1. Ms. Henriksen’s Biology
Meiosis
Ms. Henriksen’s Biology

2. Why do cells divide instead of just growing bigger?
2 Reasons:
The larger a cell becomes, the more DNA it needs.
If a cell is very big, it is hard to move enough nutrients and wastes across the membrane to meet its needs. (not enough cell membrane surface area for diffusion)

3. Homologous chromosomes
Definition: each chromosome has a matching partner from the other parent.
One from your mother, one from your father. Both have the same genes
Example: Shoes
Homologous chromosomes are like your shoes. They are very similar but not identical.
This means you have two copies of all of your genes! This is called diploid!
Homologous chromosomes
dad
dad
When doubled it looks like
mom
mom

4. Haploid and Diploid Diploid Haploid Di = 2
dad
dad
Diploid
Di = 2
Two sets of each chromosome or gene
One from mom, one from dad
Normal cells are diploid (2N)
Haploid
Only one copy of each gene
Seen in sex cells (eggs and sperm) as a result of meiosis
Key to keeping chromosome numbers the same in all offspring - not doubling
mom
mom
Haploid/diploid

5. 1
A cell that has 46 chromosomes is known to be diploid. How many chromosomes would a haploid cell from the same organism have? 92 46 23 16 3

6. Do you remember Mitosis
Prophase
The Stages of Mitosis
Mitosis
asexual reproduction
2 identical sister cells
Diploid cells (two sets of each gene)
I PMAT C
Metaphase
Interphase
Anaphase
Telophase
cytokenisis

7. Meiosis sexual reproduction happens only in reproductive organs
4 haploid cells
Important to maintaining chromosome numbers in offspring
What would happen if diploid cells fused in reproduction?
46 chromosomes + 46 chromosomes = 92
= And so on
Gives variation to new offspring because genes are separated randomly and then parental offspring is mixed to form new fertilized cell
Meiosis what it is

8. Meiosis is like Mitosis in two rounds
All chromosomes are doubled in interphase
Parent cell – chromosome pair (diploid)
Chromosomes copied, crossing over occurs
1st division - pairs split
Meiosis I
Meiosis order
Meiosis II
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes (haploid)

9. Meiosis stages
Similar to mitosis, but chromosomes line up with homologous pairs first in Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis
Meiosis order
Mitosis

10. Facts about Meiosis Meiosis happens in reproductive organs
Ovaries, testis, pistle, stamen
Produces eggs and sperm
Chromosomes can cross over to give genetic variation. Your genes are not identical to your parents
Important for maintaining chromosomes numbers
If not for haploid cells we would add DNA with each generation
Errors in meiosis can lead to problems in offspring
Down’s syndrome is caused by an extra copy of a chromosome
Crossing-over occurs when tetrads exchange portions of their chromatids.
Meiosis where

11. Down’s syndrome Can you see the extra 21st chromosome?
Is this person male or female?
Karyograph picture

12. 2 Unlike mitosis, meiosis in male mammals results in the formation of
One haploid cell
Three diploid cells
Four diploid cells
Four haploid cells 4

13. 3
Genes for the same trait (the copies from each parent) are separated from each other during the process of
Mitosis
Meiosis I
Meiosis II
Interphase 2

14. Compare but Don’t Confuse
Mitosis Results
Two identical cells
Diploid (2N) cells
Growth and replacement of cells
Asexual reproduction
Meiosis Results
Four unique cells
Haploid (N) cells
Produces gametes (sex cells) for reproduction of offspring
Sexual reproduction
Keeps chromosome numbers the same in offspring.
Allows genetic variation (different traits) in offspring (rather than clones!)

15. 4 Which of the following is not part of meiosis Produces diploid cells
Anaphase I
Sexual reproduction
Maintain chromosome # in offspring 1

16. 5 Definition: Division of the cytoplasm during cell division Mitosis
Meiosis
Cytokenisis
Telophase 3

17. 6
Definition: threadlike structure within the nucleus that contains the organisms genes.
Spindle
Chromosome
Nuclear envelop
Homologous 2

18. 7
Meiosis is the process by which gametes are produced. In which of the following human organs does meiosis occur?
Skin
Pancreas
Liver
Testis 4

19. 8
The reduction of the chromosome number during meiosis is most important for:
A keeping the amount of DNA in the cell at a minimum level
preventing the nucleus from becoming larger with each cell division
maintaining the chromosome number during sexual reproduction
allowing the growth of the cell without increasing the DNA content 3