Meiosis is the process of cell division that produces gametes, or sex cells, with half the number of chromosomes as the original parent cell. It involves two rounds of cell division, Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair and may exchange genetic material through crossing over, then separate randomly, reducing the chromosome number. Meiosis II is similar to mitosis, separating sister chromatids without DNA replication to produce four haploid daughter cells, such as sperm or egg cells, each with a unique combination of chromosomes for sexual reproduction.

1. Meiosis
Reproduction of sex cells

2. I. History
 In 1882, a scientist observed that the sex cells
of roundworms contained different numbers
of chromosomes in comparison to the body
(somatic) cells of the individuals.

3. These observations
led scientists to
believe that a
different type of cell
division occurred that
reduced the number
of chromosomes by
half

4. o In human and other eukaryotic
organisms there are two types of cells
1. Somatic cells or basic BODY cells that have
46 chromosomes or 23 PAIRS
-Diploid cells contain a FULL SET of
chromosomes

5. o In human and other eukaryotic
organisms there are two types of cells
2. Gametes or SEX cells that have 23 chromosomes
-Haploid cells have HALF the number of
chromosomes
Sperm
Egg

6. This was important because when 2 sex cells
fused together during FERTILIZATION they
produce a new DIPLOID cell called a ZYGOTE

7. II. Human Chromosome Types:
Within a diploid cell there are two different types
of chromosomes
1. _AUTOSOMES (non sex chromosomes): 44
chromosomes (22 sets)
-most are HOMOLOGOUS CHROMOSOMES
or pairs of chromosomes from mom and dad
that are alike and carry the same genetic traits

8. • Each pair have genes in the SAME location
• _MALE sex chromosomes are NOT homologous

9. Are the following homologous or nonhomologous?

10. The other type of chromosomes are…
2. SEX chromosomes: these determine the gender
a. _XX – female
(these are homologous)
b. _XY – male
(these are non homologous)

11. Chromosomes can be seen during different
stages of the cell cycle. A KARYOTYPE is a
photograph that shows these chromosomes.

13. III. The Sexual Life Cycle
A. MEIOSIS: DIVISION of a diploid cell to
produce SEX CELLS with 23 chromosomes

14. KEY POINTS:
1. _TOGETHER meiosis and fertilization
represent SEXUAL reproduction
2. _GAMETE cells produces are haploid
sperm and egg cells
3. Results in 4 genetically NON-IDENTICAL
daughter cells
4. Ensures GENETIC VARIABILITY by
mixing of genes

15. RESULT:
1. Through the process of sexual reproduction
PARENTS will pass genetic information onto
the next generation
2. The offspring will INHERIT a great variety of
gene combinations

17. IV. A Closer Look at Meiosis
• Two rounds of cell division called MEIOSIS I
and MEIOSIS II
DNA replication only occurs 1 time between
the nuclear divisions to reduce the original
chromosome number by half

18. Draw this flow map across your
paper…then add details under each
square
Ovary/Teste Interphase Meiosis I Meiosis II
Production
of Gametes

19. Ovary/Teste
• Site of Meiosis

20. Interphase
• DNA copies
• (just like in Mitosis)

21. P
M
Meiosis I
A
T
• Nuclear division
• Crossing over of homologous
chromosomes
• Independent assortment of
chromosomes

22. P
M
Meiosis II A
T
• No interphase, DNA does
NOT copy
• Otherwise, looks like
mitosis
• Nuclear division
• Chromosomes separate

23. Production of
Gametes
• 4 Haploid cells
• Sperm/egg

24. V. Phases of Meiosis I
• To understand why haploid cells are
genetically different, Meiosis I needs to be
explained
A. Interphase: chromosomes copy (s-phase)
(in humans, 46 chromosomes copy, making 92)

25. B. During Prophase:
o _HOMOLOGOUS CHROMOSOMES PAIR
and CROSSOVER
o Crossing over: ensures genetic variability in the
offspring because of RECOMBINATION or
MIXING of genetic material

26. Color code your chromosomes…

27. D. During Anaphase:
o _INDEPENDENT ASSORTMENT occurs
o After metaphase, pairs of chromosomes SEPARATE
and SORT randomly during cell division to produce
gametes containing one chromosome of each type

28. E. Telophase and Cytokinesis:
• Pairs are separated into two new cells

29. VI. Phases of Meiosis II:
• _NO INTERPHASE occurs between two
divisions, the new cells produces will have a
REDUCTION in chromosome number and only
have half the number of chromosomes
• PMAT, cytokinesis occurs: looks like mitosis
• _4 NEW cells will RESULT and will be non
identical due to crossing over and random
separation of chromosomes

31. MEIOSIS I
PMAT
MEIOSIS II
PMAT