Do All of Our Cells Divide the Same Way?

The way that gametes are formed is called meiosis.


Meiosis

Think about this: Every sexual organism gets half of its DNA from its mother and half from its father. How does that happen? If every parent gave its offspring all of its DNA, then the offspring would have double what it needs. That’s why there’s meiosis.

Meiosis is just like mitosis except for a couple of important things. Mitosis happens in all of the cells except for the sex cells. In humans, these are the sperm and egg cells; these cells divide by the process of meiosis. In a human being, meiosis begins with one cell and ends with four. Each cell starts with 46 chromosomes and ends with 23 chromosomes!

Just like mitosis, there are five phases, but the important difference is that those five phases happen twice. The first time they happen, they are called prophase I, metaphase I, anaphase I, and telophase I. Interphase of meiosis actually doubles the number of chromosomes before meiosis starts, so prophase I happens with two copies of each chromosome, for a total of 92 chromosomes (in a human). At the end of telophase I, each cell has 46 chromosomes.

In the second half of meiosis, the phases are called prophase II, metaphase II, anaphase II, and telophase II. This time, each cell begins with 46 chromosomes in prophase II, and like mitosis, ends with 23 chromosomes. Unlike mitosis, however, the chromosomes do not make copies of themselves. The reason that this is important is because these 23 chromosomes will combine with 23 from another cell. If this is a sperm cell, it will combine with an egg to form 46 total chromosomes.

We call the cell that is normally found in the body of a human a diploid cell because it has two copies of each homologous chromosome. We can also represent this by saying that it is “2n”. A haploid cell has only one copy of each homologous chromosome, represented with “n”. Body cells are diploid, but sperm and egg cells are haploid.

Questions
Remember
1. In which cells does meiosis happen?
2. What is the difference between diploid and haploid?
3. What human cells are haploid?
Put it together
4. Complete a compare/contrast chart comparing meiosis to mitosis.
5. Complete a cycle chart for meiosis. Include both the name of the phase and a summary of what happens in that phase.
Think about it
6. Draw a cell going through meiosis. Assume that the cell normally has 6 chromosomes, so that it will have twelve in Prophase I and three chromosomes by the end of Telophase II.
Review
7. Explain the “butterfly effect” in your own words.
8. How are diversity and adaptation related?
9. What is the role of chloroplasts?
10. Identify one word for each phase of mitosis that will help you remember what happens in that phase.
Cell Division
  1. Choose a living thing other than humans. Find out how many chromosomes this animal has (using books, the chart below, or the internet) and make a model (it must not be a drawing) for the following phases of mitosis:
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase
  2. Respond:
    1. How many chromosomes does the final cell have?
    2. How many would it have had if it had undergone meiosis?
    3. Where does mitosis not occur?
Organism Number of chromosomes
House Fly 12
Fruit Fly 8
Mosquito 6
Pea 14
Lettuce 18
Kangaroo 12