1. Meiosis

3. Today: We are going to start talking about the process of meiosis.
You will take notes first, then you will have an activity to work on.

6. Figure 13.9 Independent Assortment

7. I. Heredity
Heredity - the transmission of traits from one generation to the next
Genetics – the scientific study of heredity and hereditary variation
Genes – hereditary units endowed from parents
Segments of DNA
Divided into Chromosomes
46 in humans
A gene’s specific location on a chromosome is called its locus

8. II. Reproduction – 2 modes
Asexual reproduction – a single individual is the sole parent and passes copies of all its genes to its off spring
Sexual reproduction – two parents give rise to offspring that have unique combinations of genes inherited from each parent

9. 1. Asexual Reproduction 1 parent Binary Fission in bacteria
Single cell eukaryotes : mitotic cell division
DNA is copied and divided equally between daughter cells
Multicellular organisms – Budding
Hydra : Buds break off – are genetically identical to its parent
Each offspring in asexual reproduction is called a Clone

10. Figure 13.1 The asexual reproduction of a hydra

11. 2. Sexual Reproduction 2 parents
Results in greater variation than asexual reproduction
Offspring vary genetically from siblings and both parents
Behavior of chromosomes during the sexual lifecycle

12. Figure Two families

13. Human Lifecycle
Somatic cells (any cell but sperm or ovum cells) have 46 chromosomes
Can be visualized with a light microscope during mitosis
Are two of each type
Arranged in pairs
Karyotype –ordered display of an individuals chromosomes
Homologous chromosomes (homologues) – chromosomes that make up a pair that have the same length , centromere position and staining pattern

14. Figure 13.3 Preparation of a human karyotype

15. Human Lifecycle Autosomes – somatic chromosomes
If a gene for a trait is located at a particular locus on a certain chromosome, then the homologue of that chromosome will also have a gene for the same trait at the same locus
EXCEPTION: SEX CHROMOSOMES
X and Y – only a small part are homologous
Y is much shorter than the X
X has few Y counterparts , Y is lacking many X genes
XX (female) XY (male)

16. Karyotype
The occurrence of homologous pairs of chromosomes in our karyotype is a consequence of our sexual origins
A maternal set (23) and a parental set (23)

17. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

18. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

20. Next: Use the list of important terms to create flash cards.
Simply, fold a piece of paper into 6-8 pieces and cut them out.
Write the term on one side, and the definition on the other/
We have covered all terms except for the meiosis ones.

21. III. Chromosome Types
Autosome= chromosome that contains genes for characteristics not directly related to the sex of the organism. (#1-22)
Sex Chromosomes= chromosome that directly controls the development of sexual characteristics. Determines if the organism is female or male. (#23)

22. IV. Sperm and Ova Gamete= sex cell; an egg or a sperm cell.
Have a chromosome count of 23
22 autosomes – in a single set
Autosome = chromosome that contains genes for characteristics not directly related to the sex of the organism.
Plus a single sex chromosome (X or Y)
HAPLOID (n)

23. Discussion
So we have 46 chromosomes or 23 pairs in everyone of our cells. But half came from your mom and half came from your dad. How does this happen?
In Mitosis, we start with 46 and at the end we have a new cell with 46. So shouldn’t we have 92 chromosomes if we get some from mom and some from dad?
WWWWHHHHAAAATTTTTT?

24. V. Sperm and Ova –
A haploid sperm reaches and fuses with a haploid ovum
Fertilization or syngamy
Results in a fertilized egg or zygote
The zygote contains the two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines
Diploid n + n = 46 (2n)

25. How did the Gametes become Haploid?
The process of Meiosis!!

26. VI. Meiosis
Meiosis= form of nuclear division that divides a diploid cell into haploid cells; important in forming gametes for sexual reproduction.
The process that halves the number of chromosomes in a cell
Occurs in Ovaries or Testes
Gametogenesis= process by which gametes are produced through the combination of meiosis.

27. A Variety of Sexual Lifecycles
Human Life cycle
Most fungi and some protists (including some algae)
Plants and some other species of algae

28. Figure 13.4 The human life cycle

29. Alternation of generations
Figure Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy)
Alternation of generations

30. Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
Meiosis Facts:
Four daughter cells
IMPORTANT: Homologous chromosomes are different than sister chromatids
Homologous Chromosome= chromosomes that have the same length, appearance, and copies of genes, although the alleles may differ.
4 Haploid (n) cells instead of 2 diploid cells (2n)

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

34. 2. Prophase I
Lasts longer and is more complex than prophase in mitosis
Chromosomes begin to condense and homologues, each consisting of two sister chromatids, pair up

35. Prophase I
Later in prophase, each chromosome pair becomes visible in the microscope as a tetrad
A cluster of four chromatids
At various places along their length, chromatids of homologous chromosomes are crisscrossed and Crossing Over can take place.

37. Prophase I
Other cellular components prepare for division of the nucleus in a manner similar to that of mitosis
Centrosomes move away from each other and spindle microtubules form between them
The nuclear envelope and nucleoli disperse
The spindle microtubules capture the kinetochores that form on the chromosomes
The chromosomes begin moving to the metaphase plate
Can last for days or longer (over 90% of meiosis)

38. 3. Metaphase I The chromosomes are now arranged on the metaphase plate
Still in homologous pairs
Kinetochore microtubles from one pole of the cell are attached to one chromosome of each pair while microtubules from the opposite pole are attached to the homologue

40. 4. Anaphase I
The spindle apparatus 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)

42. 5. Telophase I
The members of each pair of homologous chromosomes continue to move apart until they reach the poles of the cell
Two new cells are ready to go through cytokinesis, however, they are NOT identical to the original cell (like in mitosis).
Remember, crossing over shuffles the DNA. (recombination)

43. Why are the 2 cells not IDENTICAL?
During Prophase I Crossing Over took place and therefore created a new set of chromosomes that get passed on.

44. 6. 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

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

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

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

48. 7. Prophase II
A spindle apparatus forms and the chromosomes progress toward the metaphase II plate

49. 8. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion
Kinetochores of sister chromatids of each chromosome pointing toward opposite poles

50. 9. Anaphase II The centromers of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromosomes

51. 10. 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)

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

53. Figure 13.8 A comparison of mitosis and meiosis

54. Figure 13.8 A comparison of mitosis and meiosis: summary

55. What type of cells are created at the end of Meiosis
What type of cells are created at the end of Meiosis? (based on chromosome number)
Haploid

56. What do we call the cells that go through Meiosis?
Gametes

57. When 2 gametes come together, what do we call the new cell?
Zygote

58. Explain why it is necessary for gametes to become Haploid?
A zygote is made up of 2 gametes coming together during reproduction. Both gametes need to be haploid so that when they come together they can then create a diploid zygote.

59. IX. Origins of Genetic Variation
As mentioned earlier, in species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Independent assortment of chromosomes
Crossing Over
Random Fertilization

60. Figure 13.9 Independent Assortment

61. Figure Crossing Over

62. Random Fertilization A human ovum plus a human sperm
1 of 8 million combinations possible for each ovum and sperm
223 X 223 = over 70 billion combinations
70 trillion possible combinations with out considering crossing over
YOU REALLY ARE UNIQUE!

63. X. Gametogenesis
1. Oogenesis= Females undergo oogenesis which usually produces 1 viable ova and 3 polar bodies which cannot be fertilized.
2. Spermatogenesis= Males undergo spermatogenesis which is the production of 4 sperm cells that are roughly the same size.

64. Next: Take the next 20-30 minutes to work on the chromosomes handout.
You can either work together, or on your own.
If you finish early, work on your unit 2 whiteboards.

65. Tomorrow: We will do lab 2 and take our test early!
So, we will finish unit 2 tomorrow.

66. THE END OF MEIOSIS