 47 chromosomes instead of the normal 46.
• 😀 Non-disjunction results in a chromosomal mutation called aneuploidy, which means there are too many or too few chromosomes in a cell.
• 😀 Understanding the phases of meiosis, particularly metaphase and anaphase, is essential to explaining how non-disjunction occurs.

Q & A

What is the main purpose of meiosis?

The purpose of meiosis is to reduce the chromosome number by half, resulting in the formation of four non-identical cells, each with half the number of chromosomes of the original cell.

What is non-disjunction in meiosis?

Non-disjunction occurs when chromosomes fail to separate properly during meiosis, leading to one cell having an extra chromosome and another missing one.

When does non-disjunction typically occur during meiosis?

Non-disjunction most often occurs during anaphase of meiosis I or II, when homologous chromosomes or chromatids fail to separate properly.

What is the difference between meiosis I and meiosis II?

In meiosis I, homologous chromosomes are separated into two cells. In meiosis II, chromatids of each chromosome are separated, leading to the formation of four haploid cells.

What is the result of non-disjunction in meiosis I?

In meiosis I, non-disjunction results in homologous chromosomes failing to separate, leading to one cell with an extra chromosome and another with one less.

What is the result of non-disjunction in meiosis II?

In meiosis II, non-disjunction occurs when chromatids fail to separate, leading to two cells with abnormal chromosome numbers: one with an extra chromosome and one with a missing chromosome.

What are the three possible outcomes of non-disjunction?

The three outcomes are: 1) regular meiosis with the correct number of chromosomes, 2) monosomy, where a cell is missing one chromosome, and 3) trisomy, where a cell has an extra chromosome.

How does non-disjunction lead to trisomy or monosomy?

Trisomy occurs when a cell with an extra chromosome fuses with a normal gamete, resulting in a zygote with three chromosomes of a particular type. Monosomy occurs when a cell missing a chromosome fuses with a normal gamete, leading to a zygote with one less chromosome.

What is Down syndrome and how is it linked to non-disjunction?

Down syndrome is a condition caused by non-disjunction of chromosome 21, resulting in a person having three copies of chromosome 21 instead of the usual two. This is known as trisomy 21.

What is the role of spindle fibers during meiosis?

Spindle fibers are responsible for attaching to chromosomes and pulling them apart during meiosis. Their role is crucial in ensuring that chromosomes and chromatids are correctly separated into the daughter cells.