1. M E I O S I S

2. Why don’t cells get bigger forever??
Let’s do a lab!
With some math!
Surface area = 6 (l x w)
Volume = l x w x h

3. Calculations Cell 1 Sides: .5 cm SA= Volume = Cell 2 Sides: 1 cm SA=

4. Limits to cell growth
The larger a cell becomes, the more demands the cell places on its DNA.
In addition, the cell has more trouble moving enough nutrients and wastes across its cell membrane
Activity

5. Surface Area to volume ratio
We want surface area to be bigger then volume
WHY? (Think about the lab)

6. So how do cells solve this problem?
Before they get too large…
They divide!
Into two “daughter” cells
Called CELL DIVISION

7. What steps may be involved?

8. Two stages Stage 1: MITOSIS Stage 2: CYTOKINESIS Dividing nucleus
Dividing cytoplasm

9. Chromosome We have 46! Super condensed DNA
In mitosis, each chromosome gets copied
Now called two SISTER CHROMATIDS held together by a centromere

10. Animation 1
Animation 2

11. Sketch it out!

13. The Cell Cycle
Series of events that cells go through as they grow and divide
Grows, prepares to divide, and divides
Two main parts
Interphase
Mitosis

15. G1 - “Gap” - Cell grows from last cell cycle
Divided into 2 parts
Interphase
G1 - “Gap” - Cell grows from last cell cycle
S – Synthesis of DNA (DNA makes a copy of itself)
G2 – Cell grows and gets ready for cell division
G0 – No cell division – typical of cells like nerve cells that stop dividing at maturity
Mitosis – M phase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

16. Interphase Longest phase in the Cell Cycle
– over half the time is spent in this phase

17. Mitosis Prophase – longest phase in Mitosis
Copied chromosomes pair up. Held together by Centromere (each is called a chromatid)
Shortening up of chromosomes, makes them visible
Replicated Centrioles (of animal cells) migrate to opposite poles
Nuclear envelope & nucleolus start to break down
Spindle fibers (microtubules) extend from centrioles to centromeres

18. Metaphase – shortest part of Mitosis
Chromosomes line up on the equator (middle)

19. Anaphase
– “And away they go!”
Chromatids separate and go to opposite poles. Now called chromosomes!!
Spindle fibers shorten up

20. Telophase “the end” – Final phase Chromosomes reach opposite ends
Cell membrane pinches in (cleavage furrow) or Cell plate forms (if plants)
Chromosomes uncoil
Nuclear membrane & nucleolus reappear
Spindle fibers disassembles

21. Cytokinesis MITOSIS OVERVIEW
– Dividing cellular contents. Occurs simultaneously with Telophase
MITOSIS OVERVIEW

22. Difference between Plant and Animal mitosis
No centrioles in plant cells
A cell plate forms in plant cells

23. Difference between Plant and Animal mitosis
Animal cells have a cleavage furrow that splits the two cells

24. How do cells… Know when to Found important factors grow?
Contact prevents growing
grow?

25. Cyclin is a protein that regulates the timing of the cell cycle.
Amount of cyclin in a cell rises and falls with the steps in the cell cycle.
There are other internal and external proteins that are also involved the cell cycle.
Cancer
– uncontrolled cell growth.
Cells do not respond to normal signals and grow out of control.
Cause masses called tumors.

26. Cancer
Cancer occurs when cells do not respond to signals that tell them to stop growing

28. Count those chromosomes
We have 46
If we made a baby, how many would they have?
46 from mom 46
46 from dad
92 for baby
No longer human!! 92

29. What do we do? We go through MEIOSIS
Process that takes our two sets of chromosomes (diploid number{2N}) and cuts them into one set (haploid number{1N})
4623
23 from mom + 23 from dad = 46!
Normal baby!

30. Meiosis Steps Unlike mitosis, meiosis has two stages of division
Meiosis I
Meiosis II

31. Meiosis I: Interphase I:
Replication of chromosomes. Like chromosomes are called homologous chromosomes

32. Prophase I:
The homologous chromosomes come together (Synapsis) and form a group of 4 called a tetrad.
Twisting & exchange parts of adjacent chromatids will occur – Crossing over.
This results in genetic recombination and unlinks genes that are located on the same chromosome.
HUGE GENETIC VARIETY!!!!!

33. Anaphase I & Telophase I
Chromosomes separate independently of each other.
This is known as Independent assortment.
Results in…
Random separation of chromosomes
Genetic recombination!!!!!

34. At the end of Meiosis I, there are 2 - 1N cells
No interphase
Meiosis I
Results in 2 - 1N cells
Meiosis II
Results in 4-1N cells

35. At the end of Meiosis II, there are four 1N cells.
Occurs immediately following Telophase I.
NO INTERPHASE II !!
1N cells immediately go into Prophase II, then Metaphase II, Anaphase II, and Telophase II
Split into four 1N cells, each with unpaired chromosomes
Haploid
At the end of Meiosis II, there are four 1N cells.

37. Meiosis Animations
John Kryk
Cells Alive
You Tube
McGraw
Arizona
How Mitosis and Meiosis Differ

38. Gametogenesis Creation of gametes (sex cells)
Spermatogenesis (testes)
Makes 4 haploid sperm cells from 1 - 2N primary sperm cell. Looks like normal Meiosis
Oogenesis (ovaries)
Produces ovum (eggs) from 1 diploid primary egg cell. Difference occurs during cytokinesis during meiosis I and meiosis II.
Unequal distribution of the cytoplasm resulting in 1 large ovum and 3 small polar bodies.
Polar bodies serve no function in animals but are needed in to plant embryos.

39. 1st Meiotic Division
2nd Meiotic Division

40. Comparing mitosis and meiosis
Be sure to know…
How many divisions for each?
How many starting cells?
How many cells at the end?
What N are those cells?