1. Meiosis
Formation of Gametes

2. Types of Reproduction Asexual Sexual 1 Parent Identical offspring
No fusion of gametes
Ex. Binary Fission, Budding & Fragmentation
Sexual
2 Parents
Unique offspring
Fusion of gametes

3. Organisms which reproduce sexually…
Are made up of two different types of cells
Somatic: Body cells with a diploid (2N) number of chromosomes. Ex. Skin Cells
Gametes: Sex cells with a haploid (n) number of chromosomes. Ex. Sperm and Eggs

4. Gametes Sperm Ovum (Ova= plural) Male Gamete
Produced in the male gonad…the testes
Ovum (Ova= plural)
Female Gamete
Produced in the female gonad… the ovaries

5. Fertilization: fusion of gametes
During ovulation, an ovum is released from the ovary and transported to an area where fertilization can occur.
The result of fertilization is a fertilized egg, known as a zygote
Sperm + Ovum (egg) Zygote
sperm
n=23
Egg
N=23
2n=46
zygote

6. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that is similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
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

7. Homologous Chromosomes: (Since a homologous pair consists of 4 chromatids, it is called a TETRAD)
eye color
locus
hair color
Paternal Maternal

8. Humans have 23 sets of homologous chromosomes
Each set of homologous chromosomes consists of 2 homologues

9. Homologous Chromosomes (cont.)
Sex Chromosomes
Autosomes
Chromosome sets 1-22
Code for most of the offspring’s traits
Chromosome set 23
Determine sex of offspring
XX= Female
XY= Male

10. Diploid (2n)  Haploid (n)
Meiosis
Process in which a single cell goes through two nuclear divisions to produce 4 haploid gametes
During Meiosis, 2N cells are reduced to N cells
Diploid (2n)  Haploid (n)
If Meiosis did not occur, chromosome number would double with each generation and the offspring would die

11. Oogenesis: Egg Formation
Meiosis in females
One daughter cell keeps ALL of the cytoplasm
Other 3 daughter cells only have genetic material – Polar Bodies (Barr Bodies)

12. Spermatogenesis: Sperm Formation
Meiosis in males
Cytoplasm and Nuclear Material Divides evenly.
Flagella are added last to the cells following differentiation

13. Interphase 1 Similar to interphase in mitosis
Chromosomes replicate (S Phase)
Centriole pairs also replicate
Nucleus & nucleolus are visible
Chromatin
Nuclear Membrane
Cell Membrane
Nucleolus

14. Meiosis 1
Characterized by separation of homologous pairs of chromosomes
4 stages
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1

15. Prophase 1 Chromosomes condense Nuclear envelope disappears
Spindle forms
Synapsis Occurs: Homologous chromosomes pair up to form a tetrad
Tetrad= 2 chromosomes or 4 chromatids
Crossing over occurs
centrioles
spindle fiber

16. Homologous Chromosomes
Prophase 1: Synapsis
Homologous Chromosomes
Sister Chromatids
Sister Chromatids
Tetrad

17. Prophase 1: Crossing Over
During crossing over, segments of non-sister chromatids break and exchange genetic material.
Creates genetic diversity in offspring.
Tetrad
Non-sister chromatids

18. Metaphase I
Homologous chromosomes move to the equator (metaphase plate) as tetrads
Independent Assortment occurs (orientation of homologous pairs to poles is random)
metaphase plate OR
metaphase plate

19. Anaphase I Homologues separate & move to opposite poles
Chromatids do not separate from centromeres!

20. Telophase I Nuclear envelope usually does not reform.
Chromosome number in 2 new cells is now half because chromatids did not separate.
Cytokinesis may or may not occur.

21. Meiosis II No replication of chromosomes!!!!
Proceeds just like mitosis
Starts and ends with haploid cells