1. Meiosis

2. Reproduction Two types:
1. ASEXUAL ~ single parent passes complete copy
Binary fission, fragmentation, budding, parthenogenesis
**OFFSPRING GENETICALLY SAME AS PARENT**
2. SEXUAL ~ two parents pass DNA to make genetically diff offspring
Fertilization = two gametes fuse to form zygote
Sex cells = germ cells (body cells = somatic cells)
**OFFSPRING GENETICALLY DIFF FROM PARENT**

4. Reproduction Germ cells Somatic cells
Haploid ~ has one set of chromosomes (23 for humans)
Somatic cells
Diploid ~ has two sets of chromosomes (46 for humans)
Homologous chromosomes ~ similar in size & shape & genes

5. Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the answers lie in meiosis.

6. Meiosis does two things -
1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
(This is a good idea if you’re going to combine two cells to make a new organism.) This trick is accomplished by halving chromosome number.
In meiosis, one diploid cells produces four haploid cells.

7. 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and species.

8. Discuss with the person next to you:
-Why do we need meiosis?
-Can’t we just grow with mitosis?
(Be ready to answer!)

9. Why do we need meiosis?
Meiosis is necessary to halve the number of chromosomes going into the sex cells
Why do this?
At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

10. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

11. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

12. The Stages of Meiosis:
aka: Reduction Division

13. Meiosis I : Separates Homologous Chromosomes
Interphase
Each of the chromosomes replicate
The result is two genetically identical sister chromatids which remain attached at their centromeres

14. Prophase I CRUCIAL PHASE!
During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.
Extremely IMPORTANT!!! It is during this phase that crossing over can occur.
Crossing Over is the exchange of segments during synapsis.

15. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

16. Crossing-over
During prophase I, homologous chromosomes line up next to each other. Crossing-over happens when one arm of a chromatid crosses over the arm of the other chromatid. The chromosomes break and reform.

17. Metaphase I
The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.
Still in homologous pairs

18. Anaphase I
The spindle guides the movement of the chromosomes toward the poles
Sister chromatids remain attached
Move as a unit towards the same pole
The homologous chromosome moves toward the opposite pole
Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

19. Telophase I This is the end of the first meiotic cell division.
The cytoplasm divides, forming two new daughter cells.
Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

20. Cytokinesis Occurs simultaneously with telophase I
Forms 2 daughter cells
Plant cells – cell plate
Animal cells – cleavage furrows
NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

21. In your notes, list 3 or more differences between mitosis and meiosis I.
Discuss with your partner, and be ready to share!

22. Figure 13.7 The stages of meiotic cell division: Meiosis I

23. Meiosis II : Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !

24. Prophase II
Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

25. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

26. Anaphase II The centromeres of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromatids

27. Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell and cytokinesis occurs
After completion of cytokinesis there are four daughter cells
All are haploid (n)

28. Figure 13.7 The stages of meiotic cell division: Meiosis II

29. How is meiosis II different from meiosis I
How is meiosis II different from meiosis I? Discuss and be ready to share.

30. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 = 60,000,000.
That’s a lot of diversity by this mechanism alone.

32. Mitosis vs. Meiosis

33. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and distinguishing) division of meiosis

34. Mitosis vs. Meiosis

35. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

36. Boy or Girl? The Y Chromosome “Decides”

37. Meiosis – division error
Chromosome pair

38. Meiosis error - fertilization
Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.
In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

39. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?

40. This is NOT a tiger with Down syndrome. It is inbred.

41. Summary
Two types of reproduction:
-Asexual
-Sexual
Meiosis occurs in gametes, not somatic cells; it results in genetic diversity
Meiosis has 8 parts:
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II