CR: Meiosis Lesson

Meiosis Miracles!

What is the following picture telling you about?

If you said fertilization, you are correct!

Do you have the same eye color as your parents? Hair color? Why is it that you share some traits with your parents, but not the same ones as your brother or sister (unless you are identical twins)? The process of meiosis and fertilization will answer these questions and much more.

So What is Meiosis?

Watch the presentation below to learn all about meiosis.

Remember those Stages of Meiosis?

Let's take a closer look now. Click on each stage to learn more.

Review of Chromosome Number: Haploid vs. Diploid 

In humans, all body cells (except the sex cells) have 46 chromosomes or 23 pairs of homologous chromosomes

• These cells are called somatic cells
• Somatic cells are called diploid (di means two) because they have two sets of chromosomes
• The gametes (sex cells) have half the number of chromosomes
• The gametes are called haploid cells because they have half the number of chromosomes

Problems with Meiosis

Sometimes meiosis does not proceed perfectly. When sister chromatids do not separate correctly, and the resulting cells do not receive the correct number of chromosomes, we call this nondisjunction.

If a cell has one too many chromosomes (3 copies of the chromosome) the condition is called trisomy. If a cell is lacking a chromosome (it only contains one of the two expected) it is called monosomy.

There are a number of genetic conditions that result from nondisjunction, including:

• Trisomy 21 or Down Syndrome
• Trisomy 23 or Klinefelter Syndrome: XXY
• Monosomy 23 or Turner Syndrome: XO

Notice that we name the condition based on how many chromosomes (trisomy, monosomy) and which chromosome is lacking/extra (21, 23).

Let's Walk Through This Again

Use the video below for an overview of Meiosis.

Still trying to figure it out? Watch the playlist below and be prepared to answer the following questions:

1. Why is meiosis called "reduction division?"
2. How many times does interphase occur?
3. What are the stages of meiosis?
4. What is crossing over?
5. How many divisions happen in meiosis?
6. Are the gametes (sperm/egg) produced by meiosis identical?
7. What is non-disjunction?

Ready to answer those questions? Here we go!

1. Why is meiosis called "reduction division?"
   • the ending cell has 1/2 the number of chromosomes as the starting cell.
   • In humans, the starting cell has 46 chromosomes and the ending cell has 23 chromosomes.
2. How many times does interphase occur?
   • once
3. What are the stages of meiosis?
   • Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II
4. What is crossing over?
   • transfer of genetic information between homologous chromosomes in Prophase I
5. How many divisions happen in meiosis?
   • two: Meiosis I and Meiosis II
6. Are the gametes (sperm/egg) produced by meiosis identical?
   • no
7. What is non-disjunction?
   • When chromosomes don't separate correctly

Meiosis Challenge and Before You Go

Practice Time! Use the knowledge you learned from the lesson to complete the practice activities below.

Before You Go You Need To Know

The following key points are from the explore section of the lesson. You must know the following information before moving to the next lesson. This is just a summary of the key points.

1. What is the purpose of meiosis?
   • To create cells with half the number of chromosomes as the original cell.
2. What are the two differences between mitosis and meiosis?
   • Mitosis occurs in one stage, while meiosis occurs in two stages.
   • Two cells result at the end of mitosis, while four cells result at the end of meiosis.
3. What is the difference between diploid and haploid cells?
   • Diploid cells have 46 chromosomes (23 homologous pairs).
   • Haploid cells have one kind of each chromosome (23 chromosomes).

IMAGES CREATED BY GAVS