1. Meiosis and
Sexual Reproduction

2. SEXUAL REPRODUCTION
Sex cells formed by meiosis
Requires 2 parents
Offspring not identical
Meiosis
The process that results in the formation of gametes
Gametes
Sex cells, egg or sperm, which have haploid chromosomes
Haploid
Haploid means HALF, as in the number of chromosomes,
written as n

3. The union of an egg and a sperm
Fertilization n 2n
The union of an egg and a sperm
Zygote
The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes.
Somatic Cells
All body cells except for sex cells. Have diploid chromosomes
Diploid
Diploid means they have a full set of chromosomes,
written as 2n

4. Homologous Chromosomes
Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father.
* Human body cells have 46 chromosomes each
* Human body cells have 23 homologous pairs
Meiosis and Fertilization
Important for survival of many species, because these processes result in genetic variation of offspring.
Meiosis
A kind of cell division that results in gametes (sex cells) with half the number of chromosomes.

5. Gametes do not have homologous pairs,(a full set of
chromosomes) but instead they have only half the chromosomes
from each pair. Gametes are referred to as haploid (n).
B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n).
C. In meiosis, specialized cells divide 2 times producing daughter cells.

6. Meiosis
(2 Cell divisions – Meiosis I & Meiosis II)
During Meiosis, crossing over occurs between homologous chromosomes and genes are exchanged causing genetic variation

7. End of Meiosis I
**** Cells have a full set of “mixed” up chromosomes,
Diploid, and prepare to divide again.

8. End of Meiosis II
Cells divide, resulting in 4 haploid cells – gametes (sex cells)
also known as reduction division.

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

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

11. 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.

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

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

14. Mitosis vs. Meiosis

15. Mitosis vs. Meiosis