1. GLT2 – I can explain how an offspring obtains DNA from its parents.

2. Can we use mitosis to make more people?
Journal
Can we use mitosis to make more people?

3. Organisms that reproduce sexually are made up of two different types of cells.
Somatic Cells are “body” cells and contain the normal number of chromosomes. Examples would be … skin cells, brain cells, etc.
Sex cells contain only ½ the normal number of chromosomes…. Sperm cells and egg cells are sex cells.

4. Sex cells
The Male Sex Cell is the sperm and is produced in the Testes.
The Female Sex Cell is the egg and is produced in the Ovaries.

5. Fertilization egg zygote
The joining of a sperm and egg to form a zygote.
A zygote is a fertilized egg that will grow into a baby.
egg
sperm
zygote

6. Chromosomes
A zygote is made from 2 cells (one from mom and one from dad coming together).
If each cell had the full number of chromosomes (46 in humans), then what would the zygote have?
To solve this problem, sex cells are made with only half of the chromosomes (humans: sperm = 23 and egg = 23)
When they combine, the new zygote will have the right number (humans: 23+23=46). After going through mitosis millions of times, you get a baby!

7. Humans have 46 chromosomes. They make 23 pairs from the parents.
From mom
From dad

8. Chromosomes pairs
eye color
gene
hair color
from dad
from mom

9. Homologous Chromosomes
Pair of chromosomes (from mom and from dad) that are similar in shape and size.
Homologous pairs carry genes controlling the same inherited traits.
Number of fingers
Number of fingers
From dad
From mom

10. So now what?
We’ve discussed what chromosomes humans have in all of their cells.
Review: 23 chromosomes from mom’s egg + 23 chromosomes from dad’s sperm
Review: These 23 from each form pairs called homologous chromosomes
How do people make sperm and eggs with the right number of chromosomes?

11. In humans, we must go from 46 to 23.
Meiosis is the process by which sex cells are made - they have half the number of chromosomes as somatic cells (every other cell in your body)
In humans, we must go from 46 to 23.
If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. Cells need EXACTLY the right number of chromosomes

12. Meiosis Meiosis is two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes (DNA replication).

13. Interphase I Similar to mitosis interphase.
Chromosomes replicate (make a copy).

14. Meiosis I (four phases)
prophase I
metaphase I
anaphase I
telophase I

15. Prophase I Longest and most complex phase.
90% of the meiotic process is spent in Prophase I
Chromosome copies form an X – like in mitosis!
Homologous Chromosomes come together making a group of 4 called a tetrad
Nucleus breaks down

16. Prophase I Homologous chromosomes Blue eyes Green eyes Blue eyes
Chromosome and copy
Tetrad

17. Prophase I
Homologous
Chromosomes
(Tetrad)

18. Metaphase I Homologous Chromosomes (Tetrad) OR
Tetrads line up in the middle OR

19. Chromosome copies remain attached.
Anaphase I
Tetrads split up and each homologous chromosome with its copy go towards the poles.
Chromosome copies remain attached.

20. Telophase I Each side now has half of the tetrad.
Cytokinesis occurs and two daughter cells are formed.
These cells are NOT like the result of mitosis. The chromosomes have not separated from their copies.

21. Telophase I

22. Remember: Meiosis II is similar to mitosis
No interphase II
Remember: Meiosis II is similar to mitosis

23. Prophase II
same as prophase in mitosis
Chromosomes still in an X

24. Metaphase II same as metaphase in mitosis
Chromosome and copies line up in the middle

25. Anaphase II
same as anaphase in mitosis
Copies separate

26. Telophase II Same as telophase in mitosis. Nuclei form.
Cytokinesis occurs.
Remember: four daughter sex cells produced with half the number of chromosomes as the starting cell.

27. Telophase II

28. Questions?
Meiosis Song

29. Question:
A cell containing 20 chromosomes at the beginning of interphase I of meiosis would produce sex cells containing how many chromosomes?

30. Answer:
10 chromosomes

31. Question:
A cell containing 40 chromosomes at the beginning of meiosis I would produce sex cells containing how many chromosomes?

32. Answer:
10 chromosomes