1. Why do you share some but not all characters of each parent?
Meiosis
Why do you share some but not all characters of each parent?

2. Heredity Heredity Passing of genetic traits from parent to offspring
Chromosome theory of heredity
Chromosomes carry genes
Gene = unit of heredity

3. What Meiosis is all About
Meiosis allows the creation of unique individuals through sexual reproduction.

4. 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. Diploid
Gametes are the “sex” cells and contain only ½ the normal number of chromosomes. Haploid Sperm cells and ova are gametes.

5. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) similar in shape and size.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes
Homologous pairs (tetrads are duplicated) carry genes controlling the same inherited traits.

6. Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
eye color
locus
hair color
Paternal
Maternal

7. Autosomes In Humans the “Autosomes” are sets 1 – 22
(The Autosomes code for most of the offspring’s traits)

8. In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

9. Sex Chromosomes the 23rd set

10. Boy or Girl? The Y Chromosome Decides
X chromosome
Y chromosome

11. In The Beginning Two
Reproduction where the genetic material combined is called sexual reproduction
Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops
Meiosis is the process of producing sperm and eggs (gametes) – the number of chromosomes are halved

12. Two Important Accomplishments
1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
In meiosis, one diploid cells produces four haploid cells.

13. Two Important Accomplishments
2) Meiosis scrambles the genes that each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity.
This happens through independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and species.

14. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division –4 gamete cells with ½ the original number of chromosomes

15. Gametes Are Haploid
Gametes have exactly one set of chromosomes, this state is called haploid (1n)
Regular cells have two sets of chromosomes, this state is called diploid (2n)
Why?

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

18. Interphase I Similar to mitosis interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
Centriole pairs also replicate.

19. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
Meiosis
1st division

20. Synapsing
Unlike mitosis, homologous chromosomes line up next to each other during prophase
This process is called synapsing
Lined up homologues are called tetrads

21. Prophase I - Synapsis Longest and most complex phase. spindle fiber
90% of the meiotic process is spent in Prophase I
Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).
centrioles
spindle fiber
homologues

22. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

23. Prophase 1– Crossing Over
Homologous chromosomes connect at areas called chiasmata
Segments break and reform at similar locations.
This results in new genetic combinations

24. Crossing Over Non-sister chromatids Tetrad variation
chiasmata: site of crossing over
Tetrad
variation

25. Check out the Tetrads

26. Crossing-Over

27. Metaphase I INDEPENDENT ASSORTMENT Shortest phase
Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT
metaphase plate OR
metaphase plate

28. Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23
223 = 6,000,0000.
That’s a lot of diversity by this mechanism alone

29. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

30. Telophase I Each pole now has haploid set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

31. In Telophase 1, two daughter cells are formed. They are NOT identical
In Telophase 1, two daughter cells are formed. They are NOT identical! Why?

32. Meiosis II
No interphase II
(or very short - no more DNA replication)

34. Prophase II same as prophase in mitosis
spindle reforms and chromosomes move toward the metaphase plate

35. Metaphase II same as metaphase in mitosis
sister chromatids lined up on the metaphase plate
metaphase plate

36. Anaphase II same as anaphase in mitosis
sister chromatids are separated and pulled toward opposite poles of the cell

37. Telophase II Same as telophase in mitosis
Nuclei form, Cytokinesis occurs.
Four haploid daughter cells produced.
Telophase II

38. In Humans Example: 23 chromosomes in haploid n = 23
2n = 223 = ~ 6 million possible combinations!

39. Random fertilization
At least 6 million combinations from Mum and another 6 million from Dad …
>36 trillion combinations for a diploid zygote!!!

40. Meiosis in Male and Female
In males is called spermatogenesis and produces sperm.
In females is called oogenesis and produces ova.

41. Secondary Spermatocyte
Spermatogenesis
n=23
sperm
meiosis II
n=23
meiosis I
Primary Spermatocyte
n=23
2n=46
Secondary Spermatocyte
n=23
4 sperm cells are produced from each primary spermatocyte
n=23

42. Spermatogenesis The male gamete is called a spermatozoa
Four spermatocytes are formed during meiosis
Men are busy - meiosis produces roughly 250,000 sperm a day.

43. Oogenesis The female gamete is called an ovum
At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I.
A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of eggs.

44. Oogenesis Only one ovum is produced during meiosis
Oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop.

45. Any
Questions?