Chromosomal disordersWe’ve talked about trisomy, monosomy, and the wacky chromosome numbers in cancer cells.

But how do these arise?

Answer: Nondisjunction!

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division

This is how trisomy & monosomy occur!Can occur in mitosis or meiosis - Which one is worse in humans?

Nondisjunction

Mitosis - A single somatic cell in your entire body of somatic cells is affected.

The cell will die thanks to the cell cycle checkpoints, BUT if the cell cycle fails, it will keep dividing, making more messed up cells AKA a tumor!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Meiosis - Gametes are affected! The affected egg or sperm will be used to make a completely new human. All of those new human’s cells arise from that first zygote cell.

If the fist zygote cell has the wrong number of chromosomes, all future cells will, too!

Vocab

Germ cell - the cell used at the beginning of meisosis, divided twice to create gametes (egg or sperm)

2 types of nondisjunction (meiosis)

Nondisjunction I and nondisjunction II

→ These occur at different stages of meiosis…

Q: Which stage in meiosis and mitosis is responsible for dividing the chromosomes evenly between the two new cells forming?

Nondisjunction I

Abbreviated view

Nondisjunction I

- Occurs during meiosis I- All 4 daughter cells affected

- 2 have one extra copy of a chromosome- 2 have no copy of a chromosome

Nondisjunction II

- Occurs during meiosis II- Only 2 daughter cells affected

- 1 has one extra copy of a chromosome- 1 has no copy of a chromosome

Nondisjunction II

- The effect on the cell/organism depends on which number chromosome is missing/added

- If extra copy of chromosome 21 = Down’s syndrome- Only monosomy seen in humans is Turner’s

Syndrome (female with one X chromosome)- But most chromosomes duplicates/deletions are

lethal

Effects of nondisjunction

Nondisjunction practiceDraw it out! 1. Imagine that during meiotic division of one germ cell in a man,

nondisjunction I occurs. What will be the resulting chromosome numbers be in the offspring after these sperm fertilize “normal” eggs?

2. Imagine that during meiotic division of one germ cell in a man, nondisjunction II occurs. What will be the resulting chromosome numbers be in the offspring after these sperm fertilize “normal” eggs?

3. Which type of nondisjunction is worse? Explain why.