1. The formation of Sex Cells!
MEIOSIS
The formation of Sex Cells!

2. First, some terminology
Gamete: A sex cell with half the # of chromosomes as a somatic cell.
Sex Chromosomes: In humans, the 23rd pair of chromosomes. This determines the biological gender of the individual.
Biological females have two “X” chromosomes: XX
Biological males have one “X” and one “Y”: XY
Autosomes: All other chromosomes.
Haploid (n): A cell with ONE set of chromosomes.
For humans, the haploid number is n = 23.
Diploid (2n): Having TWO sets of chromosomes.
For humans, the diploid number is n = 46.
Gametes are Haploids.
Somatic cells are Diploids.

3. LABEL THE IMAGES WITH THE FOLLOWING TERMS:
Gametes
Somatic Cell
Sex Chromosomes
Autosomes
Haploid
Diploid

4. A Few more terms…
Fertilization: the fusion of male and female gametes (n + n = 2n)
Zygote: the 2n cell resulting from fertilization.
Homologous Chromosomes: A pair of chromosomes that share similar characteristics.
For instance, Chromosome 1 from the female and Chromosome 1 from the male create a pair of homologous chromosomes.
Both will code for the same set of characteristics (Eg. Eye colour), but each may have different versions of the characteristic (Eg. Brown vs. Blue).

5. DIPLOID (2n) HAPLOID (n) HAPLOID (n)  DIPLOID (2n)
WHAT IS MEIOSIS?!
The special form of cell division that produces GAMETES
Remember these have HALF the number of CHROMOSOMES as somatic cells.
MEIOSIS:
DIPLOID (2n) HAPLOID (n)
FERTILIZATION:
HAPLOID (n)  DIPLOID (2n)

6. MEIOSIS AND SEXUAL REPRODUCTION

7. in humans, meiosis occurs in the gonads
Ovary
Testicle
The formation of gametes:
Male: “SPERMATOGENESIS” - sperm
Female: OOGENESIS - egg or ova
Whether in a male or female, the process of MEIOSIS is the same.

8. Involves TWO cell divisions:
Overview of meiosis
Starts with a “Germ Cell:” A diploid(2n) cell that will become a sex cell (n).
Involves TWO cell divisions:
Meiosis I
Meiosis II
Results in the production of four haploid daughter cells

9. The germ cell starts off as a diploid, 2n. Each CHROMOSOME replicates,
INTERPHASE
The germ cell starts off as a diploid, 2n.
Each CHROMOSOME replicates,
resulting in 4n.
2n n

10. MEIOSIS i PROPHASE I The chromosomes condense.
Sister chromatids are held together by a centromere.
CROSSING OVER: Segments of chromosomes are exchanged between a pair of homologous chromosomes.
Tetrad: the entity made when homologous chromosomes cross.
- Nuclear membrane begins breaking down
Centrioles start forming spindle fibres.

11. MEIOSIS i
METAPHASE I
Homologous chromosomes line up in pairs in the middle of the cell.
Spindle fibers attach to centromeres.

12. MEIOSIS i
ANAPHASE I
Homologous chromosomes are pulled to opposite sides of the cell.

13. MEIOSIS i TELOPHASE I Spindle fibres break down
New nuclear membranes form.
If in a human, each nuclei has 23 chromosomes – same as parental cell.
Cytokinesis occurs, but the 2 daughter cells remain adjacent to each other.
In Plant Cells:
A Cell Plate forms between the cells.
In Animal Cells: Cell membrane is pinched.

14. MEIOSIS II
PROPHASE II
Spindle fibers attach to chromosomes.

15. MEIOSIS II METAPHASE II
Chromosomes line up randomly in the middle of the cell

16. MEIOSIS II ANAPHASE II Centromeres split
Sister chromatids separate and move to opposite sides of each cell.

17. MEIOSIS II TELOPHASE II Four nuclei form around the chromosomes.
Spindle fibres break down.
Cytokinesis occurs.

18. MEIOSIS & Genetic variation
Mitosis
Asexual reproduction
Daughter cells are genetically identical
Meiosis
Sexual reproduction
Gametes are genetically different – contributes to diversity
How does meiosis achieve this?
Crossing Over in Prophase I: causes different combinations of genes on chromosomes.
Random Order in Metaphase I: chromosomes line up randomly, so it’s random which daughter cell each chromosome ends up in.

19. Mitosis vs. meiosis MITOSIS MEOISIS Both are involved in cell division
DNA is copied before both mitosis and meiosis
MITOSIS
MEOISIS
# Divisions 1 2
Pairing of Chromosomes
Pairing of homologous chromosomes does not occur.
Pairing of homologous chromosomes occurs during Prophase I.
Does it create haploid of diploid cells?
Diploid
Haploid
# Daughter cells/parent 4
Genetic Variation
Daughter cells are genetically identical
Daughter cells are not genetically identical.
Parental Cell / Location
Somatic Cells in the Body
Germ Cells in the Gonads
Function in Plants & Animals
Growth & Repair
Sexual Reproduction – providing genetic variation

20. Mitosis vs. meiosis