Slide 20
At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by meiosis, rather than mitosis
Meiosis results in one set of chromosomes in each gamete
Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number
Slide 21
Fig. 13-5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Egg (n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Slide 22
The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
The three main types of sexual life cycles differ in the timing of meiosis and fertilization
Slide 23
In animals, meiosis produces gametes, which undergo no further cell division before fertilization
Gametes are the only haploid cells in animals
Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism
Slide 24
Fig. 13-6
Key
Haploid (n)
Diploid (2n)
n
n
Gametes
n
n
n
Mitosis
MEIOSIS
FERTILIZATION
MEIOSIS
2n
2n
Zygote
2n
Mitosis
Diploid
multicellular
organism
(a) Animals
Spores
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae
2n
Mitosis
Gametes
Mitosis
n
n
n
Zygote
FERTILIZATION
n
n
n
Mitosis
Zygote
(c) Most fungi and some protists
MEIOSIS
FERTILIZATION
2n
Gametes
n
n
Mitosis
Haploid multi-
cellular organism
(gametophyte)
Haploid unicellular or
multicellular organism
Slide 25
Fig. 13-6a
Key
Haploid (n)
Diploid (2n)
Gametes
n
n
n
2n
2n
Zygote
MEIOSIS
FERTILIZATION
Mitosis
Diploid
multicellular
organism
(a) Animals
Slide 26
Plants and some algae exhibit an alternation of generations
This life cycle includes both a diploid and haploid multicellular stage
The diploid organism, called the sporophyte, makes haploid spores by meiosis
Slide 27